eChapters

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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).

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.

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.

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.