eChapters

The lymphatics of the thoracic region are comprised of large number of lymph nodes which are arranged in different groups like the aortic, sternal, mediastinal and bronchial. The efferent ducts from these lymph nodes ultimately join the thoracic duct which carries lymph from all over the body except the head neck and fore limb to open in the cranial venacava.

Thoracic aortic and sternal lymph nodes

The lymphatics of the thoracic and sternal region is comprised of thoracic aortic, intercostal, cranial and caudal sternal, and the xiphoid lymph nodes and the thoracic duct which conducts the lymph collected from most of the parts of the body except the head, neck and fore limbs.

The thoracic aortic lymph nodes are located along the dorsolateral border of the aorta, ventral to the sympathetic trunk. The afferents for these lymph nodes are received from the muscles of thoracic wall, diaphragm, pleural and pericardial sacs and the intercostal lymph nodes. The efferents from the thoracic aortic lymph nodes are connected to the thoracic duct either directly or through other lymph nodes of the thoracic cavity.

Arteries of thorax

Horse

The bronchial and oesophageal arteries in horse have a common trunk of origin as broncho-oesophageal artery. It arises from the convexity of aortic arch, passes ventrally and caudally on the right side of the aorta and divides into bronchial and oesophageal arteries. The oesophageal artery passes caudally in the caudal mediastinum along the dorsal face of oesophagus and supplies branches to oesophagus, caudal mediastinal lymph nodes and pleura and anastomoses with the pleuro-oesophageal branch of the gastric artery.

The intercostal arteries are eighteen pairs in number. The first pair arises from the superior artery, the next three pairs come from subcostal artery and the remaining four arise from the thoracic aorta.

The phrenic arteries are not variable in origin as in the ox. They are small and are two to three in number. They arise on the ventral face of the aorta while it passes through the hiatus aorticus and enter the crura of the diaphragm.

The rest of the arterial branches of thorax in horse are similar to those in ox.

Spinal nerves

The thoracic spinal nerves are thirteen pairs in ox. These are named and numbered according to the vertebra caudal to which they emerge from the vertebral canal. They emerge from the vertebral canal through the respective lateral vertebral (intervertebral) foramina. Each nerve, after its emergence, divides into dorsal and ventral branches. Each dorsal branch passes up dorsally between the transverse processes of the vertebrae and divides in to medial and lateral branches. The medial branch passes upward and supplies the dorsal muscles of the back. The lateral branch passes laterally on the deep surface of the longissimus dorsi muscle. At the lateral border of this muscle it passes between this and longissimus costarum muscles and becomes a cutaneous nerve which divides into ascending and descending branches supplying the muscles and the skin of this region.. Before its division, the lateral branch supplies muscular branches to the longissimus dorsi and longissimus costarum muscles.

The ventral branches continue ventrally as intercostal nerves. The first and second thoracic nerves, before their continuation as intercostal nerves, give out a branch to the brachial plexus. At a short distance from the spinal nerve, the ventral branch of each of these nerves is connected to sympathetic trunk by a ramus communicans containing preganglionic and postganglionic sympathetic fibres. The intercostal nerves run down ventrally through the intercostal spaces, lying between the external and internal intercostal muscles close to the caudal border of the respective rib and in company with the intercostal vessels. Along this course they supply these muscles. After a variable distance, the intercostal nerves and vessels traverse through the internal intercostal muscles and become subpleural in position. The second to eighth nerves end in the transversus thoracis muscle. The rests behind these end in the rectus abdominis and other abdominal muscles and the skin in this region.

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

Surface locations for clinical examination on the wall of thorax

Lymph nodes

Prescapular lymph node is palpable as a cylindrical object just cranial to the point of shoulder. Palpation can be performed by putting the base of the hand on the cranial edge of the scapula and pressing forward with fingers. This enables to roll the skin and feel the lymph node flicking underneath the fingers.

Heart

The clinical examination of heart is performed by percussion and auscultation on the thoracic wall.

For percussion of heart, usually the fifth rib is identified opposite the olecranon of a straight vertical limb. Two fields are identified for percussion of heart. (1) The area of absolute cardiac dullness corresponds to the area of contact of the pericardium with the thoracic wall. It gives a flat sound on light percussion. (2) The area of relative cardiac dullness indicates the outline of heart which is covered by the thin margin of lung. It needs somewhat stronger percussion to define this area which is usually difficult in animals. The absolute dullness is used to test the hypertrophy, displacement and pericardial effusion. In ox, the normal area of absolute dullness is explored in the fourth and fifth intercostal spaces. Traumatic pericarditis may result in absolute dullness in these spaces extending above a horizontal line through the shoulder joint. In horse this area is a triangle bounded by the triceps in front, the sternum below and the lung along the line joining these two. On left side of the normal horse, the dullness extends about 7 cm above the olecranon in the fourth intercostal space and about 3 cm above the olecranon in the fifth space. On the right side, the dullness extends about 3 cm above the olecranon in the fifth space only. In dog, the area of absolute dullness extends about 1 to 2 cm above the sternum in the fourth and fifth intercostal spaces in the line with the corresponding costochondral junction.

The skeleton of abdomen and pelvis (Fig. 4-1) consists of the lumbar vertebrae that form the dorsal boundary or roof of the abdomen, sacral vertebrae or the sacrum that forms the dorsal boundary or the roof of the pelvic cavity, the xiphoid cartilage of the sternum that provides attachment to the strong ligaments of the abdominal floor and the pelvic bone (pelvis) that forms the floor and lateral wall of the pelvic cavity.

Lumbar vertebrae

The lumbar vertebrae (Fig. 4-1 A) in ox are six in number. These are characterized by great size and form of their transverse processes. The other characteristic features of lumbar vertebrae are as below.

The body of the vertebra is much compressed dorsoventrally. The convexity of the cranial extremity and concavity of the caudal extremity of the body of the vertebra is moderate. The arch of the first lumbar vertebra is similar in size to that of the last thoracic vertebra but behind this the arches of the second to sixth lumbar vertebra gradually increase in height and width. The caudal notches are deeper than the cranial one. The intervertebral foramina of the first few lumbar segments are often double.

Horse

The lumbar vertebrae in horse are six in number. The bodies of these vertebrae are shorter. The first three of them have a distinct ventral spine which subsides thereafter. The transverse processes increase in length from first to third or fourth and then diminish. The first one or two slightly curve caudad and the last two or three curve craniad and the third is at right angles. The caudal border of the transverse process of the fifth vertebra at its base has an oval concave facet which articulates with a convex facet on the cranial border of sixth vertebra. The sixth transverse process is thick at the base and thin, narrow at its free end. It is curved forward. It has a large concave facet on its caudal border by which it articulates with the corresponding facet on sacrum.

The sacrum of horse is formed by the fusion of five sacral vertebrae similar to that in ox. However, the fusion of the spinous processes is incomplete. On either side of the base of the each spinous process is a groove which presents the dorsal sacral foramen. There are four foramina on each side of the dorsum of the sacrum of horse. The ventral surface of sacrum of horse is not so deeply arched as that of ox. The wings of sacrum are prismatic. Each wing has a large oval convex facet for articulation with similar facet on the caudal border of the transverse process of last lumbar vertebra.

The coccygeal vertebrae are eighteen to twenty in number in horse. The laminae of the neural arch of the coccygeal vertebrae beyond fourth vertebra fail to meet dorsally.

The pelvic bone (os coxae) of horse is in general similar to that of ox. The gluteal line on the gluteal surface of the ilium is faint. The ilium is placed more obliquely. The articular surface of ilium is rough and large and extends nearly to the external angle. The internal angles of the ilium of opposite sides are more close to each other. The external angles are formed of four tuberosities each.

The ischium of the two sides meets at a greater angle because of which the pelvic floor is bowl like. The ischial arch is wide and shallow. The ridge on the ventral surface of ischium is absent. The tuber ischii is not trifid. The lower border of tuber ischii forms the ventral ischiatic spine. The ventral surface of pubis is crossed by pubic groove which leads to acetabular notch that transmits the pubic femoral ligament.

Sacroiliac articulation

The sacroiliac articulation (Fig. 4-2 A) is a synovial (diorthroidal) joint formed between the articular surface on the wing of the sacrum and pelvic surface of the proximal extremity of the ilium. The articular area of the wing of the sacrum has a rough eminence while that of the ilium has a reciprocal rough depression. Both the articular areas are covered by a thin layer of cartilage. The joint cavity is a thin like a cleft and is crossed by fibrous bands.

The joint capsule is closely fitting around the margins of the articular surfaces. The joint capsule is reinforced by the ventral sacroiliac ligament which surrounds the joint. The dorsal part of this ligament is quite strong and it occupies the angle between the ilium and wing of the sacrum. The movements are inappreciable at this joint.

Horse

In addition to the common articulations of vertebrae, the lumbar region of horse has two special articulations, the inter-transverse articulations and lumbosacral articulations.

The intertransverse articulations are bilateral joints formed between the transverse processes of the fourth and fifth and between the fifth and sixth lumbar vertebrae. The articular surfaces of these joints are the oval facets on the transverse processes of these vertebrae, the cranial ones being concave while the caudal ones the convex. These are synovial joints (diarthrodial). The capsule of the joint is reinforced ventrally.

The lumbosacral articulations are diarthrodial joints formed between the transverse processes of the sixth lumbar vertebra and the wings of the sacrum. These joints are similar to the intertransverse articulations.

The sacroiliac articulation of the pelvis is similar to that of ox. However, the sacrosciatic ligament is broader. Its lateral surface is related to middle gluteus, biceps femoris and semitendinosus muscles.

Cutaneous muscle

The cutaneous muscle of abdomen and pelvis is the continuation of cutaneous muscle of thorax. This has been described under the muscles of thorax.

Abdominal fascia

Beneath the skin in the abdominal region, there are two layers of fascia, the superficial and deep, similar to those of the fascia of thoracic region.

The superficial fascia is next to skin. It is fused dorsally with the thoracolumbar fascia. Cranially it is continuous with the superficial fascia of shoulder and arm and caudally with that of the gluteal region. In the inguinal region it forms part of fascia of penis in male and of mammary glands in female animals. At the flank, it continues as fascia of thigh region.

The deep fascia forms the abdominal tunic. It is especially well developed as a thick sheet of tunic in the ventral and the adjacent lateral parts of the abdominal floor. It extends cranially over the ribs and external intercostal and the caudal digitations of the serratus thoracis muscles. Caudally it is attached to the external angle of the ilium and thoracolumbar fascia. The suspensory ligament of prepuce in males and that of the mammary glands (udder) in females is an extension of this tunic. It strongly supports the abdominal muscles to bear the weight of abdominal viscera.

Horse

The muscles of lumbar spine in horse are more or less similar to those of ox. Of the sublumbar muscles, the psoas minor lies under the bodies of the lumbar and last three thoracic vertebrae. It arises from dorsal ends of sixteenth and seventeenth ribs and eighteenth thoracic vertebra and the first five lumbar vertebrae.

The psoas major arises from dorsal ends of last two ribs and the ventral faces of the transverse processes of the lumbar vertebrae. It is much wider than that of ox and covers the quadratus lumborum entirely.

The origin of iliacus muscle is limited to sacrum and ilium only. It has no lumbar origin and it is not so intimately united with psoas major muscle.

The quadratus lumborum is less developed.

The intertransversarii lumborum is absent between the fifth and sixth lumbar transverse processes.

The muscles of abdominal wall in horse do not have any striking differences, except that the obliquus abdominis externus is thicker and more extensive. The muscular portion of this muscle reaches the external angle of ilium and as high as lumbar transverse processes.

Stomach

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

Stomach

Horse

The stomach of horse is a simple stomach. It is ‘J’ shaped sac with its right part very much shorter than the left part. It is situated in the dorsal part of the abdominal cavity behind the diaphragm and liver mainly to the left of the median line. The parietal surface faces craniad and dorsad and to the left. It is related to the diaphragm and liver. The visceral surface faces caudad and ventrad and to the right. It is related to the small intestine and colon. Its lesser curvature is short and the greater curvature is extensive. The left extremity is called the saccus caecus. It lies ventral to the sixteenth or seventeenth rib. It is related to pancreas and terminal part of greater colon caudally and to the base of the spleen laterally. The right extremity is the pylorus. It is smaller and is continued by duodenum. The stomach is fastened by several ligaments which include the gastro-phrenic ligament, the gastro- pancreatic ligament, the greater omentum, the lesser omentum and the gastro-splenic ligament. The capacity of the equine stomach is about 8 to 15 liters.

The wall of the stomach consists of four coats, the serous, muscular, submucous and the mucous. The mucous membrane is clearly divided into two parts by sinuous ridge called the margo plicatus. The part which is towards the left extremity is nonglandular while that towards the right extremity is glandular.

The urinary organs include the kidneys, ureters, bladder and urethra (Fig. 4-5 A).

Kidneys

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

Horse

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

The ureter of the horse kidney begins at the renal pelvis and is about 70 cm in length and 6 to 8 mm in diameter.

The urinary bladder of horse is shorter but wider than that of the ox. Its capacity is about 2.8 to 3.8 liters. The rest of the features are same as those of ox.

The female genital organs (Fig. 4-6 A) comprise the ovaries where the female gametes (ova) are produced, the uterine (fallopian) tubes which convey the ova to the uterus and where the fertilization takes place, the uterus in which the ovum and the embryo develops, the dilatable passage formed by the vagina and vulva through which the foetus is expelled to the exterior and the mammary glands (udder) which is functionally associated with the female genital organs. All of these except the mammary glands are located in the abdominal and pelvic cavity while the mammary glands are located outside the cavity in the perineal region.

Ovaries

The ovaries are two small oval glands measuring about 3.5 to 4.0 cm in length, 2.5 cm in width and about 1.5 cm in thickness and they weigh about 15 to 20 gm. They are oval in form and are situated a little above the middle of the pelvic inlet and approximately 40 to 45 cm cranial to vulvar opening in a middle sized cow. They are attached to the sublumbar region by the cranial part of the broad ligament of uterus, the mesovarium. Each ovary presents for description two surfaces, two borders and two extremities. The medial and the lateral surfaces of the ovary are smooth and rounded. The attached or mesovarial border is convex. It is enclosed in a part of the broad ligament termed as mesovarium. The vessels and nerves reach the ovary by way of mesovarium to this border and enter the gland through the hilus. The free border which is also convex presents a notch in its middle. The ovarian vessels and nerves enter the gland through this notch. The cranial or tubal extremity is rounded and is related to the fimbriated end of the uterine tube. The caudal or uterine extremity is also round but narrower and is connected to the horn of the uterus by the ovarian ligament.

Horse

The ovaries in mare are bean shaped and much larger than those of cow. Each is about 7 to 8 cm long and about 3 to 4 cm thick. It weighs about 70 to 80 gm. These are situated in the sublumbar region under the fourth or fifth lumbar vertebra. They are usually in contact with the roof of the abdomen. The average distance of ovaries from the vulvar orifice is about 50 to 55 cm in the average sized mare. The notch on the free border leads to a depression, the ovulation fossa where ovulation occurs.

The uterine tubes are shorter and more flexuous in mare than those in cow. Each is about 20 to 30 cm long. At its ovarian end it is wider with a diameter of about 4 to 8 mm and becomes narrower at its uterine end with a diameter of about 2 to 3 mm.

The uterus in mare is situated partly in abdomen and partly in pelvis. The horns of the uterus are usually pressed against the sublumbar muscles by the intestine. When moderately distended they are about 25 cm long. The junction of the uterine horns with the body of the uterus is abrupt. The average length of the body is about 18 to 20 cm and its diameter is about 10 cm when moderately distended. The cervix is about 5.0 to 7.5 cm in length and 3.5 to 4.0 cm in diameter. The cervical canal is straight. The mucous membrane of the uterus has no cotyledons.

The vagina in mare is shorter than that in cow. It measures about 15 to 20 cm in length and 10 to 12 cm in diameter when slightly distended. Most of the vagina is retroperitoneal.

The male genital organs (Fig. 4-6 C) comprise the two testicles (where the male gametes are produced) with their coverings and appendages; the epididymis (which stores and transports the gametes); the ductus deferens (the ducts of the testicles); the seminal vesicles, prostate and bulbourethral glands (accessory sex glands); the urethra and the penis (the organ of copulation).

Testicles (Testes)

The testicles are the essential male gonads. They produce male gametes, the spermatozoa and the endocrine secretion which influences the development and maintainance of masculine characteristics. The testicles are paired compound tubular glands situated in the scrotum below the inguinal canals suspended by spermatic cords. Each testicle is about 15 cm long and about 6.5 to 7.5 cm in diameter. Their long axis is vertical. The left testicle is more often slightly larger and lower in position than the right testicle. In such conditions the scrotum appears asymmetrical. Each testicle is slightly compressed from side to side and presents two surfaces, two borders and two extremities. The lateral surface is convex and smooth and is covered by the tunica vaginalis propria. The medial surface is slightly flattened due to its contact with the septum scroti. The cranial border is convex and it is free and hence is called the free border. The caudal border is nearly straight and is attached to the epididymis and hence is called the attached border or the epididymal border. The dorsal and ventral extremities are blunt and rounded.

Structure: The gland is compound tubular in structure. It is covered externally by peritoneum, the tunica vaginalis propria which is very closely attached to the underlined tissue. Underneath the peritoneum is the capsule of the testicle called the tunica albuginea because of its white colour. Numerous trabeculae extend from the capsule into the organ forming the supporting tissue for the seminiferous tubules and providing a pathway for vessels. The axial strand of connective tissue extending almost the entire length of the gland is the mediastinum testes. Radiating from the mediastinum are the lobes of yellowish

Horse

The testicles in horse are oval in form and they are comparatively smaller in size than those in ox. The long axis of the testicles is parallel to the body of the animal and hence the attached border of the testicle is dorsal while its free border is ventral in position inside the scrotum. The testicle of the stallion is about 10 to 12 cm long, 5 cm wide and about 6 to 7 cm thick. It weighs about 225 to 300 gm. The size of the left and right testicle varies and the left testicle is often larger than the right one.

The epididymis is adhered to the attached (dorsal) border of the testicle. The rest of the features of epididymis in horse are similar to those of ox.

The vas deferens of horse right from its origin from the tail of epididymis to its course up to the dorsal surface of the bladder it has a uniform diameter of about 6 mm. Thereafter, it forms a fusiform enlargement, the ampulla of the vas deferens and beyond the ampulla the duct abruptly reduces in size. The ampulla is about 15 to 20 cm long and its diameter is about 2 cm. The genital fold is wider and hence the two ducts apart from each other. The vas deferens and the duct of seminal vesicle open in a common ejaculatory orifice on each side of the colliculus seminalis.

The abdominal aorta is the continuation of thoracic part of the descending aorta which enters the abdominal cavity through hiatus aorticus of the diaphragm. After entering the abdominal cavity it passes caudally below the bodies of the lumbar vertebrae and psoas minor muscle in contact with the left crus of the diaphragm and the ventral common vertebral ligament. After reaching at the fifth or sixth lumbar vertebra, it terminates into iliac quadrification by dividing into two common iliac arteries. Each of the common iliac arteries further divides into internal and external iliac artery. The internal iliac artery supplies arterial branches to the wall and the viscera of the pelvic cavity while the external iliac artery supplies arterial branches to the pelvic limb of the corresponding side. The abdominal aorta is related dorsally to the bodies of lumbar vertebrae, ventral common ligament, tendon of left crus of diaphragm and psoas minor muscle, ventrally to the pancreas and rumen, on its right to the caudal vena cava and on its left to the left adrenal, left kidney and left ureter. The abdominal aorta gives out parietal and visceral branches that supply the walls and the viscera of abdominal cavity.

Parietal branches

The parietal branches of the abdominal aorta are the six pairs of the lumbar arteries. Out of these, four or five pairs arise from the dorsal surface of the aorta while the fifth or sixth pair depending on where the aorta terminates (fifth or sixth lumbar vertebral level) arises from the internal iliac arteries.

The lumbar arteries, after arising from the dorsal surface of the aorta, pass upward across the bodies of the lumbar vertebrae, reach the intertransverse spaces between the lumbar transverse processes and divide into dorsal and ventral branches. The dorsal branches are larger. They supply spinal branches to reinforce the ventral spinal artery and the muscular branches to supply the muscles and skin over the back (loin) region of the body trunk. The ventral branches run ventrally through the fascia and muscles of the abdominal walls and supply the muscles and skin of the walls of the abdomen.

Horse

The visceral branches of the abdominal aorta are the coeliac, cranial mesenteric, renal, caudal mesenteric and testicular or ovarian.

The coeliac artery in horse is shorter than that in ox. It arises from ventral surface of the aorta and reaches the dorsal surface of the pancreas where it divides into three branches, the left gastric, hepatic and splenic.

The left gastric artery passes ventrally and cranially in the gastrophrenic ligament, gives off pancreatic and oesophageal branches and divides dorsal and caudal to the cardia into parietal and visceral branches. The parietal branch crosses the lesser curvature of the stomach just to the right of the cardia and ramifies on its parietal surface. The visceral branch is distributed in the similar manner on the visceral surface of the stomach. A long but narrow oesophageal branch arises from the parietal branch of the left gastric, passes cranially through the hiatus oesophageus and enters the thoracic cavity dorsal to the oesophagus and anastomoses with the oesophageal branch of broncho-oesophageal artery.

The hepatic artery is larger than the left gastric. It passes dorsal to the pancreas and reaches medial to the portal vein, divides into right and left branches from which the terminal branches enter the liver through portal fissure. Its collateral branches are the pancreatic, right gastric and the gastroduodenal arteries. The gastroduodenal artery further divides into right gastroepiploic and the cranial pancreaticoduodenal arteries.

The caudal vena cava is a large unpaired vein which receives blood from the pelvis, pelvic limbs, mammary glands (in females) and scrotum (in males), the abdominal walls, and the viscera of the abdomen and pelvis. It is formed under the body of the fifth or sixth lumbar vertebra by the union of two common iliac veins which in their turn are formed by the union of the internal iliac and external iliac veins of the corresponding side. It passes cranially in the sublumbar region lying slightly to the right of the median plane and to the right of the abdominal aorta. In the cranial part of the abdomen, it follows the abdominal surface of the diaphragm and is embedded in the liver prior to its entry into the thorax through the foramen vena cava in the diaphragm.

In its abdominal course, the caudal vena cava is related dorsally to the sublumbar muscles and right crus of the diaphragm, ventrally to the colon, pancreas and the liver, on its right to the right kidney and the right adrenal and on its left to the abdominal part of the aorta.

Tributaries of caudal vena cava

The common iliac veins are short vessels formed by the union of the internal and the external iliac veins. These are located at the level of the fifth or sixth lumbar vertebra. The left vein crosses the dorsal surface of the aorta to reach the vena cava.

Horse

The caudal vena cava in horse is largely embedded in the substance of the liver on its diaphragmatic surface. Here its free surface is covered only by the peritoneum. After crossing the liver, the caudal vena cava continues caudally along the right side of the abdominal aorta. During its course it gives off several branches from the dorsal and lateral surface.

The lumbar veins (parietal) are five or six pairs in number. The last one or two of these often join the internal iliac veins.

The renal veins are thin walled. The right renal vein is shorter than the left one.

The phrenic veins are two or three in number. The circumflex iliac veins join either the caudal vena cava or the internal iliac vein.

The portal vein is formed by the union of cranial and caudal mesenteric and the splenic veins in the cranial part of the sublumbar region at about the level of second lumbar vertebra. It passes cranially and to the right of the median plane continues through the portal ring of the pancreas and reaches to the portal fissure of the liver. Before entering the portal ring of the pancreas, the portal vein receives right gastroepiploic vein, right gastric vein and cranial pancreaticoduodenal vein. The cranial mesenteric, caudal mesenteric and the splenic veins are formed by the tributaries from the veins which are satellites of the branches of the respective arteries. After its entry into the hepatic portal, the portal vein divides into three principal branches. These include the right branch for right lobe, the middle branch also for the right lobe and the left branch for the left and the quadrate lobe.

The lymph nodes of the abdomen and pelvis are in two major divisions, lymph nodes of the wall of the abdomen and pelvis and those of the viscera of abdomen and pelvis. The lymph from all the lymph nodes is collected by different lymph ducts which carry and pour their contents (lymph) in the cisterna chyli near the hiatus aorticus on the abdominal surface of the diaphragm.

Lymph nodes of the wall of abdomen and pelvis

The lumbar aortic lymph nodes are a chain of small lymph nodes situated along the caudal part of the abdominal aorta. These lymph nodes receive afferents from the peritoneum, sublumbar, spinal and abdominal muscles and the lumbar vertebrae. The efferents from these lymph nodes join each other and form two lumbar trunks which later on unite in a single lumbar lymphatic trunk that opens into the cisterna chyli.

The medial iliac lymph nodes are situated near the origins of the deep circumflex iliac and the internal external iliac arteries. The afferents for these lymph nodes are received from the internal iliac, tuberal, gluteal coxal, lateral iliac, subiliac, superficial inguinal and popliteal lymph nodes. The efferents from these lymph nodes either form a lumbar trunk or join any of the lumbar aortic lymph nodes.

Horse

Lymph nodes of the wall of abdomen and pelvis

The lumbar aortic lymph nodes extend from kidneys to the deep circumflex iliac vessels. These are not clearly distinguishable from the cranial and caudal mesenteric, renal, testicular or ovarian, uterine and iliac lymph nodes.

The internal iliac lymph nodes are placed at about the origin of the iliac arteries. The external iliac lymph nodes are several small nodes situated in the triangular space between the cranial and caudal branches of the circumflex iliac artery.

The deep (internal) inguinal lymph node is not abdominal in position. It is not situated at the pelvic brim. It is one of the group of the lymph nodes of the hind limb.

The superficial (external) inguinal lymph nodes are on either side of the penis and in front of the superficial inguinal ring in male and above the mammary glands as the supramammary lymph nodes in female.

The prefemoral lymph nodes are about a dozen small glands arranged in an elongated mass placed on the medial aspect of the cranial border of tensor fasciae lata and on the caudal branch of circumflex iliac artery.

Lumbar nerves

There are six pairs of lumbar spinal nerves. Each of these, after emerging out of the intervertebral foramen, gives out firstly a meningeal branch which reenters the vertebral canal through the intervertebral foramen and supplies the meninges. Next to this it furnishes the ramus communicans to the sympathetic chain ganglion. After this, the lumbar spinal nerves divide into dorsal and ventral branches. The dorsal branches are smaller and they supply the muscles and skin on the loins and croup. The ventral branches are larger and they slightly differ from each other in their course and distribution. In their initial course, they pass under the psoas minor and psoas major muscles emerge at the lateral border of the psoas major and supply branches to the sublumbar muscles. Hereafter, they differ in their course and distribution.

The ventral branch of the first lumbar nerve passes on the medial face of the quadratus lumborum, runs down the medial face of the transversus abdominis, the obliquus abdominis internus and externus and finally reaches the rectus abdominis, supply all these muscles and the skin on the lateral wall and the floor of the abdomen.

The ventral branch of the second lumbar nerve has two divisions. The cranial division passes ventrally and caudally to reach the external angle of the ilium, supplies branches to the oblique muscles of the abdomen and the skin over this region and terminates in the rectus abdominis muscle. The caudal division passes caudally and laterally under the quadratus lumborum, supplies branches to the muscles and skin at the flank region and then crosses the circumflex iliac vessels. Here it joins a similar branch of the third lumbar nerve and forms the genitofemoral nerve.

Abdominal part

The sympathetic trunks enter the abdominal cavity between the psoas minor and the crus of the diaphragm of the corresponding side. They run caudally on the ventrolateral surfaces of the lumbar vertebrae and the ventral common vertebral ligament. The left trunk is ventrally related to the abdominal aorta and the right trunk to the caudal vena cava. The sympathetic trunks of the opposite sides in the cranial half of the lumbar part lie rather close to each other and as they pass caudally, they gradually divert from each other. The lumbar sympathetic ganglia are small spindle shaped usually six in number and are present one in each lumbar segment. The first two are larger than the rests. Each is connected with the ventral division of the corresponding lumbar spinal nerve by the ramus communicans. The interganglionic segments of the sympathetic trunk are usually single but some times they split.

Splanchnic nerves

The greater (major) splanchnic nerve is formed by the contribution of sympathetic nerve fibres from fifth or sixth thoracic to thirteenth thoracic sympathetic ganglia. It becomes larger and larger as it passes caudally along with the sympathetic trunk. It enters the abdomen at the level of the first lumbar vertebra gets separated from it, passes ventrally and laterally across the aorta (left trunk) and vena cava (right trunk) to join the coeliacomesenteric autonomic plexus on and around the coeliac and cranial mesenteric arteries.

Horse

There are six pairs of lumbar, five pairs of sacral and five pairs of coccygeal spinal nerves in horse. Their origin, course and distribution is in general similar to those in ox.

The number of sympathetic lumbar and coccygeal ganglia is just the same in horse as that of ox. However, the number of sacral sympathetic ganglia is four in horse as against five ganglia of ox.

The greater (major) splanchnic nerve in horse is formed by the fibres received from sixth or seventh to fourteenth or fifteenth thoracic sympathetic ganglia. It is smaller in its cranial thoracic part but as it proceeds caudally it gradually increases in size so much that it becomes larger than the thoracic sympathetic trunk in the caudal thoracic part. It receives a communicating branch from the lesser splanchnic nerve before terminating in the coeliacomesenteric ganglion.

The lesser (minor) splanchnic nerve is formed by fibres received from the last three or four thoracic and the first two lumbar sympathetic ganglia. It communicates with the greater splanchnic nerve and the renal and adrenal plexuses and terminates in the coeliac plexus and ganglion.

The rest of the features of spinal nerves and autonomic plexuses of the abdomen and pelvis in horse are almost similar to those of ox except those related to the distribution of fibres to the parts of ruminant versus simple stomach.

Surface anatomy of the organs of left side of abdominal cavity

Rumen

Topography of the rumen: The left side of the abdomen in cow is completely occupied by the rumen except for a small area occupied by the reticulum at its cranial end and the spleen on the craniodorsal aspect sandwiched between the dorsal border of rumen and the diaphragm. The rumen lies in contact with the left abdominal wall extending from seventh or eight intercostal space where it touches the diaphragm to the transverse plane of the tuber coxae where it almost touches the pelvic inlet.

Palpation of rumen: The rumen can be palpated in the left flank of the abdomen and is usually palpated in the dorsal third of the flank region. In ruminants, the contraction begins at reticulum and it continues in rumen in succession and hence is considered as ruminoreticular contraction cycle. It begins with double contraction of reticulum followed immediately by the contraction of rumen. The rumen contraction begins in the cranial sac and progresses to the dorsal sac and then to the ventral sac. During the contraction of the ventral sac, the dorsal sac is forced out which appears as a bulging at the paralumbar fossa. After this, there is a pause of 30 seconds and thereafter the second ruminal contraction begins which is usually associated with eructation. The ruminoreticular contraction cycle ends with the end of second ruminal contraction. Following this, the second cycle begins. Thus there are two palpable rumen contractions per minute. This rate of contraction increases to 2.8 contractions per minute during the act of feeding of the animal.

The fore limb is termed as pectoral limb or thoracic limb (Fig. 5-1). It is attached to the thorax by means of muscles of pectoral girdle (shoulder girdle). It is composed of four major segments, the shoulder girdle, arm, forearm and manus. The skeleton of shoulder girdle is formed by the shoulder bone, the scapula. The arm is formed by the arm bone the humerus. The forearm is formed by two bones the radius and ulna; and the manus is formed by the carpus consisting of six short bones –the carpal bones arranged in two layers, the metacarpus consisting of one large and one small metacarpal bones and the digits consisting of six short bones the phalanges and six small sesamoids arranged in two rows for each limb.

Shoulder girdle

The shoulder girdle (thoracic girdle or pectoral girdle) in general is composed of the scapula, coracoid and clavicle. In ruminants the scapula is well developed, the coracoid is in the form of a small rounded process fused with the ventral end of the scapula and the clavicle is absent or in some subjects it occurs embedded in the brachiocephalicus muscle cranial to the point of the shoulder. But this possibility is very rare. Hence, in ox only scapula is considered as the bone of shoulder girdle.

Scapula

The scapula is a flat triangular bone applied obliquely ventrad and craniad against the cranial part of the lateral wall of thorax. Its long axis extends from the fourth thoracic spine to the sternal end of the first rib. It presents two surfaces, three borders and three angles.

Horse

The scapula of horse is a flat triangular bone placed at the shoulder region applied against the cranial part of the lateral wall of thorax similar to that in ox. The scapular spine is placed a little further caudad from the cranial border so that the proportion of the area of supraspinous to infraspinous fossa is one third to two third of the lateral surface of the scapula. The subscapular fossa is deeper and presents well marked vascular impressions. The nutrient foramen is placed at the distal third of the infraspinous fossa. The tuber scapulae and the glenoid cavity are placed further apart. The glenoid cavity is oval in outline and its rim has a distinct notch on its craniomedial aspect. The coronoid process is better developed and the acromion process is absent.

The humerus in horse is directed obliquely distally and caudally forming an angle of about 55 degrees with the horizontal plane almost equaling to that in ox. The musculospiral groove on the shaft is deeper and more spirally curved. The nutrient foramen is placed on the distal third of the medial surface. The deltoid tuberosity is better developed. The medial and lateral tuberosities of the proximal extremity are more prominent. The intertuberal (bisipital) groove is divided by an intermediate ridge. The summit of the lateral tuberosity does not arch inward. The head is smaller than that in ox. The coronoid and olecranon fossae of the distal extremity are shallower than those in ox.

The radius of the horse is larger and longer of the two bones of the forearm. Close to the medial border of the caudal surface in the lower third of the shaft is a rough elevation for attachment of the superior check ligament (an accessory ligament of the superficial digital flexor). The caudal surface of the shaft towards its lateral border presents a rough triangular area which tapers to a point in the distal third of this surface. This is for attachment of the ulna. The distal interosseous space is absent because the shaft of ulna is shorter and does not reach to the distal part of the shaft of radius. The grooves on the cranial surface of the distal part of the shaft are better marked. The radial tuberosity at the craniomedial aspect of the proximal extremity is larger and more prominent than in ox. The distal articular surfaces are not oblique. It presents two tubercles one on each side, for ligamentous attachment. The tubercle on the lateral aspect is grooved for passage of tendon of lateral digital extensor.

Shoulder joint

The shoulder joint (Fig. 5-2 A) is an enarthrosis formed between the glenoid cavity of scapula and head of humerus.

Ligaments: The shoulder joint is enclosed by the capsular ligament which is loose enough to accommodate the wide range of movements of the joint and is attached about 1.25 cm away from the periarticular area of the two segments of the joint. The capsular ligament is supported externally by the muscles of the shoulder region.

Movements: The shoulder joint performs all kinds of movements but the extension and flexion are best marked.

Horse

The shoulder joint of horse has same features as those of ox. However, this joint in horse exhibits greater degree of mobility.

The elbow joint of horse is almost similar to that of ox except that there is only one interosseous ligament in horse against two in ox.

The carpal joint of horse has only minor differences from that of ox. The special feature of the joint in horse is that it has two additional ligaments, the superior check ligament and the inferior check ligament. The superior check ligament starts from the rough elevation on the distal third of the caudal surface, close to the medial border of the radius and is inserted to the tendon of superficial digital flexor above the carpus. The inferior check ligament is the continuation of the caudal (palmer) capsular ligament of the carpus. It joins the tendon of deep digital flexor at about midway on the palmer surface of the metacarpus. The check ligaments assist the tendons of flexor muscles of the digits after severe exertion to keep then tight and prevent flexion.

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

Muscles of shoulder girdle

The muscles of shoulder girdle have been described with the muscles of thorax.

Muscles of shoulder

The muscles of shoulder arise on the scapula, act on shoulder joint and end on the humerus. These muscles are in two groups as the lateral group of muscles and the medial group of muscles.

Horse

The muscles of shoulder region namely the supraspinatus, infraspinatus, teres major, teres minor and coracobrachialis in horse resemble those of ox. The subscapularis muscle has only a single head of origin in horse as against three heads in ox. The deltoideus muscle has two parts of origin as that of ox but in horse its acromial part is named as cranial part and it arises from scapular spine and the scapular fascia instead of the acromion process. In addition to these muscles, the shoulder in horse is composed of one more muscle named as capsularis. The capsularis is a very fine narrow muscle arising from the caudal part of the rim of the glenoid cavity of scapula and is inserted on the caudal part of the neck below the head of the humerus by insinuating itself between the fibres of brachialis muscle. This muscle is peculiar to solipeds and it raises the capsular ligament of shoulder joint during flexion of the joint.

The muscles of arm in horse are in general similar to those of ox except the following minor differences. The tendon of origin of biceps brachii is adapted to the intertuberal (bisipital) groove. The tensor fasciae antibrachii is thin in front and wide below. The medial head of triceps brachii is less developed and extends from the middle of the medial surface of shaft of humerus. The anconeus is smaller and less voluminous.

Amongst the muscles of forearm and manus, the flexors of carpus especially the flexor carpi radialis, flexor carpi ulnaris and the ulnaris lateralis resemble those of ox. The pronator teres is either vestigial or completely absent. The medial digital extensor is absent. The extensor carpi radialis resembles that of ox. The common digital extensor arises in common with the extensor carpi radialis. Its tendon descends over the synovial capsule of the carpus, passes down the metacarpus and at the fetlock it receives two slips from suspensory ligament to form the broad ligament which is inserted to the pyramidal extensor process of the distal phalanx. The lateral digital extensor is smaller. It arises from humerus and radius. Its tendon is inserted to the prominence on the dorsal (cranial) aspect of the proximal extremity of the proximal phalanx. The extensor carpi obliquus is inserted to the head of the medial small metacarpal bone. The superficial digital flexor does not divide into two parts. Its tendon is reinforced above the carpus by a strong fibrous band- the superior check ligament. The ring for the deep digital flexor is formed entirely by the tendon of superficial digital flexor. The deep digital flexor has same heads of origin as those of ox. Its tendon receives about half way down the metacarpus the inferior check ligament. It is inserted to the semilunar crest and tendinous surface of the distal phalanx.

Subclavian artery

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

The venous drainage of the fore limb of ox is conducted by two independent routes, the superficial and deep routes, in the fore limb. The major vein of the superficial route is the cephalic vein and that of the deep route is the axillary vein. The venous drainage of the fore limb begins at the digits and is carried by branches of both the routes intermingling or confluent with each other up to the level of elbow joint. From the elbow upwards the journey of the two paths is completely independent of each other. The superficial veins join the cephalic vein while the deep veins join the axillary vein to open into the cranial vena cava.

Superficial veins

The superficial veins begin in the form of distal deep and distal superficial palmer arch just above the fetlock joint. These venous arches are formed by the union of axial and abaxial digital veins. From these venous arches extend the dorsal metacarpal vein ascending first through the interdigital space and then winding around the large metacarpal bone. In this course it anastomoses with the branches of radial vein on the dorsomedial aspect of the carpometacarpal region and passes further upward as accessory cephalic vein.

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

Horse

The arteries of fore limb of horse are more or less similar in their course and distribution. However, there are some special features in respect of some of these arteries as described below.

The brachial artery descends vertically on the medial aspect of humerus to its distal extremity, crosses it obliquely and descends along the joint capsule and medial lateral ligament of elbow joint, gives off common interosseous artery at the proximal third of the forearm and then continues distally as median artery.

The median artery descends along the caudomedial aspect of the radius covered by flexor carpi radialis muscle and divides just proximal to medial condyle into radial and collateral ulnar (ulnar) arteries.

The collateral ulnar artery is smaller of the two terminal branches of median artery. It descends along the cranial surface of radius under cover of the extensor carpi radialis and terminates a little above the carpus by anastomosing with a branch of radial artery.

Brachial plexus (FIG. 5-5 A)

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

Horse

The brachial plexus in horse is formed by ventral branches of the last three thoracic and first two thoracic spinal nerves similar to that in ox. The disposition and distribution of nerve branches extending from the plexus is almost similar to that of the ox except the following two.

The radial nerve after reaching the musculospiral groove gives out a cutaneous branch that passes through the lateral head of triceps and is distributed to the skin on the lateral aspect of forearm.

The main trunk of the radial nerve after crossing the carpus descends along the medial aspect of the palmer surface of metacarpus as palmer medial metacarpal nerve and further down on the medial digit as palmer medial digital nerve.

The ulnar nerve descends the carpus and after reaching the metacarpal region joins the lateral branch of the median nerve, descends first along the lateral aspect of the palmer surface of metacarpus as palmer lateral metacarpal nerve and then along the lateral (abaxial) aspect of the lateral digit as palmer lateral digital nerve.

Site for nerve block

Brachial plexus block

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

The blocking of brachial plexus is indicated for operations for reducing the fractures of the bones of fore arm and manus.

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

Horse

The femur of horse is more massive than that in ox. The proximal third of the caudal surface of shaft of femur presents a rough eminence for attachment of biceps femoris muscle. The trochanter minor is a rough ridge like structure. The proximal third of the lateral border of shaft presents trochanter tertius for attachment of superficial gluteus muscle. The trochanter major is comprised of a convexity, a summit and a crest. The trochanteric ridge is vertical and it connects the trochanter major with the third trochanter. The lateral supracondyloid crest and the supracondyloid fossa are better developed in horse than that in ox. The patella of horse is wider and longer than that of ox.

The tibia in horse is relatively longer than that in ox. The popliteal line is prominent. It extends from the proximal third of the medial border, obliquely crosses the caudal surface and ends at the middle of the lateral border of the shaft of tibia. The medial border in its proximal third bears a small popliteal tubercle for attachment of popliteus muscle. The lateral border is attached to the shaft of the fibula and encloses an interosseous tibiofibular arch with the fibula. At the proximal extremity, the cranial tuberosity is vertically grooved. The lateral condyle on its lateral side presents facet for articulation with the proximal extremity of fibula. At the distal extremity, the grooves are oblique. The lateral malleolus is fused with the distal extremity of tibia, The medial malleolus is more prominent than the lateral one.

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

Horse

The hip joint has an additional ligament called the pubio-femoral ligament. It arises from the prepubic tendon, passes dorsally and laterally and then caudally and lodges into the subpubic (pubio-femoral) groove, penetrates the capsular ligament, passes through the acetabular notch and is inserted into the fovea capitis behind the round ligament on the head of femur. This ligament is peculiar to solipeds and it prevents the side kicking. The absence of this ligament in ruminants allows them abduction freely and that is how the ruminants can kick sideways. As against this, the equines can kick only back way. The other difference about the hip joint in horse is that the cotyloid ligament is narrower because of the deeper acetabulum.

The stifle joint of horse in general resembles that of ox except that the middle patellar ligament is not so prominent as that in ox.

The tibio-fibular articulation in horse is a diarthrosis and an amphiarthrosis. The diarthrosis is formed between the lateral condyle of tibia and the head of the fibula. It has intimate capsular ligament. The amphiarthrosis is formed between the shafts of the tibia and the fibula. The shafts of the two bones are binded by the proximal and distal interosseous ligaments.

The hock joint of horse has minor differences with those of ox owing to the difference in the number of tarsal bones and their arrangement. The synovial membrane is supported everywhere except at the dorsomedial (craniomedial) aspect where it bulges out in the cases of bog-spavin. If there is a distension of tarsal sheath, it bulges out as two rounded swellings on either sides of the tendon of deep digital flexor. The movements at the hock joint are very limited.

The fascia covering the hind limb is the superficial and deep fascia. The superficial fascia is very scanty lies beneath the skin. It has the same general disposition as in other regions of the body. The deep fascia is in the form of a thick strong sheet covering the superficial layer of muscles over the medial aspect of the hind limb. From this sheet extend several septa that pass between different muscles or muscle groups in the thigh region. The deep fascia of the leg region continues cranially with the fascia of the thoracolumbar region. Caudally it is attached to the aponeurotic tendon of insertion of sartorius and gracilis muscles. The part of the deep fascia that continues on the lateral aspect of the thigh is the fascia lata which is continuous with the gluteal fascia dorsally.

The muscles of the hind limb (Fig. 6-3 A and B) are arranged into different regional, positional as well as functional groups. The muscles of hip and thigh region are arranged in the lateral, medial and cranial groups. The muscles of leg and foot are arranged in dorsolateral and plantar groups. The dorsolateral muscles are generally the extensors of tarsus and digits while the plantar muscles are the flexors of tarsus and digits.

Horse

The tensor fasciae latae is less extensive.

The superficial gluteus exists separately. It is ‘V’ shaped. It originates from the lateral angle of ilium (tuber coxae) and gluteal fascia and is inserted to the third trochanter. The middle gluteus is the largest of the three gluteal muscles and is very extensive. It originates from the aponeurosis of the longissimus dorsi, extends as far forwards up to first lumbar vertebra and is inserted in three heads, to the trochanter major, the crest ventral to trochanter major and the trochanteric ridge. The deep gluteus is smaller than the middle gluteus and is inserted to the convexity of the trochanter major.

The biceps femoris has an additional insertion to the femur on the rough surface near the trochanter tertius.

The semitendinosus arises from the transverse processes of the first two or three coccygeal vertebrae. The semimembranosus is larger and arises from the sacrosciatic ligament also.

The sartorius does not arise from the shaft of the ilium. The gracilis is not so extensively fused to its fellow at the origin.

The obturator internus has an iliac part- the pisiformis. Its tendon passes through the lesser sciatic foramen and hence does not unite with that of the obturator externus.

Articularis genu is absent

The main arterial trunk that supplies blood to the hind limb is the external iliac artery. As this trunk passes down through different segments (regions) of the limb it is differently named according to that segment or the region. In the abdomen it is named as iliac, in the thigh region as femoral, in the leg region as tibial and popliteal and down that the metatarsal artery and lastly the digital.

External iliac artery

The external iliac artery arises from the abdominal aorta ventral to the fifth or sixth lumbar vertebra. It descends at the side of the pelvic inlet ventrally and caudally along the tendon of psoas minor, crosses the tendon, reaches the cranial border of the pubis, gains the space between the sartorius and pectineus muscles and then continues in the thigh (femoral region) as the femoral artery. In this course it is covered by peritoneum and is related laterally to the psoas minor, sartorius and iliacus muscles, medially to the common iliac vein and caudally to the external iliac vein. The following are the collateral branches of the external iliac artery.

The venous drainage of hind limb begins from two digital venous plexuses, one for each digit, situated in the corium of the foot. From each of these plexuses extend three digital veins namely the dorsal common digital vein, the medial digital vein and the lateral digital vein. The digital veins in their upward course converge and form the metatarsal veins which in their turn continue upward in the leg region as tibial and popliteal veins and then in the thigh region as femoral vein which finally joins the external iliac vein as the chief vein of the hind limb.

Digital veins

The dorsal common digital vein arises from the corresponding digital plexus in the form of proper digital vein.The two proper digital veins ascend along the medial face of the corresponding digit and unite at about the middle of the interdigital space below the metatarso-phalangeal articulation (fetlock joint) and form the dorsal common digital vein. The later, thus formed, ascends through the interdigital space, crosses vertically the metatarso-phalangeal articulation and after reaching the distal extremity of the metatarsal bone continues upward as dorsal metatarsal vein.

The medial digital vein after arising from the digital venous plexus ascends along the caudomedial aspect of the medial digit in company with the satellite artery, shares a branch each from the dorsal common digital and lateral digital veins, ascends the medial face of the fetlock joint and is then continued up as the medial plantar metatarsal vein.

The lateral digital vein after arising from the digital venous plexus of the lateral digit passes upward in the interdigital space, communicates a branch to the dorsal common digital and medial digital veins and then ascends the lateral face of the fetlock joint and then is continued up as the lateral plantar metatarsal vein.