Preface Since the beginning of veterinary education in India, Veterinary Gross Anatomy was taught in the form of Systemic Anatomy. Each of the various systems viz-Skeletal, Muscular, Blood vascular (circulatory), Nervous, Endocrine etc were taught separately in greater details. However, it was long felt necessary to switch over to regional pattern for study of anatomy. This will make easy to understand the anatomical features of all relevant parts of different systems together in the given location (region). Probably with these views the Veterinary Council of India has recently revised the course curriculum of anatomy and introduced the regional approach of teaching veterinary anatomy. The teaching fraternity of veterinary anatomy in India have widely appreciated and accepted this change in the veterinary anatomy curriculum. However there is a need to provide appropriate study and teaching material in the new format. Hope, this book entitled “Veterinary Anatomy- the Regional Gross Anatomy of Domestic Animals” shall fulfill these urgent needs and will reach to the expectations of the students and teachers. This book has been specifically designed to make the so called tough and dry subject of veterinary anatomy as more easy and interesting to learn and practice. The text of this book with regional approach of teaching and studying veterinary gross anatomy has been divided in six divisions. The first division comprises the basics of veterinary anatomy described in the conventional form of systemic anatomy while the remaining five divisions comprise regionwise description of gross anatomy. Each of these regional divisions contains the description of all anatomical structures viz. bones, joints, muscles, viscera, blood vessels, lymphatics nerves, etc located in the respective region of the animal body viz. head, neck, thorax, abdomen, pelvis and the fore and hind limb. Bovine animal, the ox, has been considered as a model species for detail description of the anatomical features, while the major differences or remarkable features in other species like horse, dog, sheep and goat, pig and fowl have been described under a separate heading as Comparative Anatomy. In addition to this, a separate section on Applied Anatomy has been introduced at the end of each regional division. This will makes it easy to understand the applications of the anatomical features that form the basis of surgical, diagnostic, medical and obstetrical techniques in the solution of clinical disorders which is the main purpose of regional pattern of learning the anatomy. Because of these distinctive features, the book has become a little voluminous and therefore, had to be split into two parts. PART-1 comprises first three divisions viz-General Systemic Anatomy, Anatomy of Head and Neck and the Anatomy of Thorax while PART-2 comprises the next three divisions viz- the Anatomy of Abdomen and Pelvis, the Anatomy of Fore limb and the Anatomy of Hind Limb.

In domestic animals, skeleton (Fig. 1-1) is a framework that is formed of bones, cartilages and ligaments binding together giving the specific shape, form and support to the body. In addition, the skeletal framework surrounds the major cavities of the body giving protection to other soft visceral organs of the body. There are three major parts of skeleton in the animal body (1) the axial skeleton (2) the appendicular skeleton and (3) the splanchnic skeleton. The axial skeleton consists of the skull, vertebral column, ribs, and sternum. The appendicular skeleton consists of the bones of fore limb and hind limb The splanchnic skeleton includes some of the bones embedded in the substance of visceral organs in the body such as os cordis in ox and os penis in dog. Bones The bones forming the skeleton of the body are of different shape, size, form and the constituting tissue material. The number of different types of bones of any species may differ in various stages of the growth of the animal. So also the numbers of various bones differ from species to species.

A branch of anatomy that deals with joints or articulations in the body is termed as arthrology or syndesmology. A joint is an articulation or union formed by two or more bones or cartilages with the help of connective tissue. To form a joint, the long bones meet at their extremities e.g. joints of fore limb and hind limb. Joints of flat bones are formed by their borders or edges e.g. joint of bones of skull, the cranial and facial bones. The short bones and irregular bones form a joint by uniting at any of their parts or surfaces e.g. intervertebral joints. Classification of joints Joints are classified on the basis of their structure i.e. the kind of tissue material by which the bones are united, the other constituent parts that help in uniting the bones and the kind of movement performed at that joint. On this basis, the following three major types of joints exist in the skeleton of animal body.

A branch of anatomy that deals with muscles and their accessory structures is called as myology and the body system concerned with myology is called muscular system. The muscular system is composed of specialized cells called muscle fibres because of their shape in the form of fibre or spindle. The plasma of these cells contains myofibrils which are contractile elements that cause contraction of muscles in a definite manner under the influence of a stimulus which is a unique property of muscles. These cells together constitute the distinctive muscular tissue. Classification of muscles Based on their morphological and functional characteristics, there are three types of muscles in the body. Smooth muscle: The muscular tissue in this type of muscles is constituted by masses of spindle shaped cells and they occur mainly in the walls of hollow visceral organs including glands, the vessels, eye ball and hair follicles. These are nonstriated muscles. Contraction in these muscles is weak but sustained and mostly consistent and rhythmic. The action of this muscle is involuntary i.e. not under the will. Cardiac muscle: The muscle fibres of this muscle are arranged in irregular masses and they form cross striations. The ends of some fibres come in contact with the sides of others forming a kind of syncytium. This is a striated muscle and its action is involuntary. This muscle occurs only in the wall of the heart and the immediate proximal ends of the major blood vessels like aorta, venacava and the pulmonary veins.

The digestive system consists of organs concerned with prehension and digestion of food, absorption of digested food and expulsion of unabsorbed food. The digestive system extends from lips to the anus and consists of the primary and accessory organs of digestion. The primary organs include the mouth, pharynx and the alimentary canal (gastrointestinal tube) in that order of sequence. The accessory organs include the tongue, teeth, salivary glands, liver and pancreas. The alimentary canal extends from pharynx to anus and consists of the oesophagus, the stomach, the small intestine and the large intestine. The wall of the alimentary canal consists of, from within outward, the mucous membrane, the muscular layer and the serous layer forming the visceral peritoneum. Mouth The mouth is bounded dorsally (roof) by hard palate and behind that the soft palate, ventrally by the body of the mandible along with the mylohyoideus muscles and laterally by the cheeks. The entrance of the mouth cavity is bounded by the lips and it is called as rima oris, while its caudal opening into the pharynx is called as aditus pharyngis. The cavity of the mouth (cavum oris) is subdivided into two parts by the teeth and alveolar processes (the alveoli) of the mandible. The inner part i.e. the space within the teeth and alveoli is termed as mouth cavity proper, while the space between the cheeks and lips and the teeth and alveoli is called the vestibule of the mouth. The mucous membrane of the mouth cavity is continuous with the margin of the lips and the common integument and caudally it continues with the mucous membrane of the pharynx.

The respiratory system is a complex biological system comprised of several organs that facilitate the inhalation of oxygen and exhalation of carbon dioxide (breathing) in living organisms. The major functions of the respiratory system is providing oxygen to the body cells and removing carbon dioxide from the body. In addition to this, the respiratory system helps in controlling body temperature (especially in dogs), eliminating water (as vapour) and assisting in voice production. The organs of respiratory system extend from nose to lungs. These consist of two major parts, the upper respiratory tract and the lower respiratory tract. The upper respiratory tract is comprised of nose, nasal cavity, the part of mouth, and the part of pharynx. The lower respiratory tract is comprised of the larynx, trachea, bronchi and lungs. Nose The nose is the externally visible part of the respiratory system. It forms the part of the face of the animal extending from the transverse level of eyes to the rostral end of the head. Exteriorly it is formed by the bones, muscles, cartilages and the skin. Internally it encloses the nasal cavity which is lined by the mucous membrane. The dorsal part of the nose (roof of the nasal cavity) is termed as dorsum nasi. It is formed by the nasal and frontal bones of the skull. The lateral parts are formed by the incisive, maxillary, lacrimal and zygomatic bones. Covering these bones externally is a muscular layer formed by the facial muscles running over these bones and acting on the upper lip and nostrils. The muscular layer is externally covered by the skin. The skin on the nose bears fine hairs except at the tip of the nose that is formed by the nostrils. The rostral end of the nose is formed by nostrils which provide entrance to the nasal cavity. The nostrils are supported by cartilages which are the dorsal and ventral extensions of the cartilage of the nasal septum. The main aim of the nose is to support and protect the nasal cavity.

Urinary system is relatively simple and consists of a pair of kidneys, a pair of ureters, the bladder and the urethra. The kidneys are the main organs that filter the blood and produce the urine while the rests of the organs of this system are simply accessory structures that are used for transport and storage of urine. For proper functioning of the metabolic processes of the body, it is essential to constantly maintain the chemical composition and volume of the tissue fluid. This is called the homeostasis. This particular task is performed by the urinary system and in particular by the kidneys. Kidneys The kidneys right and left, are bean shaped red brown relatively large glands situated in the abdominal cavity. These are placed retroperitoneal against the dorsal wall of the abdomen on either side of the spine. The right kidney is about half of its length ahead of the left kidney and it fits into the renal impression on the caudate lobe of the liver. An indentation at about the middle of the medial border is the hilus of the kidney through which pass the renal vessels, nerves and ureters. Both the kidneys are covered by a thin capsule formed of fibrous tissue which surrounds the parenchyma of the kidneys. The capsule can be easily striped away from the kidney. The function of the kidneys is to maintain the volume and composition of the plasma,regulate water, ion, and pH levels, retain nutrients and excrete fluid wastes, toxins and excess electrolytes. The kidneys achieve these functions through glomerular filtration, solute reabsorption, tubular secretion, water balance and acid-base regulation. The renal capsule provides a stiff outer shell to maintain the shape and protect the inner soft parenchyma. Deep to the capsule is the soft, dense vascular cortex. Several cone shaped pyramids form the medulla deep to the cortex. The pyramids are aligned with their bases facing outward towards the cortex and their apices point inward towards the centre of the kidney. Each apex connects to a minor calyx, a small hollow tube that collects urine. The minor calyces merge to form three major calyces, which further merge to form the hollow renal pelvis at the centre of the kidney. The urine collected into the pelvis drains into the ureter that exits through the hilus of the kidney.

Reproduction is the physiological process by which organisms produce offspring. Sexual reproduction involves production of gametes. The gametes are produced by the gonads. The rest of the process of converting the gametes into the offspring involves other reproductive organs. The male and female animals have different genital organs that contribute to reproduction. The organs of reproduction (genital organs) in male include a pair of male gonads the testes (sing.-testis), the epididymis, ductus deferens, the seminal vesicles, a prostate gland, a pair of bulbourethral glands (Cowper’s glands), the male urethra, and the penis. In female the genital organs include a pair of gonads the ovaries, the fallopian tubes, the uterus, the vagina, the vulva and the mammary glands. Male genital system The overall function of the male genital organs is to continuously produce and store the male gametes the sperms, to maintain the continuous supply of semen, to detect estrus in female, and to inseminate and fertilize the female gametes. While the testes are the site for production of sperms, the epididymis, vas deferens and the accessory glands like prostate, seminal vesicles, and bulbourethral glands are responsible for the production of different secretions which mix together and are known as seminal plasma or seminal fluid. The seminal fluid provides a source of energy, buffers, antioxidants and other compounds needed for sperm survivability, maturation and motility. Once spermatozoa are mixed with the seminal fluid, the combined product is called the semen.

Pituitary gland The pituitary gland (hypophysis) lies at the base of the skull and is housed within a bony structure called the sella turcica in the body of the basisphenoid bone. It is a small pea shaped gland which is attached to the hypothalamus by the pituitary stalk. The pituitary gland has two distinct parts called the lobes. These are the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The anterior lobe which is also called as anterior pituitary consists of five types of cells, each of which secretes different hormone or hormones. The types of cells, the hormones secreted by each type and the functions performed by them are narrated below.

Heart The heart is a cone shaped thick walled and four chambered muscular organ. It lies obliquely craniocaudal in the middle mediastinum in between the left and right lungs and is covered by pericardium inside the thoracic cavity. The major part of heart (about 60 per cent) lies to the left of the middle mediastinum. The broad upper end is the base of the heart. It lies at the junction of the dorsal and middle third of the dorsoventral diameter of thorax at the level of about third to sixth rib and it faces craniodorsally. The narrow lower end is the apex. It lies just dorsal to the sternum at the level of about sixth rib and it faces caudoventrally. The surfaces, right and left are convex. They meet cranially at the right ventricular border and caudally at the left ventricular border. The right (cranial) border is strongly convex. It runs ventrally and caudally. The greater part of this border lies parallel to the sternum. The left (caudal) border is much shorter and nearly vertical. The surfaces of the heart are marked by grooves which indicate the division of the heart into four chambers the two atria dorsally and the two ventricles ventrally. The atria are separated internally by the interatrial septum. The ventricles are separated by interventricular septum. The circular coronary groove indicates the division of the heart in to atria dorsally and ventricles ventrally. It encircles the heart in its upper third part except at the level of the origin of pulmonary artery. The coronary groove is quite deep and wide. It provides passage for the coronary artery and vein. Apart from these, it is deeply filled with fat. The right (subsinuosal) interventricular groove and left (paraconal) interventricular groove indicates the line of separation between the two (right and left) ventricles by the interventricular septum. Both of these grooves run downward from the coronary groove towards the apex parallel to the respective borders and terminate just above the apex. The right and left ventricular grooves are occupied by the respective coronary vessels along with the fat.

The great vessels that leave and enter the chambers of the heart include the Aorta, the Pulmonary Artery and the Pulmonary Veins. Aorta The Aorta is the main systemic arterial trunk. It begins as ascending aorta at the base of the left ventricle, passes craniad and dorsad between the pulmonary artery on the left and the right atrium on the right and then turns caudad inclining slightly to the left, thus describing a curve called the aortic arch with its convexity directed upward and forward and the concavity facing downward and backward. From here it proceeds caudad on the ventrolateral surface of the thoracic vertebrae and the longus colli muscle as the descending aorta. At its origin from the base of left ventricle, the caliber of the aorta is greatest forming the bulbus aorta and here it presents three dilatations called the aortic sinuses. As the descending aorta reaches the level of the seventh thoracic vertebra it continues behind as thoracic aorta running through the middle and caudal mediastinum between the left and right lungs. The thoracic aorta first lies to the left of the median plane and then gradually inclines inwards medially and becomes median in position when it reaches to the diaphragm. It then passes through the hiatus aorticus of the diaphragm and enter the abdominal cavity as abdominal aorta. From here it passes caudad below the bodies of the lumbar vertebrae and the psoas major muscle. After reaching at the level of the sixth lumbar vertebra, the abdominal aorta terminates in the iliac quadrification by dividing into two external and two internal iliac arteries.

The lymphatic system is a part of circulatory system and a vital part of the body’s immune system. It is comprised of lymph vessels, lymph ducts, and lymph nodes that move the lymph from tissues to the blood stream and the organs such as the spleen, thymus and bone marrow that produce the lymphocytes and white blood cells. Lymph is very similar to blood plasma. During circulation through the body, the blood reaches to the capillaries at the interstitial spaces. The plasma of the blood is filtered through the capillary wall. The fresh oxygen along with nutrients, hormones and other necessary fluid matters from this plasma are released in to the interstitial fluid and about 85 per cent of the plasma along with the carbon dioxide and metabolic wastes, return into the circulation through the capillary walls. The remaining 15 per cent of the plasma along with the debris of other waste products and the lymphocytes constitutes the lymph which is returned to the circulation through an accessory route of lymphatic vessels and lymph nodes constituting the lymphatic system. Lymph Vessels: The lymph vessels start as finger-like blind ending thin walled lymph capillaries in the interstitial connective tissue. These capillaries form a network and drain into the thicker walled collecting lymph vessels, lymph ducts and lymphatic trunks. Finally the larger lymph trunks and ducts empty their content the lymph into the cranial vena cava at the thoracic inlet. The lymph vessels contain numerous valves with one or two leaflets preventing the back flow of lymph through the lymph vessels. During its course in a centripetal direction through the lymph vessel, the lymph of each lymph vessel has to pass through at least one lymph node prior to its entry into the blood circulation. As a result there lies a chain of lymph nodes on the pathway of lymph vessels in the body.

Brain The brain, a command centre of the body, is a complex organ. The substance of the brain is made of the grey matter and the white matter. The grey matter is often identified as cerebral cortex but other parts of the brain also contain the grey matter. Inside the brain, the grey matter is organized into large accumulations of neurons which are called the nuclei of the brain. Each nucleus is associated with a specific function. In addition to neurons, there is other type of cells the glial cells called neuroglia. The neurons are responsible for sending and receiving impulses or signals. The glial cells are nonneuronal and provide support and nutrition to neuronal cells, form myelin and facilitate signal transmission in the nervous system. The white matter is made of millions of nerve fibres that are coming from neurons and are going to establish communications with different parts of the brain and spinal cord. Since these nerve fibres are coated with myelin sheath around them these appear white and the area occupied by these nerve fibres become the white matter of the brain. The brain has three basic components, the forebrain, midbrain and the hindbrain. Fore brain: The forebrain (Prosencephalon) is the rostral portion of the brain. It consists of cerebrum (Telencephalon) and the thalamus, hypothalamus and pineal gland together constituting the interbrain (Diencephalon). Mid brain: The midbrain (Mesencephalon) is the middle portion of the brain. It is in the centre lying between the interbrain and hindbrain. It consists of a cranial part of brain stem. Hind brain: The hindbrain (Rhombencephalon) is the caudal most portion of the brain. It consists of remaining (caudal) part of the brain stem, the medulla oblongata (Myelencephalon) and the cerebellum and pons (Metencephalon).

Peripheral nerves The peripheral nervous system (PNS) is the part of the nervous system that lies outside the brain and spinal cord. It includes cranial nerves, spinal nerves and their roots and branches, peripheral nerves and neuromuscular junctions. In the peripheral nervous system, bundles of nerve fibres or axons conduct information to and from the central nervous system. The primary role of the peripheral nervous system is to connect the central nervous system to the bones, muscles and skin of the body trunk and the limbs and the organs to allow the complex movements and behaviour. The peripheral nervous system is divided into the somatic nervous system and the autonomic (visceral) nervous system. The somatic nervous system is associated with skeletal muscle voluntary control of body movents. It consists of afferent and efferent nerves. Afferent nerves are concerned with relaying sensation from the body to the central nervous system. The efferent nerves are responsible for sending out commands from central nervous system to the body, stimulating muscle contraction. This system is made up of nerves that connect to the skin, sensory organs and all skeletal muscles. The autonomic nervous system is the part of peripheral nervous system that controls the visceral functions which occur below the level of consciousness. It consists of two divisions, the sympathetic nervous system and the parasympathetic nervous system.

The sense organs are the organs of the body that respond to external stimuli by conveying impulses to the nervous system. The sense organs allow animals to sense changes in the environment around them and in their bodies so that they can respond properly. Animals can sense a wide range of stimuli that include the sense of vision, the sense of hearing, the sense of olfaction (smell), the sense of taste and the sense of touch including pain and pressure. In animals, there are two major groups of senses, the general senses and the special senses.

The bones of skull (Fig 2-1) are divided into two groups, as cranial group of bones and facial group of bones. Most of the bones of the skull are flat bones and a few are irregular bones. Cranial bones The cranial bones form the cranium that encloses and protects the brain and organs of special senses e.g. eye and ear. These bones include the occipital, sphenoid, ethmoid, inter-parietal, parietal, temporal and frontal. The first four of these bones are single and the rests are paired. Occipital The occipital is a single bone (Fig 2 -1C) and is situated on the caudo- dorsal aspect of the skull. It consists of three parts, the lateral parts are called lateral occipital, the basal part is called the basi occipital and the dorsal (squamous) part is called the squamous occipital.

Horse The skull of horse is also pyramidal in shape like that of ox but the base of the pyramid is less wide than that of the ox. It is shorter in length and narrower in width than that of the ox. The cranium is smaller externally, however, the cavity of cranium is larger than that of the ox. The nasal cavity is divided into two chambers by nasal septum. The floor of the nasal cavity is shorter and less concave than that of the ox. The left and the right nasal chambers are completely separated and do not communicate with each other. The horn core is absent. The occipital bone is placed at the extreme caudal part of the skull, caudal condyloid foramen is absent, the basi occipital is longer and prismatic and the squamous occipital is crossed by a nuchal crest. The sphenoid bone has two parts, the pre and post sphenoid, as that in case of ox. The wing of the post sphenoid presents three notches, carotid, oval and spinosal for internal carotid artery, ventral maxillary nerve and spheno-spinous artery respectively. The foramen orbitale and the foramen rotundum are not united. There are no major differences in the ethmoid bone of ox and horse. The frontal bones are less expansive than those of ox. These are situated between parietals caudally and nasals rostrally. The supraorbital foramen perforates the root of the supra orbital process that joins the zygomatic process of the squamous temporal. The horn core is absent. The ventral borders of the frontal bones of the opposite sides form a projection which fits into the notch formed by the two nasal bones. The temporal bone in horse has three parts, the squamous, petrous and tympanic as against two parts, the squamous and petrous of the ox. The squamous part is much more developed than that of ox. It has three parts, the body, zygomatic process and the occipital (pyramidal) process. The later one arises from the caudal part of the squamous. The stylomastoid foramen is between the hyoid and the mastoid processes and the auditory bulla is smaller. The interparietal is a single bone as that of ox.

Cervical vertebrae The skeleton of neck (Fig 2-1) is formed by the cervical vertebrae. These are seven in number. Of these, the first and second cervical vertebrae are highly modified and deviate greatly from the general architectural plan of the vertebrae. The third, fourth and fifth cervical vertebrae are similar and confirm the general plan of the vertebrae. The sixth and seventh vertebrae have some special characteristics but do not deviate much from the general plan. The general plan of vertebrae and the vertebral column has been described in the chapter on skeletal system under Part I on systemic anatomy. Hence, only regional specific characters of the cervical vertebrae are described hereunder. First cervical vertebra (Atlas) The first cervical vertebra (Figure 2-2 A) is called the atlas vertebra because in man it supports the globe of the head. This is an atypical vertebra and it deviates greatly from the general plan of vertebrae. The body and the spinous process are absent. The arch forms a bony ring from which two large curved plates, the wings project laterally. The ring encloses the vertebral foramen. It forms a part of the vertebral canal through which passes the spinal cord.

Horse The first cervical vertebra (atlas) of horse is more or less similar to that of ox with a few variations as below The wings of atlas are curved ventrad and laterad. These are thinner than in ox but their borders are thick. The wings present an additional foramen, the transverse foramen in the caudal part. A faint groove on the ventral surface of the wing connects the transverse foramen to the intervertebral foramen.

Most of the bones of the skull are joined (articulated) with the adjacent bones by sutures. These are immovable joints (synarthroses). The uniting medium is a fibrous tissue and is called the sutural ligament. Most of these joints exist in the early life of the animals and as the age of the animal advances the adjacent bones fuse with each other.

The articulations of the neck are between the bones of the cervical vertebrae. The atlanto occipital articulation and the atlanto axial articulation are the special vertebral articulations while the articulations between the rest of the cervical vertebrae are common vertebral articulations. Atlanto-occipital articulation Type of joint: This is a condyloid type of hinge joint (Ginglymus). Articular surfaces: The atlanto-occipital joint (Figure 2-3 B) is formed between the condyles of the occipital bone and the condyloid cavities of the atlas vertebra.

Horse Articulations of bones of skull Most of the joints of the bones of skull are sutures similar to those in ox. However, in the adult horse many of the sutures are effaced. The ossification of the cartilage between the basilar part of the occipital and the sphenoid occurs quite earlier. The fusion of the mandibular symphysis takes place at about second to third month after birth. The temporomandibular articulation has an additional ligament which consists of elastic tissue. It connects the post-glenoid process to the caudal border of the ramys of the mandible. Hyoidean articulations In the hyoidean articulations, there is only one intercornual articulation in horse instead of the proximal and distal cornual articulations appearing in the ox. The middle cornu is generally lodged in the articulation between the great and small cornu.

The muscles of the head (Fig. 2-3 A) are described in four groups viz (1) superficial muscles including the cutaneous muscle. The other muscles in this group of muscles are the muscles of lips, cheeks, nostrils, eyelids and external ear. (2) the orbital muscles (3) the mandibular muscles and (4) the hyoid muscles. The cutaneous muscle on the head of the ox is well developed and extensive. In most of the regions of the head it is intimately adhered to the skin. The superficial fascia forms a continuous layer beneath the cutaneous muscle in this region except the natural orifices like nostrils, mouth, eyes and the external ear. The deep fascia forms a continuous layer particularly on the temporal, buccal, and the pharyngeal parts of the head. A part of the cutaneous muscle on the frontal region is called the frontalis muscle. The muscle fibres of this muscle blend with the muscles of the eyelids and the contraction of fibres of this muscle causes elevation of the superior eyelid and forms wrinkles to the skin on forehead.

Cutaneous muscle In the cervical region of ox, the cutaneus muscle, cutaneus colli is poorly developed. It is in the form of a narrow strip on the ventral part of the neck. It arises from the ventral median connective tissue strip of the superficial fascia and extends cranially covering the cervical part of the deep fascia. The fibres of this muscle fan out dorsolaterally and form the aponeurosis with the cervical fascia and join the cutaneus facei. Fascia The major groups of muscles of the cervical region are covered by the thick superficial layer of fascia. The fascia is abundant and well developed in the cervical region of the ox. The fascia of the neck is in two layers, the superficial and deep. The superficial fascia is attached to the skin, whereas the deep fascia is again in two parts, the superficial and deep parts. The superficial part covers the superficial structures in the neck viz. the trapezius and brachiocephalicus muscles in the dorsolateral aspect and the jugular canal with jugular vein, and the rhomboideus muscles on the ventrolateral aspect of the neck. The superficial parts of the deep layer of cervical fascia of two sides meet at the median line both, dorsally and ventrally. The deep part of the deep fascia is attached to the vertebrae and the longus colli muscle on either side at the median line and covers the trachea, oesophagus, carotid artery etc before joining ventrally.

Horse The cutaneous muscle in horse is thin and incompletely developed in the superficial fascia. The cutaneous muscle of the facial region (cutaneous fasciei) is incomplete and poorly developed over the mandibular space and the massetor muscle. Rostrally it passes to the angle of the mouth and blends with the orbicularis oris. The cutaneous muscle of the cervical region (cutaneous colli) is situated along the ventral region of the neck. The frontalis muscle is absent. The orbicularis oris forms incomplete ring around the rima oris. The levator nasolabialis is less extensive and is divided into the superior and inferior branches. Between the two branches pass the dilator naris lateralis and levator labii superioris. The levator labii superioris extends from malar, lacrimal and the maxillary bones to the upper lip in a common tendon with the muscle of the opposite side. The zygomaticus is thin, short and weak. It arises from the fascia covering the massetor. The incisivus maxillaris muscle which is absent in ox exists in horse and it lies under the mucous membrane of the upper lip. It originates on the alveolar border of the incisive bone from the level of lateral incisor to the first premolar and it inserts to the upper lip. Its action is to depress the upper lip. The incisivus mandibularis which is also absent in ox exists in horse under the mucous membrane of the lower lip. It originates at the alveolar border of the body of the mandible from the level of lateral incisor to the first premolar and is inserted on the skin of the lower lip and the chin. Its action is to raise the lower lip. The incisivus maxillaris and the incisivus mandibularis muscles acting together cause the closing of rima oris. The depressor labii inferioris is thicker and extends further back. The buccinator is almost similar to that of ox.

Horse The fascia in the cervical region of horse has two layers, the superficial and deep. Each of these is also double layered. The superficial fascia contains the cutaneus muscle the cutaneus colli. The fascia of the two sides meets at the median line along the nuchal ligament and ventrally at the fibrous raphe. The brachiocephalicus is undivided. It arises from the mastoid process, nuchal crest and wing of the atlas and the transverse process of the second, third and fourth cervical vertebrae. The sternocephalicus is also undivided. It arises from the cartilage of the manubrium sterni and inserts at the angle of the lower jaw. The dorsal border of this muscle forms the ventral border of the jugular furrow. The sternothyrohyoideus muscle consists of two separate but unequal parts, the sternothyroideus and the sternohyoideus. The sternothyroideus is and the sternohyoideus is larger. The first part inserts on the thyroid cartilage of the larynx and the second part inserts to the body of the hyoid bone. The omohyoideus is more extensive in horse than that in ox. It arises from the subscapular fascia near the shoulder joint and inserts at the body and the lingual process of the hyoid bone. The scalinus has two parts like those of ox, but the dorsal part in horse is smaller. It arises on the transverse process of the seventh cervical vertebra and inserts on the first rib. The ventral part is very extensive. It arises from the transverse processes of the third to sixth cervical vertebrae and inserts on the medial surface of the cranial border of the first rib. The longus capitis (rectus capitis ventralis major) arises from the transverse process of third, fourth and fifth cervical vertebrae and is inserted on the tubercle at the junction of basilar part of occipital bone with the basi sphenoid. It does not form aponeurosis either at the origin or at its insertion. The rectus capitis ventralis (rectus capitis ventralis minor) is smaller in horse than that in ox. The intertransversarii cervicis (intertransversalis colli) are smaller and present six fasciculi. The most cranial of these is separated from the common mass formed by the rest of these fascicles.

Mouth The mouth is bounded dorsally (roof) by hard palate (2-5 B) and behind that the soft palate, ventrally (floor) by the body of the mandible along with the mylohyoide muscles and laterally by the cheeks (Fig. 2-5 C). The entrance of mouth cavity is bounded by lips and it is called as rima oris, while its caudal opening into the pharynx is called as aditus pharyngis or isthmus faucium. The cavity of the mouth (cavum oris) is subdivided into two parts by the teeth and alveolar processes (the alveoli) of the mandible. The inner part i.e. the space within the teeth and alveoli is termed as mouth cavity proper, while the space between the cheeks and lips and that between the teeth and alveoli is called the vestibule of the mouth. The mucous membrane of the mouth cavity is continuous with the margin of the lips and the common integument and caudally it continues with the mucous membrane of the pharynx.

Nose The nose is the externally visible part of the respiratory system. It forms the part of the face of the animal extending from the transverse level of eyes to the rostral end of the head. Exteriorly it is formed by the bones, muscles, cartilages and the skin. Internally it encloses the nasal cavity which is lined by the mucous membrane. The dorsal part of the nose (roof of the nasal cavity) is termed as dorsum nasi. It is formed by the nasal and frontal bones of the skull. The lateral parts are formed by the incisive, maxillary, lacrimal and zygomatic bones. Covering these bones externally is a muscular layer formed by the facial muscles running over these bones and acting on the upper lip and nostrils. The muscular layer is externally covered by the skin. The skin on the nose bears fine hairs except at the tip of the nose that is formed by the nostrils. The rostral end of the nose is formed by nostrils which provide entrance to the nasal cavity. The nostrils are supported by cartilages which are the dorsal and ventral extensions of the cartilage of the nasal septum. The main aim of the nose is to support and protect the nasal cavity.

The brain, (Figure 2-7 A) a command centre of the body, is a complex organ. The substance of the brain is made of the grey matter and the white matter. The grey matter is often identified as cerebral cortex but other parts of the brain also contain the grey matter. Inside the brain, the grey matter is organized into large accumulations of neurons which are called the nuclei of the brain. Each nucleus is associated with a specific function. In addition to neurons, there is other type of cells the glial cells called neuroglia. The neurons are responsible for sending and receiving impulses or signals. The glial cells are nonneuronal and provide support and nutrition to neuronal cells, form myelin and facilitate signal transmission in the nervous system. The white matter is made of millions of nerve fibres that are coming from neurons and are going to establish communications with different parts of the brain and spinal cord. Since these nerve fibres are coated with myelin sheath around them these appear white and the area occupied by these nerve fibres become the white matter of the brain. The brain has three basic components, the forebrain, midbrain and the hindbrain. Fore brain: The forebrain (Prosencephalon) is the rostral portion of the brain. It consists of cerebrum (Telencephalon) and the thalamus, hypothalamus and pineal gland together constituting the interbrain (Diencephalon). Mid brain: The midbrain (Mesencephalon) is the middle portion of the brain. It is in the centre lying between the interbrain and hindbrain. It consists of a cranial part of brain stem. Hind brain: The hindbrain (Rhombencephalon) is the caudal most portion of the brain. It consists of remaining (caudal) part of the brain stem, the medulla oblongata (Myelencephalon) and the cerebellum and pons (Metencephalon).

Organ of vision (Eye) The vision is by far the most used of the five senses and is one of the primary means that is used to gather information from the surroundings. The organ of vision or the bulb of the eye is contained in the bony socket called the orbit in the skull. The bulb of the eye (eyeball) is embedded in the fat of the orbit. Associated with the eyeball within the orbit are certain accessory structures viz. the muscles, fasciae, eyelids, conjunctiva and lacrimal apparatus. Orbit The orbit is the cavity or socket in the skull wherein the eyeball and its accessory structures (appendages) are situated. The orbit is continuous caudally with the temporal fossa. The axis of the orbit which is usually considered as running between the optic foramen and the middle of the socket is directed rostrad, laterad and dorsad. The medial wall of the orbit is formed by the frontal, lacrimal and the presphenoid (wing) bones of the skull. It is concave and smooth. The medial wall in its rostral part presents a fossa that is called the lacrimal fossa which accommodates the lacrimal sac along with the lacrimal gland. Caudal to the lacrimal fossa is a small depression for the attachment of obliquus ventralis muscle. The dorsal wall of the orbit is formed by the frontal and partly by the lacrimal bone. The supraorbital foramen, an opening of the canal perforating the root of the zygomatic process, is situated in the dorsal wall of the orbit. The ventral wall is slightly rough and is formed by the zygomatic bone, the zygomatic process of the temporal bone, the zygomatic process of the frontal bone and the part of the maxilla bone. The lateral wall (rim) of the orbit is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone. Several foramina and/ or fissures for the passage of the vessels and nerves are located in the walls of the orbit. These include the optic, alar, oval, supraorbital, ethmoidal, lacrimal, maxillary, sphenopalatine, and the orbitorotundum foramen.

Pituitary gland (Hypophysis cerebi) The pituitary gland (hypophysis cerebri) lies at the base of the skull and is housed within a bony structure called the sella turcica in the body of the besisphenoid bone. It is a small pea shaped gland which is attached to the hypothalamus by the pituitary stalk. The pituitary gland has two distinct parts called the lobes. These are the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The anterior lobe which is also called as anterior pituitary consists of five types of cells, each of which secretes different hormone or hormones. The types of cells, the hormones secreted by each type and the functions performed by them are narrated below. The thyrotrophs secrete thyroxin called as Thyroid Stimulating Hormone (TSH). It stimulates production of thyroid hormone. The gonadotrophs secrete leuteinizing hormone (LH) and Follicle Stimulating Hormone (FSH). The LH stimulates the production of oestrogen and progesterone by the ovaries in female and testosterone by the testes in male. The FSH stimulates the production of oestrogen and the growth of egg cells (oocytes) in the ovaries in female and sperm cells (spermatozoa) in the testes in male. The corticotrophs secrete the Adreno-Cortico-Trophic Hormone (ACTH) which stimulates the production of cortisol and adrenogenic hormones by the adrenal cortex. The somatotrophs secret Growth Hormone (GH). The GH is a growth promoting hormone which regulates the growth and differentiation of the skeletal system. The lactotrophs secrete the Prolactine Hormone. The prolactine hormone is responsible for initiation of milk secretion by the acinar cells of the mammary gland.

Mouth Horse The mouth cavity (cavum oris) of horse is longer and narrower. The vestibule of the mouth cavity is less capacious than that of ox. Dog The form and size of the mouth cavity vary greatly in different breeds of dog. In some breeds the mouth is long and narrow while in others it is short and wide. Rima oris in general in all types of dogs is extensive. Sheep and Goat The mouth cavity is elongated and narrow. The rima oris is short. The buccal vestibule is capacious but the labial vestibule is small.

Arteries of neck Common carotid artery The right and left common carotid arteries arise from the bicarotid trunk as it bifurcates on the ventral surface of the trachea at the thoracic inlet. In their course to reach the head, both of these, the right and left, common carotid arteries ascend through the neck along the ventrolateral aspect of the trachea and oesophagus respectively. During this course each of these arteries is accompanied by the vagosympathetic trunk dorsal to it, recurrent laryngeal nerve ventral to it and the internal jugular vein lateral to it. All these structures are enclosed in the carotid sheath. The external jugular vein is separated from the common carotid artery by the omohyoideus and sternocephalicus muscles. The right artery crosses the lateral surface of the trachea and passes along its dorsolateral surface and finally reaches to the lateral surface of the larynx and pharynx in front of the digastricus muscle ventral to the jugular process of the occipital bone. The left artery crosses the trachea and oesophagus and continues along its dorsolateral surface to the level of the third cervical vertebra. Here the oesophagus inclines towards the median plane and the artery follows the lateral surface of oesophagus till it reaches to the lateral surface of the larynx and pharynx just in front of the digastricus muscle and ventral to the jugular processes of the occipital bone. At this point the common carotid artery terminates by giving off the occipital, and pharyngeal arteries and then further continues as the external carotid artery. Along its course through the neck, the common carotid artery gives out several branches as below.

The venous blood from the head and neck is carried by two jugular veins (Figure 2-11 B), the internal and external jugular veins on each side. Both of these extend from the base of the head to the thoracic inlet and are located on the ventrolateral part of the neck. The internal jugular vein is smaller while the external jugular is larger one. The internal jugular vein is formed by the union of the thyroid, laryngeal and occipital veins beneath the wing of the atlas between the mandibular salivary gland and the rectus capitis ventralis major muscle. It passes down the neck in company with the common carotid artery, vagus and sympathetic nerves enclosed within a carotid sheath, receives in its course the muscular venous branches, tracheal vein and the oesophageal vein and terminates by joining the external jugular vein just in front of the thoracic inlet. The external jugular vein is the larger of the two jugular veins and is formed by the union of the superficial temporal and the maxillary veins just behind the caudal border of the mandible and on the deep face of the parotid salivary gland below the temporomandibular joint. It becomes superficial by emerging through the parotid gland, passes down the neck subcutaneously in the jugular furrow, receives the internal jugular vein forming a common jugular vein in front of the thoracic inlet. It receives along its course, the external maxillary vein, parotid veins, auricular veins, small veins from the muscles of the cervical region and the cephalic vein at its termination. While in the jugular groove it is bounded dorsally by the brachiocephalicus muscle, ventrally by sternocephalicus and medially by the sternomastoid part of the sternocephalicus muscle. The vein is covered by skin and fascia and is separated from common carotid artery, vagosympathetic trunk, recurrent laryngeal nerve and internal jugular vein by the upper border of the sternomastoid muscle. The right and left common jugular veins unite and form the jugular confluence which joins the cranial vena cava ventral to the trachea slightly to the right of the median plane at the level of the cranial border of the first rib.

The lymphatics of the head and neck region is comprised of the lymph nodes and the lymph ducts. The lymph nodes of the head are the parotid, pterygoid, mandibular, the lateral and medial retropharyngeal, and the rostral and caudal hyoid. The lymph nodes in the cervical region are the superficial and accessory superficial, cranial, middle and caudal deep cervical and the costocervical and subrhomboidal lymph nodes. The lymph from these lymph nodes is finally collected and carried to the common jugular vein by the tracheal and right lymphatic ducts. Lymph nodes of head Parotid lymph node The parotid lymph node is large and elongated gland usually single but sometimes two small glands connected closely with each other by a fibrous tissue lying on the upper part of the lateral surface in the caudal half of the masseter muscle. It is ventral to the temporomandibular joint, partly or completely covered by the dorsal part of the parotid salivary gland and is situated lateral to the maxillary vessels and nerve. The afferent lymph vessels are received to this gland (node) from the muscles of head, ocular and auricular muscles, the parotid salivary gland, the gums, lips, appendages of the eye, ear, lacrimal gland, and from the bones of the head such as frontal, malar, nasal, maxilla, incisive, turbinate, mandible etc. The afferent from the parotid lymph node joins the tracheal duct of the corresponding side.

Arteries Horse The common carotid artery in horse has a similar origin, course and distribution as that in ox except that it has no relation with the internal jugular vein since this vein is absent in horse. Each common carotid artery at its termination in front of the digastricus gives out internal carotid artery in horse in addition to the occipital and pharyngeal arteries given off by the common carotid artery in ox. The internal carotid artery which is absent in ox is present in horse. It is considered as chief source of blood for the brain in horse. It passes over the guttural pouch and enters the cranial cavity through the foramen lacerum and gains the cavernous sinus. The internal carotid arteries of the opposite sides are connected by a large transverse branch called the intercarotid artery that lies behind the pituitary gland (further course of this artery will be described with the blood supply of the brain in horse). The external carotid artery which is a direct continuation of the common carotid artery after its termination in front of the digastricus muscle is related to guttural pouch during its course along with other structures described in ox.

The nerves of the head connect the central nervous system i.e. brain with the structures like skin, muscles, glands, blood vessels, organs of special senses etc. so as to monitor or control their functioning. These nerves are the cranial nerves and their branches. There are twelve pairs of cranial nerves. Olfactory nerve (Cranial nerve I) The olfactory nerve is the first cranial nerve and is known as the nerve of sense of smell. It consists of fibres which are the axons of olfactory cells of olfactory part of nasal mucous membrane. These fibres are nonmedulated and they form number of small bundles which are enclosed into connective tissue sheaths derived from the meningeal layers of the brain. These nerve fibres bundles pass caudally upward through the cribriform plate of the ethmoid bone and enter the cranium to join the olfactory bulbs of the brain. Besides the olfactory nerve bundles there are two more nerves that extend from the nasal mucous membrane, traverse through the cribriform plate and join the parts of the brain. These include the vomeronasal nerve that is formed by the nerve fibres arising on the vomeronasal organ and terminating on the accessory olfactory bulb of the brain. The other one is the terminal nerve that after reaching the brain joins the olfactory trigone.

There are eight pairs of cervical spinal nerves in ox. The first of these emerges out through the lateral vertebral (alar) foramen of the atlas vertebra. The second through the lateral vertebral foramen (at the cranial border of the arch) of the axis vertebra and the eighth nerve emerges through the intervertebral foramen between the last cervical and first thoracic vertebra. The third to seventh nerves emerge through the corresponding intervertebral foramen. Each of these nerves divides into dorsal and ventral branch immediately after their exit from the vertebral canal through respective foramina. The dorsal branches are larger than the ventral except for the first and last two and most of them divide in to medial and lateral branches. They supply the dorsal (dorsolateral) cervical muscles, skin and the cervical part of the nuchal ligament. The ventral branches are distributed to the ventral cervical muscles and skin, The ventral branches of the last two cervical nerves are large.

Horse The optic and oculomotor nerves in horse are thinner than those in ox. The oculomotor nerve leaves the cranial cavity with the ophthalmic and abducent nerves through the orbital foramen and the trochlear nerve leaves the cranial cavity through a small foramen immediately above the orbital fissure in horse as against the foramen orbitorotundum in ox. The ophthalmic branch of the trigeminal nerve leaves the cranium through the orbital fissure, the maxillary branch through the foramen ovale and the mandibular branch through the foramen rotundum. As against this, the ophthalmic and maxillary branches in ox emerge through the foramen orbitorotundum and the mandibular branch through the foramen ovale. The lacrimal nerve, a branch of the ophthalmic, runs rostrad on the rectus dorsalis and levator palpebrae superioris and ramifies chiefly in the lacrimal gland and the upper eyelid. One of its branches communicates with the orbital branch of the maxillary nerve, penetrates the periorbita, gains the temporal region behind the supraorbital process, forms a plexus with the branches of the frontal and auriculopalpebral nerves and divides to distribute fibres to the skin of the temporal region. The continuation of the maxillary nerve as infraorbital nerve gives out the caudal maxillary alveolar and middle maxillary alveolar branches to supply the molar and premolar teeth and before emerging through the infraorbital foramen gives out the rostral maxillary alveolar branch for the incisor and canine teeth.

Teeth Estimation of age from appearance of teeth The teeth are used as a tool for estimation of age of animal. The time of eruption and the stage of wear of incisors are usually considered as parameters for this purpose. The time of eruption of the incisor teeth in ox has been described under the chapter on gross anatomy of head. The eruption time of incisors in other species is described under the chapter on comparative anatomy of head and neck. To differentiate the deciduous teeth from the permanent teeth is important criteria in this process. The deciduous teeth are smaller, their neck is visible just above the gum line and these are usually whitish in colour. The permanent teeth are larger, their crowns are slightly wider and the neck which separates the crown from the root is concealed inside the gums. These are usually yellowish in colour. In ox, all the four incisors (deciduous) are erupted at the age of 4 years. The permanent teeth start wearing from fifth year onward. The process of wearing commences on mesiolabial side of the masticatory surface. When the entire masticatory surface is in wear, the outer and inner enamel rings are completely separated by yellow dentin and that particular tooth is considered as leveled. In the process of wearing, the gum starts receding and the neck of the tooth is exposed. In ox, usually the wearing begins at I-1 at the age of 6 years, this is followed by I-2 at 7 years, I-3 at 8 years and I-4 at 9 years. Up to the age of 14-15 years usually all incisors fall off but those remained are reduced to small round pegs.

The skeleton of thorax (Fig. 3-1) consists of a bony cage formed by thirteen thoracic vertebrae constituting its dorsal boundary and roof, thirteen pairs of ribs forming its lateral walls, the costal cartilages which assist the ribs to form the lateral walls and the sternum forming its floor. The general architecture of the vertebrae and the vertebral column is described in the chapter on skeletal system under PART ONE (Systemic anatomy) of this book. Only the deviations from the general characteristics of vertebrae as occur in the thoracic vertebrae are described here in the following account on the thoracic vertebrae. Thoracic vertebrae These are thirteen in number in ox (Fig. 3-1 A and B) The characteristic features of these vertebrae are the presence of capitular facets for articulation with the head of the rib and the greater development of dorsal spinous processes (neural spines).for attachment of muscles of the back and the supraspinous ligament that helps in the ligamentum nuchae to extend and suspend the head with the body trunk.

Horse The bony cage of thorax is longer but narrower in horse than that in ox. It is compressed laterally The thoracic vertebrae are eighteen in number. Their bodies are shorter. The spinous process of the first one is shorter and it is curved backward. The height of the spinous process increases up to third and fourth and there after it diminishes gradually up to fifteenth beyond which it is almost same. The backward inclination of the spinous process is most pronounced in the second. The spine of the sixth vertebra is straightly vertical and that of the last two vertebrae is directed slightly forward. The ribs are in eighteen pairs. First eight pairs are sternal and the remaining ten pairs are asternal. These are shorter, narrower and more curved. The length of the ribs increases up to sixth and then it diminishes. The cranial borders of the ribs from second to sixth are sharp and from ninth to eighteenth they are rounded and thick. The lower edges of the ribs are excavated for receiving the costal cartilages. The first rib inclines little forwards, the second rib is vertical and the rests slope backwards. The first pair of costal cartilages articulates at their distal ends with each other and with the sternum. The upper ends of the cartilages form synarthroidial joints with the ribs. The sternum of horse is composed of seven sternebrae. It is canoe or boat shaped and presents three faces, three borders and two extremities. It is compressed laterally in front and dorsoventrally behind. The dorsal surface is narrow and triangular. It is concave craniocaudally. The lateral surfaces are convex above and slightly concave below. The lateral borders separate the dorsal surface from the lateral surfaces and they give attachment to two lateral sternal ligaments. The ventral border is the direct continuation of the cariniform cartilage of the cranial extremity of the sternum. It is thin, sharp and convex from before backwards. It is very prominent and can be felt by palpation in live animal. It is also called as sternal crest or keel. The cranial extremity presents a laterally compressed cariniform cartilage which presents a deep notch dorsally for articulation with the first pair of costal cartilages. The first sternebra is fused with second and the manubrium is made up of cariniform cartilage and the first sternebra.

Articulations of thoracic vertebrae These are (1) Intercentral articulations which are amphiarthroses formed between the bodies of the adjacent vertebrae and (2) Inter neural articulations which are diarthroses formed between the articular processes of the vertebrae. These articulations are similar to the articulations described for typical vertebrae of the cervical region. The only difference is that the supraspinous ligament in the thoracic part of vertebral column is single as against two parts the funicular and lamellar parts of the cervical part of this ligament. The supra spinous ligament in the thoracic part is the backward continuation of the funicular division of its cervical part which extends medially behind from the occipital eminence , passes through the cervical region as nuchal ligament and continues behind over the summits of the spinous processes of the thoracic and lumbar vertebrae and finally terminates on the sacrum.

Cutaneous muscle The cutaneous muscle of the thoracic region is a part of the cutaneous muscle of the trunk. It arises from the interbrachial fascia on the medial surface of the arm and elbow and the lateral aspect of the arm and shoulder. It extends as a broad sheet between the shoulder and arm cranially and the stifle and thigh caudally and then upward to the middle of the dorsum and the tuber coxae. Fascia of thorax The fascia of the thoracic region is the caudal continuation of cervical fascia. It is in two layers, the superficial and deep. The superficial fascia is continuous with that of the skin and the cutaneous muscle. The deep fascia forms a well developed sheath for aponeurosis with the longissimus muscle. It is attached medially to the supraspinous ligament and the summits of the spinous processes of the thoracic vertebrae while laterally it is attached to the lateral surface of the ribs. From here it continues ventrolaterally and passes medial to the scapula under the rhomboideus thoracis muscle.

Muscles of the back (spine), muscles of thorax and muscles of thoracic girdle of Horse, Dog, Sheep and Goat, and Pig almost resemble those of ox except for a few miner differences due to variations in the number of bony segments of the vertebrae, ribs and sternebrae. Fowl Muscles of back (spine) Spinalis thoracis is the thoracic part of the spinales complexus. It originates by aponeurosis on the spinous processes of the thoracic vertebrae including the dorsal median ridge of the synsacrum, dorsal surface of the bases of the transverse processes and the cranial border of the ilium. The muscle inserts by multiple tendinous slips on the spinous processes, vertebral arches, transverse processes of several thoracic vertebrae and the vertebral ribs. This muscle helps in straightening of the head and neck of the fowl. Iliocostalis et longissimus dorsi is the segmented muscle originating on the transverse processes of the thoracic vertebrae and inserting on the vertebral parts of the ribs. This muscle assists in elevating the thorax.

Boundaries The cavity of thorax (thoracic cavity) is formed like a truncated cone which is compressed laterally in its cranial part and some what wider in its caudal part. The dorsal wall or roof of the thorax is formed by the thoracic vertebrae and the ligaments and muscles connected with them. The lateral walls are formed by the ribs and intercostal muscles and other muscles supporting the walls internally and externally. The floor or the ventral wall is formed by the sternum and costal cartilages along with the ligaments and muscles attached to them. The caudal wall or the base of the thorax is formed by the diaphragm. It slopes craniad and ventrad. It is bound dorsally by the last thoracic vertebra, ventrally by xiphoid cartilage and laterally by the last pair of ribs and the costal arch formed by the union of costal cartilages of asternal ribs. The apex of the cone is narrow, nearly oval or triangular in form and it is directed cranially. The cavity of the apex is bound dorsally by the first thoracic vertebra, laterally by the first pair of ribs and ventrally by the first sternebra along with manubrium sterni. The apex is fully occupied by the longus colli muscle, oesophagus, trachea, vessels, nerves and lymph nodes. The width of the thorax is greatest at the level of ninth ribs. The wall of the thorax is internally lined by the serous membrane which is a part of the pleura.

The thoracic cavity (Fig. 3-3 A, B and C) contains lungs and pleura, the heart and pericardium, the great vessels connected with the heart (the aortae, azygos veins and venae cavae), the oesophagus, trachea, the lymph nodes, vessels, nerves and lymphatics. All of these contents except the lungs and pleural sacs are located in the mediastinum, while the lungs along with the pleural sacs are situated on either sides of the mediastinum. Lungs The right and left lungs (Fig. 3-3 A and B) are paired organs which occupy greater space of the thoracic cavity. Each lung is enclosed in a double layered bag of serous membrane called the pleural sac. The right lung is larger, almost double in size than the left one. The difference in size is mainly due to an additional lobe called the intermediate lobe and the greater size of an apical lobe of the right lung. Each lung is divided into different lobes formed by deep fissures extending from the ventral border to almost the area of the root of the lung. Each lung is soft, light weighted, spongy and elastic. The surface of the lung is smooth and glistening and it is mapped out into irregular areas indicating the lobules of the lungs. In the fresh, unpreserved cadaver the lung collapses to about one third of its original size and looses its normal shape. The colour in fresh state is pink, in bled animals it is pale grey and in unbled animals it is dark red. In embalmed specimens, the outer surface of the lungs is marked by the imprints of other organs and structures that are related to lungs in the thoracic cavity.

Lungs and pleura Horse The lungs of horse are comparatively larger and longer than those of ox. These are not divided into lobes by deep fissures. The surface markings of lobules of lungs are indistinct due to reduced amount of interlobular connective tissue. The upper part of the medial surface of left lung has two grooves which are the markings of the aorta and oesophagus. These markings are comparatively faint and less marked. The ventral border of both lungs presents cardiac notch. The left cardiac notch is quadrilateral and more extensive than the right one. It extends from third to sixth ribs. The right cardiac notch is smaller and triangular. It extends from third to fifth ribs. Dog The lungs of dog are short and wide in order to be accommodated in the correspondingly similar shape and size of the thoracic cavity of dog. The fissures in the ventral border of the lungs are deeper and extend up to the hilus of the lung giving it an appearance of independent lobes. The right lung has four lobes and the left lung has two lobes. The intermediate (accessory) lobe of the right lung presents on its lateral surface a groove for caudal vena cava and right phrenic nerve. The cardiac impression on the mediastinal surface of the cardiac lobe is deeper in the right lung than that in the left lung. The right cardiac notch is triangular and it lies at the level of fourth and fifth interchondral spaces. The left cardiac notch is more ventral in position at the level of fifth and sixth interchondral space. The lobules are small and the surface lobulation is not distinct because of less amount of interlobular connective tissue.

The wall of the thorax and its contents (the viscera) are supplied by the following branches of the thoracic aorta. Brachiocephalic trunk The brachiocephalic trunk (Fig. 3-4 A) is a large vessel distributing arterial branches to the head, neck, thoracic limbs (fore limbs) and the cranial part of thorax especially to the region of the withers, thoracic floor and parts of the cranial third of the lateral thoracic wall. It arises from the convexity of the aortic arch between the right auricle and pulmonary artery within the pericardium. It is directed cranially and dorsally in the cranial mediastinum ventral to the trachea. It is partly covered on the right side by the cranial vena cava and on the left it is crossed by the vagus nerve. The recurrent laryngeal nerve runs between it and the trachea. It gives off the left subclavian artery opposite the second intercostal space or the third rib and the right subclavian artery opposite the first rib and then continues cranially as the common bicarotid trunk. The subclavian arteries entirely furnish blood supply to the thoracic limbs. Except for different (separate) origins and the left artery being longer than the right artery, the right and left subclavian arteries are identical in their course and distribution. A short portion of each artery is within the thoracic cavity and is called as intrathoracic course. Each of these arteries leave the thorax by encircling and crossing the cranial border of the first rib below the insertion of the scalinus ventralis muscle and above the brachial vein. The intrathoracic branches of the subclavian arteries are the costocervical, caudal cervical and internal thoracic arteries.

The venous drainage from the wall of the thorax is performed by the dorsal intercostal veins. These veins are distributed as below. The dorsal intercostal veins are twelve pairs in number. Of these, the fourth to twelfth intercostal veins from right side and fifth to twelfth intercostal veins from the left side join the vena azygos.

The lymphatics of the thoracic region are comprised of large number of lymph nodes which are arranged in different groups like the aortic, sternal, mediastinal and bronchial. The efferent ducts from these lymph nodes ultimately join the thoracic duct which carries lymph from all over the body except the head neck and fore limb to open in the cranial venacava. Thoracic aortic and sternal lymph nodes The lymphatics of the thoracic and sternal region is comprised of thoracic aortic, intercostal, cranial and caudal sternal, and the xiphoid lymph nodes and the thoracic duct which conducts the lymph collected from most of the parts of the body except the head, neck and fore limbs. The thoracic aortic lymph nodes are located along the dorsolateral border of the aorta, ventral to the sympathetic trunk. The afferents for these lymph nodes are received from the muscles of thoracic wall, diaphragm, pleural and pericardial sacs and the intercostal lymph nodes. The efferents from the thoracic aortic lymph nodes are connected to the thoracic duct either directly or through other lymph nodes of the thoracic cavity.

Arteries of thorax Horse The bronchial and oesophageal arteries in horse have a common trunk of origin as broncho-oesophageal artery. It arises from the convexity of aortic arch, passes ventrally and caudally on the right side of the aorta and divides into bronchial and oesophageal arteries. The oesophageal artery passes caudally in the caudal mediastinum along the dorsal face of oesophagus and supplies branches to oesophagus, caudal mediastinal lymph nodes and pleura and anastomoses with the pleuro-oesophageal branch of the gastric artery. The intercostal arteries are eighteen pairs in number. The first pair arises from the superior artery, the next three pairs come from subcostal artery and the remaining four arise from the thoracic aorta. The phrenic arteries are not variable in origin as in the ox. They are small and are two to three in number. They arise on the ventral face of the aorta while it passes through the hiatus aorticus and enter the crura of the diaphragm. The rest of the arterial branches of thorax in horse are similar to those in ox.

Spinal nerves The thoracic spinal nerves are thirteen pairs in ox. These are named and numbered according to the vertebra caudal to which they emerge from the vertebral canal. They emerge from the vertebral canal through the respective lateral vertebral (intervertebral) foramina. Each nerve, after its emergence, divides into dorsal and ventral branches. Each dorsal branch passes up dorsally between the transverse processes of the vertebrae and divides in to medial and lateral branches. The medial branch passes upward and supplies the dorsal muscles of the back. The lateral branch passes laterally on the deep surface of the longissimus dorsi muscle. At the lateral border of this muscle it passes between this and longissimus costarum muscles and becomes a cutaneous nerve which divides into ascending and descending branches supplying the muscles and the skin of this region.. Before its division, the lateral branch supplies muscular branches to the longissimus dorsi and longissimus costarum muscles. The ventral branches continue ventrally as intercostal nerves. The first and second thoracic nerves, before their continuation as intercostal nerves, give out a branch to the brachial plexus. At a short distance from the spinal nerve, the ventral branch of each of these nerves is connected to sympathetic trunk by a ramus communicans containing preganglionic and postganglionic sympathetic fibres. The intercostal nerves run down ventrally through the intercostal spaces, lying between the external and internal intercostal muscles close to the caudal border of the respective rib and in company with the intercostal vessels. Along this course they supply these muscles. After a variable distance, the intercostal nerves and vessels traverse through the internal intercostal muscles and become subpleural in position. The second to eighth nerves end in the transversus thoracis muscle. The rests behind these end in the rectus abdominis and other abdominal muscles and the skin in this region.

The thoracic spinal nerves of the horse, dog, sheep and goat, pig and fowl almost resemble to those of ox in origin, disposition and distribution. The number of pairs of thoracic spinal nerves, however, vary and it corresponds to the number of thoracic vertebrae present in those particular species of animal.

Surface locations for clinical examination on the wall of thorax Lymph nodes Prescapular lymph node is palpable as a cylindrical object just cranial to the point of shoulder. Palpation can be performed by putting the base of the hand on the cranial edge of the scapula and pressing forward with fingers. This enables to roll the skin and feel the lymph node flicking underneath the fingers. Heart The clinical examination of heart is performed by percussion and auscultation on the thoracic wall. For percussion of heart, usually the fifth rib is identified opposite the olecranon of a straight vertical limb. Two fields are identified for percussion of heart. (1) The area of absolute cardiac dullness corresponds to the area of contact of the pericardium with the thoracic wall. It gives a flat sound on light percussion. (2) The area of relative cardiac dullness indicates the outline of heart which is covered by the thin margin of lung. It needs somewhat stronger percussion to define this area which is usually difficult in animals. The absolute dullness is used to test the hypertrophy, displacement and pericardial effusion. In ox, the normal area of absolute dullness is explored in the fourth and fifth intercostal spaces. Traumatic pericarditis may result in absolute dullness in these spaces extending above a horizontal line through the shoulder joint. In horse this area is a triangle bounded by the triceps in front, the sternum below and the lung along the line joining these two. On left side of the normal horse, the dullness extends about 7 cm above the olecranon in the fourth intercostal space and about 3 cm above the olecranon in the fifth space. On the right side, the dullness extends about 3 cm above the olecranon in the fifth space only. In dog, the area of absolute dullness extends about 1 to 2 cm above the sternum in the fourth and fifth intercostal spaces in the line with the corresponding costochondral junction.

The skeleton of abdomen and pelvis (Fig. 4-1) consists of the lumbar vertebrae that form the dorsal boundary or roof of the abdomen, sacral vertebrae or the sacrum that forms the dorsal boundary or the roof of the pelvic cavity, the xiphoid cartilage of the sternum that provides attachment to the strong ligaments of the abdominal floor and the pelvic bone (pelvis) that forms the floor and lateral wall of the pelvic cavity. Lumbar vertebrae The lumbar vertebrae (Fig. 4-1 A) in ox are six in number. These are characterized by great size and form of their transverse processes. The other characteristic features of lumbar vertebrae are as below. The body of the vertebra is much compressed dorsoventrally. The convexity of the cranial extremity and concavity of the caudal extremity of the body of the vertebra is moderate. The arch of the first lumbar vertebra is similar in size to that of the last thoracic vertebra but behind this the arches of the second to sixth lumbar vertebra gradually increase in height and width. The caudal notches are deeper than the cranial one. The intervertebral foramina of the first few lumbar segments are often double.

Horse The lumbar vertebrae in horse are six in number. The bodies of these vertebrae are shorter. The first three of them have a distinct ventral spine which subsides thereafter. The transverse processes increase in length from first to third or fourth and then diminish. The first one or two slightly curve caudad and the last two or three curve craniad and the third is at right angles. The caudal border of the transverse process of the fifth vertebra at its base has an oval concave facet which articulates with a convex facet on the cranial border of sixth vertebra. The sixth transverse process is thick at the base and thin, narrow at its free end. It is curved forward. It has a large concave facet on its caudal border by which it articulates with the corresponding facet on sacrum. The sacrum of horse is formed by the fusion of five sacral vertebrae similar to that in ox. However, the fusion of the spinous processes is incomplete. On either side of the base of the each spinous process is a groove which presents the dorsal sacral foramen. There are four foramina on each side of the dorsum of the sacrum of horse. The ventral surface of sacrum of horse is not so deeply arched as that of ox. The wings of sacrum are prismatic. Each wing has a large oval convex facet for articulation with similar facet on the caudal border of the transverse process of last lumbar vertebra. The coccygeal vertebrae are eighteen to twenty in number in horse. The laminae of the neural arch of the coccygeal vertebrae beyond fourth vertebra fail to meet dorsally. The pelvic bone (os coxae) of horse is in general similar to that of ox. The gluteal line on the gluteal surface of the ilium is faint. The ilium is placed more obliquely. The articular surface of ilium is rough and large and extends nearly to the external angle. The internal angles of the ilium of opposite sides are more close to each other. The external angles are formed of four tuberosities each. The ischium of the two sides meets at a greater angle because of which the pelvic floor is bowl like. The ischial arch is wide and shallow. The ridge on the ventral surface of ischium is absent. The tuber ischii is not trifid. The lower border of tuber ischii forms the ventral ischiatic spine. The ventral surface of pubis is crossed by pubic groove which leads to acetabular notch that transmits the pubic femoral ligament.

Sacroiliac articulation The sacroiliac articulation (Fig. 4-2 A) is a synovial (diorthroidal) joint formed between the articular surface on the wing of the sacrum and pelvic surface of the proximal extremity of the ilium. The articular area of the wing of the sacrum has a rough eminence while that of the ilium has a reciprocal rough depression. Both the articular areas are covered by a thin layer of cartilage. The joint cavity is a thin like a cleft and is crossed by fibrous bands. The joint capsule is closely fitting around the margins of the articular surfaces. The joint capsule is reinforced by the ventral sacroiliac ligament which surrounds the joint. The dorsal part of this ligament is quite strong and it occupies the angle between the ilium and wing of the sacrum. The movements are inappreciable at this joint.

Horse In addition to the common articulations of vertebrae, the lumbar region of horse has two special articulations, the inter-transverse articulations and lumbosacral articulations. The intertransverse articulations are bilateral joints formed between the transverse processes of the fourth and fifth and between the fifth and sixth lumbar vertebrae. The articular surfaces of these joints are the oval facets on the transverse processes of these vertebrae, the cranial ones being concave while the caudal ones the convex. These are synovial joints (diarthrodial). The capsule of the joint is reinforced ventrally. The lumbosacral articulations are diarthrodial joints formed between the transverse processes of the sixth lumbar vertebra and the wings of the sacrum. These joints are similar to the intertransverse articulations. The sacroiliac articulation of the pelvis is similar to that of ox. However, the sacrosciatic ligament is broader. Its lateral surface is related to middle gluteus, biceps femoris and semitendinosus muscles.

Cutaneous muscle The cutaneous muscle of abdomen and pelvis is the continuation of cutaneous muscle of thorax. This has been described under the muscles of thorax. Abdominal fascia Beneath the skin in the abdominal region, there are two layers of fascia, the superficial and deep, similar to those of the fascia of thoracic region. The superficial fascia is next to skin. It is fused dorsally with the thoracolumbar fascia. Cranially it is continuous with the superficial fascia of shoulder and arm and caudally with that of the gluteal region. In the inguinal region it forms part of fascia of penis in male and of mammary glands in female animals. At the flank, it continues as fascia of thigh region. The deep fascia forms the abdominal tunic. It is especially well developed as a thick sheet of tunic in the ventral and the adjacent lateral parts of the abdominal floor. It extends cranially over the ribs and external intercostal and the caudal digitations of the serratus thoracis muscles. Caudally it is attached to the external angle of the ilium and thoracolumbar fascia. The suspensory ligament of prepuce in males and that of the mammary glands (udder) in females is an extension of this tunic. It strongly supports the abdominal muscles to bear the weight of abdominal viscera.

Horse The muscles of lumbar spine in horse are more or less similar to those of ox. Of the sublumbar muscles, the psoas minor lies under the bodies of the lumbar and last three thoracic vertebrae. It arises from dorsal ends of sixteenth and seventeenth ribs and eighteenth thoracic vertebra and the first five lumbar vertebrae. The psoas major arises from dorsal ends of last two ribs and the ventral faces of the transverse processes of the lumbar vertebrae. It is much wider than that of ox and covers the quadratus lumborum entirely. The origin of iliacus muscle is limited to sacrum and ilium only. It has no lumbar origin and it is not so intimately united with psoas major muscle. The quadratus lumborum is less developed. The intertransversarii lumborum is absent between the fifth and sixth lumbar transverse processes. The muscles of abdominal wall in horse do not have any striking differences, except that the obliquus abdominis externus is thicker and more extensive. The muscular portion of this muscle reaches the external angle of ilium and as high as lumbar transverse processes.

Stomach The stomach of ruminants (Fig. 4-4 A) is compound stomach with four parts; the rumen, reticulum, omasum and abomasum. The first three compartments comprise the forestomach or proventriculus. These parts have a nonglandular mucous membrane lined by a stratified squamous epithelium and are concerned with maceration, fermentation and cellulose digestion while the last part, the abomasum has a glandular mucous membrane which is concerned with digestion of the food material with the help of gastric juices. The relative sizes of the four compartments change with the age of the animal. In new bourn calf the rumen and reticulum together have about half the capacity of abomasum. But, as the diet of the animal is restricted to milk during this period, these two compartments remain collapsed and functionless. At eight weeks of age, the total capacity of rumen and reticulum equals to that of the abomasum and at twelve weeks of age the combined capacity of rumen and reticulum doubles to that of the abomasum. During this period the growth of the omasum is slow and steady. At about one and half years of age the omasum and abomasum equals in their capacity. At this age the four parts reach to their definitive size, shape and capacity. The relative capacity of the four compartments at this stage is rumen eighty per cent, reticulum five per cent, omasum seven per cent and abomasum eight per cent. The total capacity of average sized cattle varies between hundred fifteen to hundred fifty litre.

Stomach Horse The stomach of horse is a simple stomach. It is ‘J’ shaped sac with its right part very much shorter than the left part. It is situated in the dorsal part of the abdominal cavity behind the diaphragm and liver mainly to the left of the median line. The parietal surface faces craniad and dorsad and to the left. It is related to the diaphragm and liver. The visceral surface faces caudad and ventrad and to the right. It is related to the small intestine and colon. Its lesser curvature is short and the greater curvature is extensive. The left extremity is called the saccus caecus. It lies ventral to the sixteenth or seventeenth rib. It is related to pancreas and terminal part of greater colon caudally and to the base of the spleen laterally. The right extremity is the pylorus. It is smaller and is continued by duodenum. The stomach is fastened by several ligaments which include the gastro-phrenic ligament, the gastro- pancreatic ligament, the greater omentum, the lesser omentum and the gastro-splenic ligament. The capacity of the equine stomach is about 8 to 15 liters. The wall of the stomach consists of four coats, the serous, muscular, submucous and the mucous. The mucous membrane is clearly divided into two parts by sinuous ridge called the margo plicatus. The part which is towards the left extremity is nonglandular while that towards the right extremity is glandular.

The urinary organs include the kidneys, ureters, bladder and urethra (Fig. 4-5 A). Kidneys The kidneys are two compound tubular glands situated retroperitoneally in the sublumbar region. They are elongated, the right kidney being elliptical in outline and the left one slightly oval. They are red brown in colour. The weight of each kidney varies from 500 to 700 gm, the left kidney being about 25 gm heavier than the right kidney. The outer surface of the bovine kidneys is marked by polygonal lobes which are about twenty in number and are separated from each other by deep grooves (fissures). The grooves are filled in by fat. Both the kidneys are embedded in a large amount of perirenal fat termed as adipose capsule.

Horse The kidneys of horse slightly differ in form and size than those of ox. The outer surfaces of kidneys of horse are smooth and plane as against lobulated kidneys of ox. The right kidney is like the heart of the plying cards while left kidney is bean shaped. The right kidney is placed under the vertebral ends of the last two ribs and first lumbar transverse process. The left kidney is situated under the last rib and the first two or three lumbar transverse processes. The hilus of both the kidneys is at about the middle of the medial border of the respective kidneys. The cranial extremity of the right kidney is thicker while the caudal extremity of the left kidney is wider and thicker. The hilus of the kidneys lead into the renal sinus in the interior for the lodgment of the renal pelvis which is the dilated origin of the ureter. The inner central part of the medulla forms a concave ridge called the renal crest. It presents numerous openings through which the large ducts open into the renal pelvis. This part is called the area cribrosa. The ureter of the horse kidney begins at the renal pelvis and is about 70 cm in length and 6 to 8 mm in diameter. The urinary bladder of horse is shorter but wider than that of the ox. Its capacity is about 2.8 to 3.8 liters. The rest of the features are same as those of ox.

The female genital organs (Fig. 4-6 A) comprise the ovaries where the female gametes (ova) are produced, the uterine (fallopian) tubes which convey the ova to the uterus and where the fertilization takes place, the uterus in which the ovum and the embryo develops, the dilatable passage formed by the vagina and vulva through which the foetus is expelled to the exterior and the mammary glands (udder) which is functionally associated with the female genital organs. All of these except the mammary glands are located in the abdominal and pelvic cavity while the mammary glands are located outside the cavity in the perineal region. Ovaries The ovaries are two small oval glands measuring about 3.5 to 4.0 cm in length, 2.5 cm in width and about 1.5 cm in thickness and they weigh about 15 to 20 gm. They are oval in form and are situated a little above the middle of the pelvic inlet and approximately 40 to 45 cm cranial to vulvar opening in a middle sized cow. They are attached to the sublumbar region by the cranial part of the broad ligament of uterus, the mesovarium. Each ovary presents for description two surfaces, two borders and two extremities. The medial and the lateral surfaces of the ovary are smooth and rounded. The attached or mesovarial border is convex. It is enclosed in a part of the broad ligament termed as mesovarium. The vessels and nerves reach the ovary by way of mesovarium to this border and enter the gland through the hilus. The free border which is also convex presents a notch in its middle. The ovarian vessels and nerves enter the gland through this notch. The cranial or tubal extremity is rounded and is related to the fimbriated end of the uterine tube. The caudal or uterine extremity is also round but narrower and is connected to the horn of the uterus by the ovarian ligament.

Horse The ovaries in mare are bean shaped and much larger than those of cow. Each is about 7 to 8 cm long and about 3 to 4 cm thick. It weighs about 70 to 80 gm. These are situated in the sublumbar region under the fourth or fifth lumbar vertebra. They are usually in contact with the roof of the abdomen. The average distance of ovaries from the vulvar orifice is about 50 to 55 cm in the average sized mare. The notch on the free border leads to a depression, the ovulation fossa where ovulation occurs. The uterine tubes are shorter and more flexuous in mare than those in cow. Each is about 20 to 30 cm long. At its ovarian end it is wider with a diameter of about 4 to 8 mm and becomes narrower at its uterine end with a diameter of about 2 to 3 mm. The uterus in mare is situated partly in abdomen and partly in pelvis. The horns of the uterus are usually pressed against the sublumbar muscles by the intestine. When moderately distended they are about 25 cm long. The junction of the uterine horns with the body of the uterus is abrupt. The average length of the body is about 18 to 20 cm and its diameter is about 10 cm when moderately distended. The cervix is about 5.0 to 7.5 cm in length and 3.5 to 4.0 cm in diameter. The cervical canal is straight. The mucous membrane of the uterus has no cotyledons. The vagina in mare is shorter than that in cow. It measures about 15 to 20 cm in length and 10 to 12 cm in diameter when slightly distended. Most of the vagina is retroperitoneal.

The male genital organs (Fig. 4-6 C) comprise the two testicles (where the male gametes are produced) with their coverings and appendages; the epididymis (which stores and transports the gametes); the ductus deferens (the ducts of the testicles); the seminal vesicles, prostate and bulbourethral glands (accessory sex glands); the urethra and the penis (the organ of copulation). Testicles (Testes) The testicles are the essential male gonads. They produce male gametes, the spermatozoa and the endocrine secretion which influences the development and maintainance of masculine characteristics. The testicles are paired compound tubular glands situated in the scrotum below the inguinal canals suspended by spermatic cords. Each testicle is about 15 cm long and about 6.5 to 7.5 cm in diameter. Their long axis is vertical. The left testicle is more often slightly larger and lower in position than the right testicle. In such conditions the scrotum appears asymmetrical. Each testicle is slightly compressed from side to side and presents two surfaces, two borders and two extremities. The lateral surface is convex and smooth and is covered by the tunica vaginalis propria. The medial surface is slightly flattened due to its contact with the septum scroti. The cranial border is convex and it is free and hence is called the free border. The caudal border is nearly straight and is attached to the epididymis and hence is called the attached border or the epididymal border. The dorsal and ventral extremities are blunt and rounded. Structure: The gland is compound tubular in structure. It is covered externally by peritoneum, the tunica vaginalis propria which is very closely attached to the underlined tissue. Underneath the peritoneum is the capsule of the testicle called the tunica albuginea because of its white colour. Numerous trabeculae extend from the capsule into the organ forming the supporting tissue for the seminiferous tubules and providing a pathway for vessels. The axial strand of connective tissue extending almost the entire length of the gland is the mediastinum testes. Radiating from the mediastinum are the lobes of yellowish

Horse The testicles in horse are oval in form and they are comparatively smaller in size than those in ox. The long axis of the testicles is parallel to the body of the animal and hence the attached border of the testicle is dorsal while its free border is ventral in position inside the scrotum. The testicle of the stallion is about 10 to 12 cm long, 5 cm wide and about 6 to 7 cm thick. It weighs about 225 to 300 gm. The size of the left and right testicle varies and the left testicle is often larger than the right one. The epididymis is adhered to the attached (dorsal) border of the testicle. The rest of the features of epididymis in horse are similar to those of ox. The vas deferens of horse right from its origin from the tail of epididymis to its course up to the dorsal surface of the bladder it has a uniform diameter of about 6 mm. Thereafter, it forms a fusiform enlargement, the ampulla of the vas deferens and beyond the ampulla the duct abruptly reduces in size. The ampulla is about 15 to 20 cm long and its diameter is about 2 cm. The genital fold is wider and hence the two ducts apart from each other. The vas deferens and the duct of seminal vesicle open in a common ejaculatory orifice on each side of the colliculus seminalis.

The abdominal aorta is the continuation of thoracic part of the descending aorta which enters the abdominal cavity through hiatus aorticus of the diaphragm. After entering the abdominal cavity it passes caudally below the bodies of the lumbar vertebrae and psoas minor muscle in contact with the left crus of the diaphragm and the ventral common vertebral ligament. After reaching at the fifth or sixth lumbar vertebra, it terminates into iliac quadrification by dividing into two common iliac arteries. Each of the common iliac arteries further divides into internal and external iliac artery. The internal iliac artery supplies arterial branches to the wall and the viscera of the pelvic cavity while the external iliac artery supplies arterial branches to the pelvic limb of the corresponding side. The abdominal aorta is related dorsally to the bodies of lumbar vertebrae, ventral common ligament, tendon of left crus of diaphragm and psoas minor muscle, ventrally to the pancreas and rumen, on its right to the caudal vena cava and on its left to the left adrenal, left kidney and left ureter. The abdominal aorta gives out parietal and visceral branches that supply the walls and the viscera of abdominal cavity. Parietal branches The parietal branches of the abdominal aorta are the six pairs of the lumbar arteries. Out of these, four or five pairs arise from the dorsal surface of the aorta while the fifth or sixth pair depending on where the aorta terminates (fifth or sixth lumbar vertebral level) arises from the internal iliac arteries. The lumbar arteries, after arising from the dorsal surface of the aorta, pass upward across the bodies of the lumbar vertebrae, reach the intertransverse spaces between the lumbar transverse processes and divide into dorsal and ventral branches. The dorsal branches are larger. They supply spinal branches to reinforce the ventral spinal artery and the muscular branches to supply the muscles and skin over the back (loin) region of the body trunk. The ventral branches run ventrally through the fascia and muscles of the abdominal walls and supply the muscles and skin of the walls of the abdomen.

Horse The visceral branches of the abdominal aorta are the coeliac, cranial mesenteric, renal, caudal mesenteric and testicular or ovarian. The coeliac artery in horse is shorter than that in ox. It arises from ventral surface of the aorta and reaches the dorsal surface of the pancreas where it divides into three branches, the left gastric, hepatic and splenic. The left gastric artery passes ventrally and cranially in the gastrophrenic ligament, gives off pancreatic and oesophageal branches and divides dorsal and caudal to the cardia into parietal and visceral branches. The parietal branch crosses the lesser curvature of the stomach just to the right of the cardia and ramifies on its parietal surface. The visceral branch is distributed in the similar manner on the visceral surface of the stomach. A long but narrow oesophageal branch arises from the parietal branch of the left gastric, passes cranially through the hiatus oesophageus and enters the thoracic cavity dorsal to the oesophagus and anastomoses with the oesophageal branch of broncho-oesophageal artery. The hepatic artery is larger than the left gastric. It passes dorsal to the pancreas and reaches medial to the portal vein, divides into right and left branches from which the terminal branches enter the liver through portal fissure. Its collateral branches are the pancreatic, right gastric and the gastroduodenal arteries. The gastroduodenal artery further divides into right gastroepiploic and the cranial pancreaticoduodenal arteries.

The caudal vena cava is a large unpaired vein which receives blood from the pelvis, pelvic limbs, mammary glands (in females) and scrotum (in males), the abdominal walls, and the viscera of the abdomen and pelvis. It is formed under the body of the fifth or sixth lumbar vertebra by the union of two common iliac veins which in their turn are formed by the union of the internal iliac and external iliac veins of the corresponding side. It passes cranially in the sublumbar region lying slightly to the right of the median plane and to the right of the abdominal aorta. In the cranial part of the abdomen, it follows the abdominal surface of the diaphragm and is embedded in the liver prior to its entry into the thorax through the foramen vena cava in the diaphragm. In its abdominal course, the caudal vena cava is related dorsally to the sublumbar muscles and right crus of the diaphragm, ventrally to the colon, pancreas and the liver, on its right to the right kidney and the right adrenal and on its left to the abdominal part of the aorta. Tributaries of caudal vena cava The common iliac veins are short vessels formed by the union of the internal and the external iliac veins. These are located at the level of the fifth or sixth lumbar vertebra. The left vein crosses the dorsal surface of the aorta to reach the vena cava.

Horse The caudal vena cava in horse is largely embedded in the substance of the liver on its diaphragmatic surface. Here its free surface is covered only by the peritoneum. After crossing the liver, the caudal vena cava continues caudally along the right side of the abdominal aorta. During its course it gives off several branches from the dorsal and lateral surface. The lumbar veins (parietal) are five or six pairs in number. The last one or two of these often join the internal iliac veins. The renal veins are thin walled. The right renal vein is shorter than the left one. The phrenic veins are two or three in number. The circumflex iliac veins join either the caudal vena cava or the internal iliac vein. The portal vein is formed by the union of cranial and caudal mesenteric and the splenic veins in the cranial part of the sublumbar region at about the level of second lumbar vertebra. It passes cranially and to the right of the median plane continues through the portal ring of the pancreas and reaches to the portal fissure of the liver. Before entering the portal ring of the pancreas, the portal vein receives right gastroepiploic vein, right gastric vein and cranial pancreaticoduodenal vein. The cranial mesenteric, caudal mesenteric and the splenic veins are formed by the tributaries from the veins which are satellites of the branches of the respective arteries. After its entry into the hepatic portal, the portal vein divides into three principal branches. These include the right branch for right lobe, the middle branch also for the right lobe and the left branch for the left and the quadrate lobe.

The lymph nodes of the abdomen and pelvis are in two major divisions, lymph nodes of the wall of the abdomen and pelvis and those of the viscera of abdomen and pelvis. The lymph from all the lymph nodes is collected by different lymph ducts which carry and pour their contents (lymph) in the cisterna chyli near the hiatus aorticus on the abdominal surface of the diaphragm. Lymph nodes of the wall of abdomen and pelvis The lumbar aortic lymph nodes are a chain of small lymph nodes situated along the caudal part of the abdominal aorta. These lymph nodes receive afferents from the peritoneum, sublumbar, spinal and abdominal muscles and the lumbar vertebrae. The efferents from these lymph nodes join each other and form two lumbar trunks which later on unite in a single lumbar lymphatic trunk that opens into the cisterna chyli. The medial iliac lymph nodes are situated near the origins of the deep circumflex iliac and the internal external iliac arteries. The afferents for these lymph nodes are received from the internal iliac, tuberal, gluteal coxal, lateral iliac, subiliac, superficial inguinal and popliteal lymph nodes. The efferents from these lymph nodes either form a lumbar trunk or join any of the lumbar aortic lymph nodes.

Horse Lymph nodes of the wall of abdomen and pelvis The lumbar aortic lymph nodes extend from kidneys to the deep circumflex iliac vessels. These are not clearly distinguishable from the cranial and caudal mesenteric, renal, testicular or ovarian, uterine and iliac lymph nodes. The internal iliac lymph nodes are placed at about the origin of the iliac arteries. The external iliac lymph nodes are several small nodes situated in the triangular space between the cranial and caudal branches of the circumflex iliac artery. The deep (internal) inguinal lymph node is not abdominal in position. It is not situated at the pelvic brim. It is one of the group of the lymph nodes of the hind limb. The superficial (external) inguinal lymph nodes are on either side of the penis and in front of the superficial inguinal ring in male and above the mammary glands as the supramammary lymph nodes in female. The prefemoral lymph nodes are about a dozen small glands arranged in an elongated mass placed on the medial aspect of the cranial border of tensor fasciae lata and on the caudal branch of circumflex iliac artery.

Lumbar nerves There are six pairs of lumbar spinal nerves. Each of these, after emerging out of the intervertebral foramen, gives out firstly a meningeal branch which reenters the vertebral canal through the intervertebral foramen and supplies the meninges. Next to this it furnishes the ramus communicans to the sympathetic chain ganglion. After this, the lumbar spinal nerves divide into dorsal and ventral branches. The dorsal branches are smaller and they supply the muscles and skin on the loins and croup. The ventral branches are larger and they slightly differ from each other in their course and distribution. In their initial course, they pass under the psoas minor and psoas major muscles emerge at the lateral border of the psoas major and supply branches to the sublumbar muscles. Hereafter, they differ in their course and distribution. The ventral branch of the first lumbar nerve passes on the medial face of the quadratus lumborum, runs down the medial face of the transversus abdominis, the obliquus abdominis internus and externus and finally reaches the rectus abdominis, supply all these muscles and the skin on the lateral wall and the floor of the abdomen. The ventral branch of the second lumbar nerve has two divisions. The cranial division passes ventrally and caudally to reach the external angle of the ilium, supplies branches to the oblique muscles of the abdomen and the skin over this region and terminates in the rectus abdominis muscle. The caudal division passes caudally and laterally under the quadratus lumborum, supplies branches to the muscles and skin at the flank region and then crosses the circumflex iliac vessels. Here it joins a similar branch of the third lumbar nerve and forms the genitofemoral nerve.

Abdominal part The sympathetic trunks enter the abdominal cavity between the psoas minor and the crus of the diaphragm of the corresponding side. They run caudally on the ventrolateral surfaces of the lumbar vertebrae and the ventral common vertebral ligament. The left trunk is ventrally related to the abdominal aorta and the right trunk to the caudal vena cava. The sympathetic trunks of the opposite sides in the cranial half of the lumbar part lie rather close to each other and as they pass caudally, they gradually divert from each other. The lumbar sympathetic ganglia are small spindle shaped usually six in number and are present one in each lumbar segment. The first two are larger than the rests. Each is connected with the ventral division of the corresponding lumbar spinal nerve by the ramus communicans. The interganglionic segments of the sympathetic trunk are usually single but some times they split. Splanchnic nerves The greater (major) splanchnic nerve is formed by the contribution of sympathetic nerve fibres from fifth or sixth thoracic to thirteenth thoracic sympathetic ganglia. It becomes larger and larger as it passes caudally along with the sympathetic trunk. It enters the abdomen at the level of the first lumbar vertebra gets separated from it, passes ventrally and laterally across the aorta (left trunk) and vena cava (right trunk) to join the coeliacomesenteric autonomic plexus on and around the coeliac and cranial mesenteric arteries.

Horse There are six pairs of lumbar, five pairs of sacral and five pairs of coccygeal spinal nerves in horse. Their origin, course and distribution is in general similar to those in ox. The number of sympathetic lumbar and coccygeal ganglia is just the same in horse as that of ox. However, the number of sacral sympathetic ganglia is four in horse as against five ganglia of ox. The greater (major) splanchnic nerve in horse is formed by the fibres received from sixth or seventh to fourteenth or fifteenth thoracic sympathetic ganglia. It is smaller in its cranial thoracic part but as it proceeds caudally it gradually increases in size so much that it becomes larger than the thoracic sympathetic trunk in the caudal thoracic part. It receives a communicating branch from the lesser splanchnic nerve before terminating in the coeliacomesenteric ganglion. The lesser (minor) splanchnic nerve is formed by fibres received from the last three or four thoracic and the first two lumbar sympathetic ganglia. It communicates with the greater splanchnic nerve and the renal and adrenal plexuses and terminates in the coeliac plexus and ganglion. The rest of the features of spinal nerves and autonomic plexuses of the abdomen and pelvis in horse are almost similar to those of ox except those related to the distribution of fibres to the parts of ruminant versus simple stomach.

Surface anatomy of the organs of left side of abdominal cavity Rumen Topography of the rumen: The left side of the abdomen in cow is completely occupied by the rumen except for a small area occupied by the reticulum at its cranial end and the spleen on the craniodorsal aspect sandwiched between the dorsal border of rumen and the diaphragm. The rumen lies in contact with the left abdominal wall extending from seventh or eight intercostal space where it touches the diaphragm to the transverse plane of the tuber coxae where it almost touches the pelvic inlet. Palpation of rumen: The rumen can be palpated in the left flank of the abdomen and is usually palpated in the dorsal third of the flank region. In ruminants, the contraction begins at reticulum and it continues in rumen in succession and hence is considered as ruminoreticular contraction cycle. It begins with double contraction of reticulum followed immediately by the contraction of rumen. The rumen contraction begins in the cranial sac and progresses to the dorsal sac and then to the ventral sac. During the contraction of the ventral sac, the dorsal sac is forced out which appears as a bulging at the paralumbar fossa. After this, there is a pause of 30 seconds and thereafter the second ruminal contraction begins which is usually associated with eructation. The ruminoreticular contraction cycle ends with the end of second ruminal contraction. Following this, the second cycle begins. Thus there are two palpable rumen contractions per minute. This rate of contraction increases to 2.8 contractions per minute during the act of feeding of the animal.

The fore limb is termed as pectoral limb or thoracic limb (Fig. 5-1). It is attached to the thorax by means of muscles of pectoral girdle (shoulder girdle). It is composed of four major segments, the shoulder girdle, arm, forearm and manus. The skeleton of shoulder girdle is formed by the shoulder bone, the scapula. The arm is formed by the arm bone the humerus. The forearm is formed by two bones the radius and ulna; and the manus is formed by the carpus consisting of six short bones –the carpal bones arranged in two layers, the metacarpus consisting of one large and one small metacarpal bones and the digits consisting of six short bones the phalanges and six small sesamoids arranged in two rows for each limb. Shoulder girdle The shoulder girdle (thoracic girdle or pectoral girdle) in general is composed of the scapula, coracoid and clavicle. In ruminants the scapula is well developed, the coracoid is in the form of a small rounded process fused with the ventral end of the scapula and the clavicle is absent or in some subjects it occurs embedded in the brachiocephalicus muscle cranial to the point of the shoulder. But this possibility is very rare. Hence, in ox only scapula is considered as the bone of shoulder girdle. Scapula The scapula is a flat triangular bone applied obliquely ventrad and craniad against the cranial part of the lateral wall of thorax. Its long axis extends from the fourth thoracic spine to the sternal end of the first rib. It presents two surfaces, three borders and three angles.

Horse The scapula of horse is a flat triangular bone placed at the shoulder region applied against the cranial part of the lateral wall of thorax similar to that in ox. The scapular spine is placed a little further caudad from the cranial border so that the proportion of the area of supraspinous to infraspinous fossa is one third to two third of the lateral surface of the scapula. The subscapular fossa is deeper and presents well marked vascular impressions. The nutrient foramen is placed at the distal third of the infraspinous fossa. The tuber scapulae and the glenoid cavity are placed further apart. The glenoid cavity is oval in outline and its rim has a distinct notch on its craniomedial aspect. The coronoid process is better developed and the acromion process is absent. The humerus in horse is directed obliquely distally and caudally forming an angle of about 55 degrees with the horizontal plane almost equaling to that in ox. The musculospiral groove on the shaft is deeper and more spirally curved. The nutrient foramen is placed on the distal third of the medial surface. The deltoid tuberosity is better developed. The medial and lateral tuberosities of the proximal extremity are more prominent. The intertuberal (bisipital) groove is divided by an intermediate ridge. The summit of the lateral tuberosity does not arch inward. The head is smaller than that in ox. The coronoid and olecranon fossae of the distal extremity are shallower than those in ox. The radius of the horse is larger and longer of the two bones of the forearm. Close to the medial border of the caudal surface in the lower third of the shaft is a rough elevation for attachment of the superior check ligament (an accessory ligament of the superficial digital flexor). The caudal surface of the shaft towards its lateral border presents a rough triangular area which tapers to a point in the distal third of this surface. This is for attachment of the ulna. The distal interosseous space is absent because the shaft of ulna is shorter and does not reach to the distal part of the shaft of radius. The grooves on the cranial surface of the distal part of the shaft are better marked. The radial tuberosity at the craniomedial aspect of the proximal extremity is larger and more prominent than in ox. The distal articular surfaces are not oblique. It presents two tubercles one on each side, for ligamentous attachment. The tubercle on the lateral aspect is grooved for passage of tendon of lateral digital extensor.

Shoulder joint The shoulder joint (Fig. 5-2 A) is an enarthrosis formed between the glenoid cavity of scapula and head of humerus. Ligaments: The shoulder joint is enclosed by the capsular ligament which is loose enough to accommodate the wide range of movements of the joint and is attached about 1.25 cm away from the periarticular area of the two segments of the joint. The capsular ligament is supported externally by the muscles of the shoulder region. Movements: The shoulder joint performs all kinds of movements but the extension and flexion are best marked.

Horse The shoulder joint of horse has same features as those of ox. However, this joint in horse exhibits greater degree of mobility. The elbow joint of horse is almost similar to that of ox except that there is only one interosseous ligament in horse against two in ox. The carpal joint of horse has only minor differences from that of ox. The special feature of the joint in horse is that it has two additional ligaments, the superior check ligament and the inferior check ligament. The superior check ligament starts from the rough elevation on the distal third of the caudal surface, close to the medial border of the radius and is inserted to the tendon of superficial digital flexor above the carpus. The inferior check ligament is the continuation of the caudal (palmer) capsular ligament of the carpus. It joins the tendon of deep digital flexor at about midway on the palmer surface of the metacarpus. The check ligaments assist the tendons of flexor muscles of the digits after severe exertion to keep then tight and prevent flexion.

The muscles of fore limb (Fig. 5-3 A and B) are distributed in different regions of fore limb as muscles of shoulder girdle, shoulder, arm and those of forearm and manus. By means of the muscles of shoulder girdle the forelimb is attached with the trunk of the body by synsarcosis and bears the weight of the body. The rest of the groups of muscles act on different joints of the fore limb as extensors or flexors of these joints. The group of muscles acting on these joints originates on the bone of the proximal segment and insert on the bone of distal segment of the concerned joint. Muscles of shoulder girdle The muscles of shoulder girdle have been described with the muscles of thorax. Muscles of shoulder The muscles of shoulder arise on the scapula, act on shoulder joint and end on the humerus. These muscles are in two groups as the lateral group of muscles and the medial group of muscles.

Horse The muscles of shoulder region namely the supraspinatus, infraspinatus, teres major, teres minor and coracobrachialis in horse resemble those of ox. The subscapularis muscle has only a single head of origin in horse as against three heads in ox. The deltoideus muscle has two parts of origin as that of ox but in horse its acromial part is named as cranial part and it arises from scapular spine and the scapular fascia instead of the acromion process. In addition to these muscles, the shoulder in horse is composed of one more muscle named as capsularis. The capsularis is a very fine narrow muscle arising from the caudal part of the rim of the glenoid cavity of scapula and is inserted on the caudal part of the neck below the head of the humerus by insinuating itself between the fibres of brachialis muscle. This muscle is peculiar to solipeds and it raises the capsular ligament of shoulder joint during flexion of the joint. The muscles of arm in horse are in general similar to those of ox except the following minor differences. The tendon of origin of biceps brachii is adapted to the intertuberal (bisipital) groove. The tensor fasciae antibrachii is thin in front and wide below. The medial head of triceps brachii is less developed and extends from the middle of the medial surface of shaft of humerus. The anconeus is smaller and less voluminous. Amongst the muscles of forearm and manus, the flexors of carpus especially the flexor carpi radialis, flexor carpi ulnaris and the ulnaris lateralis resemble those of ox. The pronator teres is either vestigial or completely absent. The medial digital extensor is absent. The extensor carpi radialis resembles that of ox. The common digital extensor arises in common with the extensor carpi radialis. Its tendon descends over the synovial capsule of the carpus, passes down the metacarpus and at the fetlock it receives two slips from suspensory ligament to form the broad ligament which is inserted to the pyramidal extensor process of the distal phalanx. The lateral digital extensor is smaller. It arises from humerus and radius. Its tendon is inserted to the prominence on the dorsal (cranial) aspect of the proximal extremity of the proximal phalanx. The extensor carpi obliquus is inserted to the head of the medial small metacarpal bone. The superficial digital flexor does not divide into two parts. Its tendon is reinforced above the carpus by a strong fibrous band- the superior check ligament. The ring for the deep digital flexor is formed entirely by the tendon of superficial digital flexor. The deep digital flexor has same heads of origin as those of ox. Its tendon receives about half way down the metacarpus the inferior check ligament. It is inserted to the semilunar crest and tendinous surface of the distal phalanx.

Subclavian artery The fore limb is chiefly supplied by the subclavian artery (Fig. 5-4 A). It originates from the brachiocephalic trunk within the thoracic cavity. The left subclavian detaches from the brachiocephalic trunk opposite the second rib, passes craniodorsally towards the thoracic inlet winds around the cranial border of the first rib and then courses caudoventrally through the axillary space between the subscapular and teres major muscles as the axillary artery. It gives off the subscapular branch and continues as the brachial artery. The right subclavian is the direct continuation of the brachiocephalic trunk after the later gives off the bicarotid trunk opposite the first rib. It gets exit through the thoracic inlet, winds around the cranial border of the first rib and then continues as the right axillary artery and thereafter as the brachial artery similar to the artery of the left side.

The venous drainage of the fore limb of ox is conducted by two independent routes, the superficial and deep routes, in the fore limb. The major vein of the superficial route is the cephalic vein and that of the deep route is the axillary vein. The venous drainage of the fore limb begins at the digits and is carried by branches of both the routes intermingling or confluent with each other up to the level of elbow joint. From the elbow upwards the journey of the two paths is completely independent of each other. The superficial veins join the cephalic vein while the deep veins join the axillary vein to open into the cranial vena cava. Superficial veins The superficial veins begin in the form of distal deep and distal superficial palmer arch just above the fetlock joint. These venous arches are formed by the union of axial and abaxial digital veins. From these venous arches extend the dorsal metacarpal vein ascending first through the interdigital space and then winding around the large metacarpal bone. In this course it anastomoses with the branches of radial vein on the dorsomedial aspect of the carpometacarpal region and passes further upward as accessory cephalic vein.

The lymph from the fore limb is collected and carried into the circulation through two main lymph nodes, the prescapular and the axillary lymph nodes. There may be two more lymph nodes in fore limb but those are inconsistent and only occasionally found in ox. These include the accessory axillary lymph node and infraspinous lymph nodes and their afferents are mostly from the thoracic wall.

Horse The arteries of fore limb of horse are more or less similar in their course and distribution. However, there are some special features in respect of some of these arteries as described below. The brachial artery descends vertically on the medial aspect of humerus to its distal extremity, crosses it obliquely and descends along the joint capsule and medial lateral ligament of elbow joint, gives off common interosseous artery at the proximal third of the forearm and then continues distally as median artery. The median artery descends along the caudomedial aspect of the radius covered by flexor carpi radialis muscle and divides just proximal to medial condyle into radial and collateral ulnar (ulnar) arteries. The collateral ulnar artery is smaller of the two terminal branches of median artery. It descends along the cranial surface of radius under cover of the extensor carpi radialis and terminates a little above the carpus by anastomosing with a branch of radial artery.

Brachial plexus (FIG. 5-5 A) The fore limb is supplied by nerves that extend from the brachial plexus. The brachial plexus is a large wide, flat and thick fascicular anastomosis of nerves formed by the convergence of ventral branches of last three cervical and first two thoracic spinal nerves in ox. It is placed between the scalenus muscle on the lateral wall of thorax and subscapularis and deep pectoral muscles on the costal (medial) surface of the scapula. From this plexus extend the following eleven branches (nerves) which are distributed mainly to the fore limb (thoracic limb) and most of the muscles of the thoracic girdle.

Horse The brachial plexus in horse is formed by ventral branches of the last three thoracic and first two thoracic spinal nerves similar to that in ox. The disposition and distribution of nerve branches extending from the plexus is almost similar to that of the ox except the following two. The radial nerve after reaching the musculospiral groove gives out a cutaneous branch that passes through the lateral head of triceps and is distributed to the skin on the lateral aspect of forearm. The main trunk of the radial nerve after crossing the carpus descends along the medial aspect of the palmer surface of metacarpus as palmer medial metacarpal nerve and further down on the medial digit as palmer medial digital nerve. The ulnar nerve descends the carpus and after reaching the metacarpal region joins the lateral branch of the median nerve, descends first along the lateral aspect of the palmer surface of metacarpus as palmer lateral metacarpal nerve and then along the lateral (abaxial) aspect of the lateral digit as palmer lateral digital nerve.

Site for nerve block Brachial plexus block The brachial plexus is the key source of nerve supply to the fore limb. Loss of sensation below the elbow is achieved by blocking the nerve roots contributing to this plexus. In ox it is formed by the ventral branches of 6th to 8th cervical and 1st and 2nd thoracic spinal nerves. These nerve roots pass over the lateral aspect of the middle third of the first rib before anastomosing to form the plexus. All these nerve roots can be blocked by injecting anaesthetic solution on these nerves at the middle third of the lateral surface of the first rib. To reach this site, the needle is inserted between the thoracic wall and the medial face of the shoulder at the level of acromion process of the scapula about 12 to 14 cm cranial to it horizontally in the caudal direction. The blocking of brachial plexus is indicated for operations for reducing the fractures of the bones of fore arm and manus.

The hind limb (Fig. 6-1) is termed as pelvic limb and it consists of four segments-the pelvic or hip region, thigh, leg and the pes region. The skeleton of pelvic region is formed by the pelvic girdle (os coxae). The skeleton of thigh region is mainly formed by a long bone (thigh bone) the femur and at its lower extremity is a short sesamoid bone, the patella. The leg region is formed by two long bones, the larger one the tibia and the smaller one the fibula. The last segment, the pes has three subdivisions as tarsus, metatarsus and digits. The tarsus has five short bones arranged in two rows on par with the carpus of fore limb. The metatarsus is constituted by two bones of which one is a long bone called the large metatarsal and the other a short small rudimentary bone the small metatarsal bone. The last part of the hind limb is the digits which are constituted by six phalanges and six sesamoids like those of the forelimb.

Horse The femur of horse is more massive than that in ox. The proximal third of the caudal surface of shaft of femur presents a rough eminence for attachment of biceps femoris muscle. The trochanter minor is a rough ridge like structure. The proximal third of the lateral border of shaft presents trochanter tertius for attachment of superficial gluteus muscle. The trochanter major is comprised of a convexity, a summit and a crest. The trochanteric ridge is vertical and it connects the trochanter major with the third trochanter. The lateral supracondyloid crest and the supracondyloid fossa are better developed in horse than that in ox. The patella of horse is wider and longer than that of ox. The tibia in horse is relatively longer than that in ox. The popliteal line is prominent. It extends from the proximal third of the medial border, obliquely crosses the caudal surface and ends at the middle of the lateral border of the shaft of tibia. The medial border in its proximal third bears a small popliteal tubercle for attachment of popliteus muscle. The lateral border is attached to the shaft of the fibula and encloses an interosseous tibiofibular arch with the fibula. At the proximal extremity, the cranial tuberosity is vertically grooved. The lateral condyle on its lateral side presents facet for articulation with the proximal extremity of fibula. At the distal extremity, the grooves are oblique. The lateral malleolus is fused with the distal extremity of tibia, The medial malleolus is more prominent than the lateral one.

The articulations of hind limb (pelvic limb) comprises the joints of pelvic region which include the sacro-iliac articulation, pelvic symphysis and the hip joint; the articulation of thigh region named as stifle joint; the articulation of tarsal or hock joint and the articulations of digits which include the metatarso-phalangeal joint (fetlock) joint, the proximal interphalangeal joint (pastern joint) and the distal interphalangeal joint (coffin joint). The joints of pelvic region especially the sacro-iliac joint and the pelvic symphysis have been described with the articulations of abdomen and pelvis.

Horse The hip joint has an additional ligament called the pubio-femoral ligament. It arises from the prepubic tendon, passes dorsally and laterally and then caudally and lodges into the subpubic (pubio-femoral) groove, penetrates the capsular ligament, passes through the acetabular notch and is inserted into the fovea capitis behind the round ligament on the head of femur. This ligament is peculiar to solipeds and it prevents the side kicking. The absence of this ligament in ruminants allows them abduction freely and that is how the ruminants can kick sideways. As against this, the equines can kick only back way. The other difference about the hip joint in horse is that the cotyloid ligament is narrower because of the deeper acetabulum. The stifle joint of horse in general resembles that of ox except that the middle patellar ligament is not so prominent as that in ox. The tibio-fibular articulation in horse is a diarthrosis and an amphiarthrosis. The diarthrosis is formed between the lateral condyle of tibia and the head of the fibula. It has intimate capsular ligament. The amphiarthrosis is formed between the shafts of the tibia and the fibula. The shafts of the two bones are binded by the proximal and distal interosseous ligaments. The hock joint of horse has minor differences with those of ox owing to the difference in the number of tarsal bones and their arrangement. The synovial membrane is supported everywhere except at the dorsomedial (craniomedial) aspect where it bulges out in the cases of bog-spavin. If there is a distension of tarsal sheath, it bulges out as two rounded swellings on either sides of the tendon of deep digital flexor. The movements at the hock joint are very limited.

The fascia covering the hind limb is the superficial and deep fascia. The superficial fascia is very scanty lies beneath the skin. It has the same general disposition as in other regions of the body. The deep fascia is in the form of a thick strong sheet covering the superficial layer of muscles over the medial aspect of the hind limb. From this sheet extend several septa that pass between different muscles or muscle groups in the thigh region. The deep fascia of the leg region continues cranially with the fascia of the thoracolumbar region. Caudally it is attached to the aponeurotic tendon of insertion of sartorius and gracilis muscles. The part of the deep fascia that continues on the lateral aspect of the thigh is the fascia lata which is continuous with the gluteal fascia dorsally. The muscles of the hind limb (Fig. 6-3 A and B) are arranged into different regional, positional as well as functional groups. The muscles of hip and thigh region are arranged in the lateral, medial and cranial groups. The muscles of leg and foot are arranged in dorsolateral and plantar groups. The dorsolateral muscles are generally the extensors of tarsus and digits while the plantar muscles are the flexors of tarsus and digits.

Horse The tensor fasciae latae is less extensive. The superficial gluteus exists separately. It is ‘V’ shaped. It originates from the lateral angle of ilium (tuber coxae) and gluteal fascia and is inserted to the third trochanter. The middle gluteus is the largest of the three gluteal muscles and is very extensive. It originates from the aponeurosis of the longissimus dorsi, extends as far forwards up to first lumbar vertebra and is inserted in three heads, to the trochanter major, the crest ventral to trochanter major and the trochanteric ridge. The deep gluteus is smaller than the middle gluteus and is inserted to the convexity of the trochanter major. The biceps femoris has an additional insertion to the femur on the rough surface near the trochanter tertius. The semitendinosus arises from the transverse processes of the first two or three coccygeal vertebrae. The semimembranosus is larger and arises from the sacrosciatic ligament also. The sartorius does not arise from the shaft of the ilium. The gracilis is not so extensively fused to its fellow at the origin. The obturator internus has an iliac part- the pisiformis. Its tendon passes through the lesser sciatic foramen and hence does not unite with that of the obturator externus. Articularis genu is absent

The main arterial trunk that supplies blood to the hind limb is the external iliac artery. As this trunk passes down through different segments (regions) of the limb it is differently named according to that segment or the region. In the abdomen it is named as iliac, in the thigh region as femoral, in the leg region as tibial and popliteal and down that the metatarsal artery and lastly the digital. External iliac artery The external iliac artery arises from the abdominal aorta ventral to the fifth or sixth lumbar vertebra. It descends at the side of the pelvic inlet ventrally and caudally along the tendon of psoas minor, crosses the tendon, reaches the cranial border of the pubis, gains the space between the sartorius and pectineus muscles and then continues in the thigh (femoral region) as the femoral artery. In this course it is covered by peritoneum and is related laterally to the psoas minor, sartorius and iliacus muscles, medially to the common iliac vein and caudally to the external iliac vein. The following are the collateral branches of the external iliac artery.

The venous drainage of hind limb begins from two digital venous plexuses, one for each digit, situated in the corium of the foot. From each of these plexuses extend three digital veins namely the dorsal common digital vein, the medial digital vein and the lateral digital vein. The digital veins in their upward course converge and form the metatarsal veins which in their turn continue upward in the leg region as tibial and popliteal veins and then in the thigh region as femoral vein which finally joins the external iliac vein as the chief vein of the hind limb. Digital veins The dorsal common digital vein arises from the corresponding digital plexus in the form of proper digital vein.The two proper digital veins ascend along the medial face of the corresponding digit and unite at about the middle of the interdigital space below the metatarso-phalangeal articulation (fetlock joint) and form the dorsal common digital vein. The later, thus formed, ascends through the interdigital space, crosses vertically the metatarso-phalangeal articulation and after reaching the distal extremity of the metatarsal bone continues upward as dorsal metatarsal vein. The medial digital vein after arising from the digital venous plexus ascends along the caudomedial aspect of the medial digit in company with the satellite artery, shares a branch each from the dorsal common digital and lateral digital veins, ascends the medial face of the fetlock joint and is then continued up as the medial plantar metatarsal vein. The lateral digital vein after arising from the digital venous plexus of the lateral digit passes upward in the interdigital space, communicates a branch to the dorsal common digital and medial digital veins and then ascends the lateral face of the fetlock joint and then is continued up as the lateral plantar metatarsal vein.

The lymphatics of hind limb is comprised of four lymph nodes namely the deep inguinal, epigastric, coxal and popliteal lymph nodes and the afferents and efferent ducts joining and leaving these lymph nodes. Deep inguinal lymph node The deep inguinal lymph node is located in the region of deep femoral artery near the origin of the caudal epigastric and the pudic branches from it. The lymph node receives afferents from the muscles in the prepubic and the thigh regions. The efferent vessel from this lymph node joins and opens in the medial iliac lymph node. Epigastric lymph node The epigastric lymph node is inconstant. When present, it is located on the course of the caudal epigastric artery near the pubis on the medial surface of the rectus abdominis muscle. The afferents for this lymph node are received from the abdominal muscles and the peritoneum in the vicinity. The efferent vessel of this lymph node joins the medial iliac lymph node.

Horse The arteries of the hind limb of horse have in general similar architecture as that of ox except the special features mentioned below. The deep circumflex iliac artery which is a branch of the external iliac artery in ox has a variable origin in horse. It may originate directly from the aorta or the common iliac trunk just prior to the origin of the external iliac artery. The prepubic and deep femoral arteries arise either by a common trunk or separately from the femoral artery. The obturator artery is a well developed independent branch of the femoral artery in horse as against its origin as a branch of deep femoral artery in ox. The saphenous artery is a small branch of femoral artery and it supplies the skin on the medial aspect of the thigh and leg. The popliteal artery terminates by its division into cranial and caudal tibial arteries at a lower level in horse than that in ox. The caudal tibial artery is larger and longer in horse than that in ox. From the proximal plantar arch extend four caudal metatarsal arteries instead of four, two of them are superficial and the other two deep branches. The large metatarsal artery descends along the groove between the large and lateral small metatarsal bone and divides just above the fetlock into lateral and medial digital arteries. Each digital artery divides at the middle of the wing of the third phalanx into preplantar and plantar digital arteries. The preplantar digital arteries supply the sensitive laminae of the sole. The plantar digital arteries of the opposite sides form anastomosis. Small laminar branches from the anastomosis form the circumflex artery of the toe which supplies branches to the sole.

Pelvic (Lumbosacral) plexus The hind limb is supplied by the nerve branches extending from the Pelvic (lumbosacral) plexus (Fig. 6-4 A and B) which is formed by the convergence of the ventral divisions of the last three lumbar and first two sacral nerves. It is formed in two parts, the cranial and caudal. The two parts are connected by anastomotic branches. The cranial (lumbar) part is formed by the fourth and fifth lumbar nerves and lies between the psoas major and psoas minor muscles while the caudal (sacral) part is formed by the sixth lumbar and first and second sacral nerves. Some times a root from third sacral also joins this part. It lies on the pelvic surface of the sacrosciatic (ischial) ligament. The nerves extending from the cranial part of the plexus are the lateral cutaneous femoral, the femoral and the obturator. The nerves extending from the caudal part are the cranial gluteal, caudal gluteal and the sciatic (ischiatic). Lateral cutaneous femoral nerve The lateral cutaneous femoral nerve while leaving the cranial part of the plexus, receives fibres from the fourth lumbar nerve, passes caudally between the psoas major and psoas minor muscles and reaches the tuber coxae. Here it perforates the obliquus abdominis externus muscle in company with the circumflex iliac artery and reaches the skin of the lateral aspect of the thigh. In this course, the nerve supplies the psoas major, psoas minor, iliacus and external oblique muscles of the abdomen and the skin and cutaneous muscle on the lateral face of the thigh.

Horse The lumbosacral plexus in horse is formed by last three lumbar and first two sacral nerves, similar to that in ox. The caudal gluteal nerve divides into two branches. The medial branch supplies the superficial gluteal muscle in addition to the middle and deep gluteal and biceps femoris muscles. The lateral branch reaches the level of tuber ischii and terminates into the semitendinosus muscle at the perineal region. The sciatic nerve is almost similar to that of ox in the disposition and distribution of its branches. The superficial and deep peroneal nerves are also similar to those of ox. The tibial nerve divides at the distal third of the leg into medial and lateral plantar nerves. Each plantar nerve divides at the fetlock into cranial, middle and caudal digital nerves. Dog The lumbosacral plexus in dog is formed by the last four or five lumbar and first two or three sacral nerves. The cranial gluteal nerve receives fibres from sixth and seventh lumbar and first and second sacral nerves. The caudal gluteal nerve arises from sixth and seventh lumbar and first sacral nerves. The sciatic nerve derives fibres from sixth and seventh lumbar and first and second sacral nerves. The lateral cutaneous nerve arises either from sciatic or from the peroneal nerve. The peroneal nerve divides into superficial and deep branches, similar to those of ox. The tibial nerve divides into medial and lateral plantar nerves at the level of distal third of the leg. The medial plantar nerve divides at the metatarsus into medial and lateral branches. The medial branch becomes the first plantar common digital nerve. It divides to form the plantar proper digital nerves of the first and second digits. The lateral branch divides into three plantar metatarsal nerves which unite, in the intermetatarsal space with the three plantar common digital nerves of the lateral plantar nerve. Each of these united plantar common digital nerves descends and after reaching at the metatarso-phalangeal articulation into three plantar proper digital nerves. The lateral plantar nerve, divides into three plantar common digital nerves which join the three plantar metatarsal nerves and form the plantar (caudal) proper digital nerves. The united proper digital nerves supply the second to fifth digit and terminate into the corresponding intermetatarsal space.

Sites for surgical interventions Luxation of patella in cattle and horses The luxation of patella is a condition wherein the patella of the stifle joint (femoropatellar articulation) is locked above the medial ridge of the trochlea of the femur. Anatomy of femoropatellar articulation of the stifle joint The stifle joint is a composite joint formed in two parts. One of these is an articulation between the condyles of distal extremity of femur and the condyles of the proximal extremity of tibia (femoro-tibial articulation) and the other is the articulation between the trochlea of the distal extremity of the femur and the articular surface of the patella (femoro-patellar articulation). The trochlea of the femur consists of two asymmetrical ridges separated by a groove. The medial ridge is more prominent, wider and a little more extended dorsally than the lateral ridge. The two ridges converge ventrally. The articular surface of the patella is more or less quadrilateral or triangular with the base above and apex below. It has a vertical ridge in the middle and it separates the two concave areas of which the medial one is larger than the lateral. The patella is fastened against the trochlea with the help of three types of ligaments. One of these is the capsular ligament which is attached to the periphery of the articular areas of the two segments of the joint. The other one is the medial and lateral collateral ligaments. The third set of ligaments is the patellar (straight) ligaments.

Bibliography Akers, R. M. and D. M. Denbow. 2013. Anatomy and Physiology of Domestic Animals. 2nd ed., Wiley- Blackwell. Bhamburkar, V. R. 1987. Anatomical Studies on the Sympathetic Nervous System in Goat. Ph.D. Thesis Submitted to G. B. Pant University, Pantnagar (Nainital) U.P. India. Bhamburkar, V. R. and R. V. Pandit. 1989. Localization of the central neurons of cranial cervical ganglion in buffalo. Indian J. Vet. Anat. 1: 14-16. Bhamburkar, V. R. and Prem Prakash. 1990. Anatomical disposition of the thoracic sympathetic nervous system in goat. Indian J. Vet. Anat. 2: 50-51. Bhamburkar, V. R. and Prem Prakash. 1990. Distribution of sympathetic nerves to the stomach in goat. Indian J. Vet. Anat. 2: 51 Bhamburkar, V. R. and Prem Prakash. 1991. Autonomic innervation of pelvic organs in goat. Indian J. Vet. Anat. 3: 51 Bhamburkar, V. R. and R. S. Dalvi. 1994. Comparative gross and histomorphological study of the external ear of domestic animals. Indian J. Vet. Anat. 6: 45- 46. Bhardwaj, R. L., R. Rajput and K. S. Roy. 2002. Applied Anatomy of Domestic Animals. Kalyani Publishers, Ludhiana, New Delhi, India. Colville, T. and J. M. Bassert. 2009. Clinical Anatomy and Physiology: A Laboratory Manual. Mosby. Elsevier. St. Louis, Missouri. de Lahunta, A. and R. E. Habel. 1986. Applied Veterinary Anatomy. W. B. Saunders Company, Philadelphia. Dyce, K. M., W. O. Sack and T. G. Wensing. 2010. Text Book of Veterinary Anatomy. 4th ed. Elsevier. Evans, H. E. and de Lahunta. 2013. Miller’s Anatomy of the Dog. Elsevier. Saunders, St. Louis. Missouri. Getty, R. 2002. The Anatomy of the Domestic Animals. 5th ed. (First East-West Press Edition) Affiliated East-West Press Pvt. Ltd. New Delhi, India. Ghoshal, N. G., T. Koch and P. Popesko. 1981. The Venous Drainage of the Domestic Animals. W. B. Saunders Company. Philadelphia. Habel, R. E. 1963. Applied Anatomy: A Laboratory Guide for Veterinary Students. Published by Author Ithaca, New York. Habel, R. E. 1970. Guide to the dissection of Domestic Ruminants. Published by the Author. Ithaca, New York. Kaupp, B. F. 1918. The Anatomy of the Domestic Fowl. W. B. Saunders Co. , Philadelphia. Lumb, W. V. and E. W. Jones. 1984. Veterinary Anaesthesia. 2nd ed. Lea and Febiger, Philadelphia. May, N. D. S. 1970. The Anatomy of the Sheep. 3rd ed. University of Queensland Press, Brisbane, Australia. McFadyean, J. 1922. The Anatomy of the Horse. A Dissection Guide. 3rd ed. W. & A. K. Johnston Ltd. Edinburgh. McFadyean, J. 1953. Osteology and Arthrology of the Domesticated Animals. 4th ed. Edited by H. V. Hughes and J. W. Dransfield. Bailliere Tindal & Cox. London. McKibben, J. and R. Getty. 1968. A comparative morphologic study of the cardiac innervation in domestic animals: I Canine. Am. J. Anat. 122(2): 533-544. McKibben, J. and R. Getty. 1968. A comparative morphologic study of the cardiac innervation in domestic animals: II Feline. Am. J. Anat. 122(2): 545-554. McKibben, J. and R. Getty. 1969. A study of the cardiac innervation in domestic animals: Cattle. Anat. Rec. 165 (2): 141-152. McKibben, J. and R. Getty. 1969. Innervation of heart of domestic animals: Horse. Am. J. Vet. Res. 30 (2): 193-202.