eChapters

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 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 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 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 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 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 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 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.

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.

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.

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.

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

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.

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

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.

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.

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 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.

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.

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.

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.

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 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.

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 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.

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.

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.

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.

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.

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.

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.