Veterinary · Pre-clinical and foundation · also Comparative Anatomy, Osteology-Arthrology, Splanchnology, MSVE VAN-111, MSVE VAN-121, MSVE VAN-221
Structure of the domestic animal body, with the ox as the type species compared across horse, sheep, goat, pig, dog, fowl and laboratory animals. Applied anatomy (injection sites, nerve blocks, surgical approaches) links it directly to the clinic.
In the exams: ICAR AICE-JRF core group 07; ICAR AIEEA-PG 13 (Anatomy); PRC 10%; Nepal NLEV 6 items (with histology); BCSE 18-20 items; China basic paper 20 (with histology and embryology); AVE Paper 1 at least 2
Veterinary gross anatomy is the study of the structure of the domestic animal body as seen with the naked eye: bones, joints, muscles, vessels, nerves, sense organs and viscera, and how they lie inside the body cavities. It is taught comparatively. One type species, traditionally the ox in South Asian, many African and some European curricula (the dog in most North American ones), is described in full, and the horse, sheep, goat, pig, dog, cat, fowl and laboratory animals are learned by their differences from it.
Almost every clinical act is applied anatomy. A jugular injection, a caudal epidural, a cornual block, a flank laparotomy, a rectal examination and a post-mortem all depend on knowing where a structure lies relative to landmarks that can be seen or felt. Species differences have consequences: an epidural is placed at the lumbosacral space of a dog but at the tail head of a cow because the spinal cord ends at different levels.
Anatomy is a named subject in India's ICAR AIEEA (PG) Veterinary Science paper and in core group 07 of the ICAR AICE-JRF/SRF, and carries a fixed share in the Philippine licensure examination, the North American BCSE, Nepal's NLEV, China's practising-veterinarian examination and the Australasian AVE. Questions range from recall of formulae and nerve supply to applied problems: which nerve to block, which landmark to palpate, which structure is at risk. This manual follows the curriculum units in order. Where textbooks disagree or individuals vary, it gives a range or says so.
Veterinary anatomy uses the Nomina Anatomica Veterinaria (NAV). Because quadrupeds stand horizontally, cranial (towards the head) and caudal (towards the tail) replace anterior and posterior; on the head, rostral means towards the nose. Dorsal is towards the back, ventral towards the belly, medial towards the median plane and lateral away from it; proximal and distal describe position along a limb. At and below the carpus the back of the limb is palmar, and at and below the tarsus plantar. Axial and abaxial describe position relative to the axis of a digit or, in cloven-hoofed animals, the line between the two main digits. The median plane divides the body into equal halves; sagittal planes are parallel to it; transverse planes cut across the long axis; dorsal planes are parallel to the back.
Bones are long, short, flat, irregular, sesamoid (patella, proximal sesamoids) or pneumatic (containing air, such as the frontal bone and, in birds, the humerus). A growing long bone has a diaphysis, epiphyses and a cartilaginous growth plate (physis) with the metaphysis beside it; the periosteum supplies the bone and heals fractures. The skeleton is axial (skull, vertebrae, ribs, sternum), appendicular (limbs and girdles) and visceral, the last formed in soft organs: the os cordis of the ox heart, the os penis of the dog and the rostral bone of the pig snout.
The generally accepted vertebral formulae are: ox C7 T13 L6 S5 Cy18-20; horse C7 T18 L6 S5 Cy15-21; sheep and goat C7 T13 L6-7 with four or five sacral vertebrae and a variable caudal count; pig C7 T14-15 L6-7 S4 Cy20-23; dog C7 T13 L7 S3 Cy20-23; cat C7 T13 L7 S3. Every domestic mammal has seven cervical vertebrae, and the thoracic count equals the pairs of ribs. Lumbar and caudal counts vary between individuals; some Arabian horses have five lumbar vertebrae.
The atlas has no body and wide palpable wings; the axis carries the dens, the pivot of the atlantoaxial joint. In the ox the lumbar transverse processes are long and horizontal and form the dorsal boundary of the paralumbar fossa, which makes them the landmarks for paravertebral nerve blocks. In the horse the last lumbar transverse processes articulate with each other and with the sacral wings.
The ox, sheep, goat, dog and cat have 13 pairs of ribs, the horse 18 and the pig 14-15. Sternal ribs reach the sternum through their cartilages; asternal ribs join the cartilage in front to form the costal arch. The ox and horse have eight pairs of sternal ribs, the dog nine. The intercostal vessels and nerve run along the caudal border of each rib, which is why a thoracocentesis needle is passed along the cranial border.
In the ox the frontal bone forms the whole roof of the cranium and extends caudally as the intercornual protuberance, the highest point of the skull. The horn core projects from it, and in the adult the frontal sinus extends into the horn core, so dehorning an adult opens the sinus. The temporal line, running from the orbit to the horn base, is the landmark for the cornual nerve. The ox has no upper incisors; a dental pad replaces them.
The horse skull is long, with a facial crest on the maxilla and the infraorbital foramen above its rostral end. The roots of the caudal upper cheek teeth project into the maxillary sinus, so a tooth-root infection can present as a one-sided nasal discharge. The orbit is closed by a complete bony bar in the horse and ruminants, but open laterally and completed by a ligament in the dog, cat and pig. Dog skulls are dolichocephalic, mesaticephalic or brachycephalic by breed. At the vascular notch on the ventral border of the horse mandible the facial artery, vein and parotid duct turn onto the face; the pulse is taken there.
Domestic ungulates have no clavicle (the dog has at most a small remnant), so the forelimb is joined to the trunk by muscles alone, a synsarcosis. The scapular spine ends in an acromion in the ox and dog but not in the horse or pig. The horse humerus has an intermediate tubercle that divides the groove for the biceps tendon. The radius and ulna are separate in the dog and pig, partly fused in the ox, and in the horse the ulna is reduced and fused distally, so the forearm cannot rotate. The ox carpus has six bones (radial, intermediate, ulnar, accessory; fused second-and-third and fourth carpals), the horse seven or eight, the dog seven because radial and intermediate fuse. The ox cannon bone is the fused third and fourth metacarpals; the horse bears weight on the third metacarpal with the second and fourth reduced to splint bones; the pig has four metacarpals and the dog five. The horse's distal sesamoid is the navicular bone.
The hip bone is formed by ilium, ischium and pubis. In the cow the tuber coxae (hook bone) and tuber ischiadicum (pin bone) are key landmarks. The mare's pelvic inlet is wide and nearly round; the cow's is taller than wide, with a narrower ischial arch and a pelvic floor rising caudally, so calving is mechanically harder. The horse femur has a third trochanter, which the ox and dog lack. The fibula is complete in the dog and pig; in the ox it is reduced to a proximal remnant and a separate malleolar bone. The tarsus has six bones in the horse, five in the ox (central and fourth fused as the centroquartal bone) and seven in the dog.
Ruminants and pigs bear weight on digits III and IV; II and V are dewclaws, complete in the pig. The horse has one functional digit. Ungulates are unguligrade, carnivores digitigrade and the human plantigrade; fewer, longer distal bones increase stride at the cost of manipulation.
Fibrous joints include sutures, syndesmoses and gomphoses (teeth in sockets). Cartilaginous joints are synchondroses of hyaline cartilage, such as growth plates, or symphyses of fibrocartilage, such as the pelvic symphysis and intervertebral discs. Synovial joints have articular cartilage, a capsule of fibrous and synovial layers, synovial fluid and sometimes menisci or intra-articular ligaments. By shape they are plane, hinge (elbow, fetlock), pivot (atlantoaxial), condylar (stifle), ellipsoidal, saddle and ball-and-socket (shoulder, hip); a composite joint, like the carpus, involves more than two bones.
The shoulder of large animals has no true collateral ligaments; the infraspinatus and subscapularis tendons act as collaterals. In the hip the ligament of the femoral head is present in all species, and the horse also has an accessory ligament from the prepubic tendon, which the ox lacks; this helps explain why hip luxation is commoner in cattle. The stifle comprises femoropatellar and femorotibial joints, with menisci and cranial and caudal cruciate ligaments. The ox and horse have three patellar ligaments, the dog and pig one. Cranial cruciate rupture in the dog is shown by the cranial drawer test.
The nuchal ligament of the horse and ox has a cord-like funicular part to the withers and a sheet-like lamellar part to the cervical spines; the dog has only a funicular part, and the pig and cat none. In the horse, infection of the atlantal bursa and the supraspinous bursa produces poll evil and fistulous withers. An intervertebral disc has an anulus fibrosus and a nucleus pulposus; chondrodystrophic dog breeds suffer early disc extrusion.
Bones act as levers and joints as fulcra. Most limb muscles insert close to the joint, trading force for speed and range. The trunk is slung between the forelimbs by the serratus ventralis and pectoral muscles. In the standing horse the forelimbs carry more weight than the hindlimbs, commonly about 60 percent, because of the head and neck.
The stay apparatus lets the horse stand, even doze, with little muscular effort. In the forelimb, the tendinous biceps brachii and lacertus fibrosus link to the extensor carpi radialis and stop the shoulder and elbow collapsing; the accessory (check) ligaments of the superficial digital flexor (from the radius) and the deep digital flexor (from the palmar carpal ligament) make those tendons passive supports; and the suspensory apparatus, made of the interosseous muscle, the proximal sesamoid bones and the distal sesamoidean ligaments, stops the fetlock over-extending.
In the hindlimb the reciprocal apparatus couples stifle and hock: the fibularis (peroneus) tertius runs cranially and the superficial digital flexor caudally between femur and tarsus, so the two joints flex and extend together. If the fibularis tertius ruptures, the stifle flexes while the hock stays extended and the common calcanean tendon slackens. The stifle locks when the medial patellar ligament and parapatellar fibrocartilage hook over the enlarged medial ridge of the femoral trochlea. Failure to unlock is upward fixation of the patella, relieved when necessary by medial patellar desmotomy, a procedure also used in cattle and buffalo.
The less mobile attachment of a muscle is its origin and the more mobile its insertion. Parallel-fibred muscles shorten far; pennate muscles produce more force. Bursae and synovial sheaths reduce friction where tendons cross bone or pass through tight canals.
The muscles of mastication (masseter, temporalis, pterygoids) are supplied by the mandibular division of the trigeminal nerve. The masseter dominates in herbivores for grinding, the temporalis in carnivores for a scissor bite. The digastric opens the jaw; its rostral belly is supplied by the trigeminal and its caudal belly by the facial nerve. All facial muscles are supplied by the facial nerve, so facial paralysis droops the ear, eyelid and lip on one side.
The diaphragm, supplied by the phrenic nerve (mainly C5-C7), has a tendinous centre and muscular costal, sternal and lumbar parts. The aortic hiatus between the crura transmits the aorta, azygos vein and thoracic duct; the oesophageal hiatus the oesophagus and vagal trunks; the caval foramen in the tendinous centre the caudal vena cava.
The abdominal wall has the external oblique (fibres caudoventral), internal oblique (cranioventral), transversus abdominis and rectus abdominis beside the linea alba. Large animals have an elastic tunica flava abdominis. The inguinal canal lies between the oblique muscles; its superficial ring is a slit in the external oblique aponeurosis and its deep ring lies between the internal oblique and the inguinal ligament.
The extrinsic forelimb muscles include the trapezius, rhomboideus, omotransversarius, brachiocephalicus, latissimus dorsi, pectorals and serratus ventralis. The jugular groove is bounded dorsally by the brachiocephalicus and ventrally by the sternocephalicus. In the cranial half of the horse neck the omohyoideus separates the jugular vein from the common carotid artery, making injection there safer.
The supraspinatus and infraspinatus are supplied by the suprascapular nerve; their atrophy after injury is called sweeney in horses. The triceps extends the elbow (radial nerve); the biceps brachii and brachialis flex it (musculocutaneous nerve). Carpal and digital extensors on the craniolateral forearm are supplied by the radial nerve, flexors caudally by the median and ulnar nerves.
In the hindlimb the gluteals extend the hip. The hamstrings (biceps femoris, semitendinosus, semimembranosus) are supplied by the sciatic nerve; in ruminants and the pig the biceps femoris fuses with the superficial gluteal as the gluteobiceps. The quadriceps extends the stifle (femoral nerve). The gastrocnemius extends the hock (tibial nerve); the cranial tibial, fibularis and long digital extensor muscles flex the hock and extend the digits (common fibular nerve).
Intramuscular sites are chosen for muscle mass, few nerves and vessels, drainage and, in food animals, carcass value. Quality-assurance programmes in several countries recommend the lateral neck of cattle, in the triangle bounded by the nuchal ligament, the cervical vertebrae and the cranial border of the scapula, rather than the rump or thigh. The horse is injected in the neck, pectorals, triceps or caudal thigh; the pig behind the ear; the dog in the quadriceps, lumbar epaxials or triceps, avoiding the sciatic nerve in the caudal thigh.
The heart lies in the middle mediastinum, mostly left of the median plane, roughly between the third and sixth ribs in the ox and horse. The right atrium receives both venae cavae and the coronary sinus; the tricuspid valve opens to the right ventricle and pulmonary trunk. The left atrium receives the pulmonary veins; the mitral valve opens to the thick left ventricle and aorta. Chordae tendineae and papillary muscles hold the cusps; the septomarginal trabecula carries conduction fibres. The ox has two ossa cordis in the fibrous skeleton; the horse has cartilage that may ossify with age. Ruminants and the pig have a left azygos vein draining into the coronary sinus; the horse and dog a right azygos vein.
In the horse and ruminants a single brachiocephalic trunk leaves the aortic arch and gives both subclavian arteries and the bicarotid trunk; in the dog and pig the left subclavian arises separately from the arch. In adult ruminants the internal carotid artery regresses and the brain is supplied mainly through a rete mirabile fed by the maxillary artery. The abdominal aorta gives the coeliac and cranial mesenteric arteries (sometimes a common trunk in ruminants), renal and gonadal arteries, the caudal mesenteric, and the external and internal iliac arteries.
The cow's middle uterine artery arises from the umbilical artery and enlarges in pregnancy, when its pulse develops a palpable thrill (fremitus); the mare's uterine artery arises from the external iliac. The pulse is taken from the facial or transverse facial artery in the horse, the facial or median caudal artery in cattle and the femoral artery in the dog.
The external jugular vein is the standard venipuncture site in the horse and ruminants. The dog and cat use the cephalic and saphenous veins as well as the jugular; the pig the jugular or cranial vena cava at the thoracic inlet, with small injections into ear veins; cattle also the median caudal vein on the ventral tail. The subcutaneous abdominal (milk) vein of the cow is avoided routinely because it bleeds and forms haematomas.
The portal vein carries blood from gut, pancreas and spleen to the liver. In the fetus the umbilical vein brings placental blood to the liver and the ductus venosus lets part bypass it; the ductus venosus disappears before birth in the horse and pig. The foramen ovale and ductus arteriosus close after birth; the ductus becomes the ligamentum arteriosum. Patent ductus arteriosus and portosystemic shunt are important congenital defects in dogs.
Lymph passes through nodes to the thoracic duct, which begins at the cisterna chyli and empties into the great veins at the thoracic inlet. Ox nodes are large and single; horse nodes are clustered; pig nodes are inverted, with germinal centres central. Ruminants also have haemal nodes. Palpable nodes in healthy cattle include the parotid, mandibular, superficial cervical, subiliac and, in cows, mammary nodes; in the dog the mandibular, superficial cervical and popliteal. Meat inspection relies on knowing which region each node drains. The ox spleen lies on the left of the rumen; the horse spleen is sickle-shaped against the left body wall.
The brain comprises cerebrum, diencephalon with the attached pituitary, midbrain, pons and cerebellum, and medulla oblongata. It is covered by dura mater, arachnoid and pia mater; cerebrospinal fluid circulates in the subarachnoid space and is sampled from the cerebellomedullary cistern through the atlanto-occipital space, or at the lumbosacral space.
The spinal cord is shorter than the vertebral canal. In the dog it ends at about the sixth or seventh lumbar vertebra (further caudally in small dogs and cats); in the horse and ox it ends within the sacrum, the nerve roots beyond forming the cauda equina. Hence caudal epidural anaesthesia in cattle and horses is given at the sacrocaudal or first intercoccygeal space, and lumbosacral epidural in the dog, cat, pig and small ruminants. There are eight cervical nerves but seven cervical vertebrae, because the first nerve leaves through the atlas.
The brachial plexus arises from C6 to T1 or T2. Its nerves are the suprascapular, subscapular, axillary, musculocutaneous, radial, median and ulnar. Radial paralysis is the most important: the elbow drops and the animal cannot extend the limb to bear weight. The lumbosacral plexus gives the femoral nerve (quadriceps; injury collapses the stifle and abolishes the patellar reflex), the obturator nerve (adductors) and the sciatic nerve, which divides into the tibial and common fibular nerves. Fibular paralysis makes the animal knuckle at the fetlock with the hock extended; tibial paralysis drops the hock. Paralysis after difficult calving was long blamed on the obturator nerve, but compression of the sixth lumbar root of the sciatic nerve in the pelvis is now recognised as a major contributor.
The cranial nerves are: I olfactory; II optic; III oculomotor (most extraocular muscles, the levator of the upper lid, pupillary constriction); IV trochlear (dorsal oblique; the only nerve leaving the dorsal brainstem); V trigeminal (ophthalmic, maxillary and mandibular divisions; facial sensation and chewing); VI abducent (lateral rectus, retractor bulbi); VII facial (facial muscles, tears, salivation, taste rostrally); VIII vestibulocochlear; IX glossopharyngeal; X vagus; XI accessory (trapezius and parts of neck muscles); XII hypoglossal (tongue).
The cornual nerve of cattle is a branch of the zygomaticotemporal branch of the lacrimal nerve (ophthalmic division of V). The infraorbital (maxillary) and mental (mandibular) nerves supply the upper and lower lips. The auriculopalpebral nerve, from VII, is motor to the orbicularis oculi; blocking it stops blinking without sensory anaesthesia.
The horse's left recurrent laryngeal nerve loops round the aortic arch and is longer than the right; it is the one mainly affected in recurrent laryngeal neuropathy, which paralyses the cricoarytenoideus dorsalis, the only abductor of the arytenoid, causing roaring at exercise. The horse's guttural pouch, a diverticulum of the auditory tube, has nerves IX to XII, the sympathetic trunk and the internal carotid artery in its wall; fungal infection can cause fatal nosebleed, dysphagia and Horner's syndrome.
The sympathetic outflow is thoracolumbar. Preganglionic fibres synapse in the sympathetic trunk ganglia or in prevertebral ganglia such as the coeliac and mesenteric. In the neck the sympathetic trunk travels with the vagus as the vagosympathetic trunk, and the cervicothoracic (stellate) ganglion lies at the thoracic inlet. Loss of sympathetic supply to the head causes Horner's syndrome: small pupil, drooping upper lid, protruding third eyelid and sunken eye, with facial and neck sweating on the affected side in the horse.
The parasympathetic outflow is craniosacral, through cranial nerves III, VII, IX and X and the sacral pelvic nerves. The vagus supplies the heart, lungs and gut as far as the transverse colon. In ruminants it forms dorsal and ventral vagal trunks behind the diaphragm supplying the forestomachs and abomasum; damage to them, for example from adhesions after a penetrating reticular foreign body, causes vagal indigestion. The pelvic nerves supply the bladder, rectum and genital organs.
The eyeball has a fibrous tunic (cornea and sclera), a vascular tunic or uvea (choroid; ciliary body, which holds the lens by zonular fibres; and iris), and a nervous tunic, the retina, whose axons leave at the optic disc. In the ox the pupil is a horizontal oval, with small granula iridica on the pupillary margin (larger in the horse, where they are called corpora nigra). A reflective tapetum lucidum lies in the dorsal choroid: fibrous in ruminants and the horse, cellular in carnivores, absent in the pig.
The third eyelid is supported by T-shaped cartilage; the ox has a superficial gland of the third eyelid and a deep Harderian gland. The lacrimal gland lies dorsolaterally; tears drain through two puncta into the nasolacrimal duct. The extraocular muscles are four recti, two obliques and the retractor bulbi: the lateral rectus and retractor bulbi are supplied by the abducent nerve, the dorsal oblique by the trochlear and the rest by the oculomotor. The orbicularis oculi, which closes the lids, is supplied by the facial nerve.
The external ear is the auricle of elastic cartilage and the external acoustic meatus, closed by the tympanic membrane. The middle ear, with a large tympanic bulla in the ox, holds the malleus, incus and stapes and connects to the nasopharynx by the auditory tube. The inner ear contains the cochlea for hearing and the vestibule and semicircular ducts for balance. Middle ear infection in calves may involve the nearby facial nerve, giving a head tilt with facial paralysis on the same side.
The ox tongue has a raised torus linguae behind a transverse groove, the fossa linguae, where feed and foreign bodies lodge. Mechanical papillae are filiform, conical and lenticular. Gustatory papillae are fungiform, vallate and foliate; ruminants lack foliate papillae, which the horse and dog have. Taste from the rostral tongue travels in the chorda tympani of the facial nerve, from the root in the glossopharyngeal nerve.
Olfactory epithelium lies caudally on the ethmoidal conchae. The vomeronasal organ opens into the incisive duct and detects pheromones; the flehmen response draws fluid towards it. The moist planum nasolabiale of cattle carries a ridge pattern unique to each animal, used as a muzzle print for identification.
The dental formula lists for one side the incisors, canines, premolars and molars, upper over lower; the total is twice the sum. Permanent formulae: ox, sheep and goat 2(I 0/3, C 0/1, P 3/3, M 3/3) = 32 (some texts write I 0/4, C 0/0 because the lower canine looks like an incisor); horse 2(I 3/3, C 1/1, P 3-4/3, M 3/3) = 40-42, mares usually lacking canines; pig 2(I 3/3, C 1/1, P 4/4, M 3/3) = 44; dog 2(I 3/3, C 1/1, P 4/4, M 2/3) = 42; cat 2(I 3/3, C 1/1, P 3/2, M 1/1) = 30. Deciduous totals: ruminants 20, horse 24, pig and dog 28. The horse's first upper premolar is the wolf tooth; the dog's upper P4 and lower M1 are the carnassial teeth.
Brachydont teeth (dog, pig) have short crowns and stop growing; hypsodont cheek teeth (horse, ruminants) have long crowns that keep erupting as they wear. Because the horse's upper arcade is wider than the lower, sharp enamel points form on the buccal edges of the upper and lingual edges of the lower cheek teeth and are rasped (floated). Cattle age is estimated from permanent incisor eruption; the central pair usually erupts at about one and a half to two years and the corner pair at about three and a half to four, varying with breed and nutrition.
The dog's parotid duct crosses the masseter to open opposite the upper fourth premolar; in the ox and horse it passes round the ventral border of the mandible. Ruminant saliva, produced in large volume, buffers the rumen. The horse's long soft palate lies below the epiglottis, so the horse breathes through its nose, and displacement of the palate disturbs breathing at exercise.
The rumen fills most of the left half of the abdomen; pillars divide it into dorsal and ventral sacs with caudal blind sacs, and its papillae absorb volatile fatty acids. The reticulum, lined by a honeycomb of folds, lies most cranially against the diaphragm, roughly opposite the sixth to eighth ribs and close to the heart. Heavy swallowed objects fall into it and may penetrate the wall and diaphragm, causing traumatic reticuloperitonitis and pericarditis. Rumen and reticulum form the main fermentation chamber. The omasum, on the right, is filled with laminae that absorb water; the abomasum, the glandular true stomach, lies on the abdominal floor and extends to the right, ending at a muscular torus pyloricus. In the newborn calf the abomasum is the largest compartment, and the reticular groove closes when the calf suckles so milk bypasses the rumen.
The loosely anchored abomasum can be displaced by gas to the left, between rumen and left body wall (most often in dairy cows soon after calving), or to the right, where it may twist; a high-pitched ping on percussion under the last ribs is the key sign. The greater omentum of ruminants forms superficial and deep walls enclosing a supraomental recess that holds most of the intestine.
The horse stomach is small, lies left against the diaphragm and has a squamous non-glandular region separated from the glandular region by the margo plicatus, where ulcers are common. A strong cardiac sphincter makes vomiting nearly impossible, so gastric distension needs a nasogastric tube. The pig stomach has a gastric diverticulum and torus pyloricus. The dog stomach is simple; in large deep-chested breeds it can rotate (gastric dilatation-volvulus), and gastropexy fixes the pyloric antrum to the right body wall.
The ox ascending colon forms a flat spiral loop in the mesentery on the right; the pig's spiral colon is a cone on the left. The horse is a hindgut fermenter: its caecum has a base in the right flank and an apex near the xiphoid, and its large colon runs right ventral, sternal flexure, left ventral, pelvic flexure, left dorsal, diaphragmatic flexure, right dorsal, then the transverse and small colon. The narrow pelvic flexure is a common impaction site, and the loosely attached left colon can become trapped over the nephrosplenic ligament. The dog has a short caecum and simple colon.
The ox liver lies wholly on the right and is indistinctly lobed; it has a gall bladder, which the horse and rat lack. The pig liver shows a visible lobular pattern; the dog liver has deep fissures. In the ox and pig pancreatic juice usually enters the duodenum by an accessory duct alone; in small ruminants the pancreatic duct joins the bile duct; the horse and dog usually keep both ducts.
The horse has a blind nasal diverticulum (false nostril) in each nostril. The larynx is built of epiglottic, thyroid, cricoid and paired arytenoid cartilages; the horse has lateral laryngeal ventricles. Ruminants and the pig have a tracheal bronchus supplying the right cranial lobe before the tracheal bifurcation. The ox lung is deeply fissured: left cranial (two parts) and caudal lobes; right cranial (two parts), middle, caudal and accessory lobes. It is strongly lobulated by thick interlobular septa, which limit collateral ventilation. The horse lung is scarcely lobed apart from the accessory lobe. The mediastinum is thick and usually complete in ruminants, but thin and often fenestrated in the horse and dog, so air or fluid on one side may reach the other.
The kidneys are retroperitoneal. The ox kidney is externally lobated with about twenty lobes and has no renal pelvis; calyces drain directly to the ureter. The right kidney lies under the last rib; the left is pendulous, pushed right of the midline by the rumen and palpable per rectum. Sheep, goat, dog and cat kidneys are smooth and bean-shaped with a single renal crest. The pig kidney is smooth and flattened but multipyramidal with calyces and a pelvis. The horse's right kidney is heart-shaped and more cranial. The cow and sow have a suburethral diverticulum beneath the external urethral opening, a trap for careless catheterisation; the bull has a urethral recess at the ischial arch; rams and bucks have a urethral process where calculi commonly lodge.
The testes descend through the inguinal canal before or around birth; failure is cryptorchidism. The ox testis hangs vertically, the horse testis lies nearly horizontally, and the boar's lies below the anus with the tail of the epididymis dorsal. The vaginal tunic, an outpouching of peritoneum, surrounds testis and cord and opens into the abdomen at the vaginal ring. The pampiniform venous plexus around the testicular artery cools arterial blood by countercurrent exchange.
The penis of the bull, ram, buck and boar is fibroelastic with a sigmoid flexure and retractor penis muscles; the boar's glans is corkscrew-shaped and it has a preputial diverticulum. The stallion's penis is musculocavernous without a sigmoid flexure. The dog has an os penis and a bulbus glandis that forms the copulatory tie; the tomcat's glans bears spines.
The bull and stallion have ampullae, vesicular glands, prostate and bulbourethral glands; the boar has very large vesicular and bulbourethral glands; the dog has a large prostate and no vesicular or bulbourethral glands, and benign prostatic enlargement is common in older entire dogs.
The ovary is almond-shaped in the cow, kidney-shaped with an ovulation fossa in the mare and mulberry-like in the sow. Ovary, tube and uterus hang from the broad ligament (mesovarium, mesosalpinx, mesometrium). The uterus is bipartite in the cow and ewe, with horns joined by intercornual ligaments and caruncles that join fetal cotyledons as placentomes; the mare's has a large body and short horns; the sow's horns are long and coiled; rabbits, rats and mice have a duplex uterus with two cervices. The cow's cervix is firm and ridged by annular rings, palpable as a cylinder on the pelvic floor; the mare's is soft with longitudinal folds; the sow's has interdigitating pads.
The cow's udder has four independent quarters, each with teat canal, teat cistern and gland cistern, supported by the elastic median and the lateral suspensory ligaments and supplied mainly by the external pudendal artery. Blood leaves through the external pudendal and subcutaneous abdominal veins. The keratin plug and sphincter of the teat canal are the main barrier against mastitis. The ewe and goat have two glands, the mare two glands with two duct systems each, and the sow usually six to eight pairs.
The thoracic cavity is bounded by the thoracic vertebrae dorsally, the ribs and intercostal muscles laterally, the sternum ventrally and the diaphragm caudally; the diaphragm domes so far forward that much of the abdomen lies inside the rib cage. The thoracic inlet is bounded by the first thoracic vertebra, the first ribs and the manubrium. Each pleural sac has parietal pleura (costal, diaphragmatic, mediastinal) and visceral pleura on the lung, continuous at the root of the lung and below it as the pulmonary ligament. In the right sac the plica venae cavae carries the caudal vena cava and right phrenic nerve and forms a recess for the accessory lobe.
The line of pleural reflection lies caudal to the basal border of the lung, leaving the costodiaphragmatic recess. The clinical lung field of cattle is small: bounded cranially by the caudal border of the triceps, dorsally by the back muscles, and caudoventrally by a line from about the eleventh intercostal space dorsally to about the sixth costochondral junction. In the horse that line reaches about the sixteenth intercostal space dorsally. Texts differ by about one space, and the lines move with breathing. A needle through the lower caudal intercostal spaces enters the abdomen, not the thorax.
The pericardium is a fibrous sac lined by serous membrane, anchored by a sternopericardial ligament in large animals and a phrenicopericardial ligament in the dog. In cattle, a foreign body from the reticulum causes traumatic pericarditis with muffled heart sounds, jugular distension and brisket oedema.
The abdominal cavity is bounded by the diaphragm cranially, the lumbar vertebrae and sublumbar muscles dorsally, the abdominal muscles and lower ribs laterally and ventrally, and opens caudally into the pelvic cavity. The pelvic cavity is bounded by the sacrum and first caudal vertebrae dorsally, the ilium and, in ungulates, the sacrosciatic ligament laterally, and the pubis and ischium ventrally. Its inlet is formed by the sacral promontory, ilia and pubic brim; its outlet by the caudal vertebrae, sacrosciatic ligaments and ischial arch.
The peritoneum is parietal, visceral or connecting (mesenteries, omenta, ligaments). Retroperitoneal organs include the kidneys, adrenals, aorta and caudal vena cava. The greater omentum leaves the greater curvature of the stomach, the lesser omentum passes from the lesser curvature to the liver, and between them lies the omental bursa, entered by the epiploic foramen, bounded dorsally by the caudal vena cava and ventrally by the portal vein. In the horse small intestine can herniate through this foramen and strangulate.
In the pelvis the peritoneum forms the rectogenital, vesicogenital and pubovesical pouches; the caudal rectum is retroperitoneal. A rectal tear in the peritoneal part opens into the abdomen and causes peritonitis; one further caudal leaks into pelvic connective tissue.
On the left the adult ox abdomen is almost entirely rumen, with the spleen craniodorsally. On the right the liver lies under the ribs, the omasum below it, the abomasum on the floor, and the intestines caudally within the supraomental recess; the duodenum runs caudally along the right flank.
The paralumbar fossa is bounded dorsally by the lumbar transverse processes, cranially by the last rib and caudoventrally by the ridge of the internal oblique muscle from the tuber coxae. The left flank gives access to the rumen for rumenotomy, is the site for emergency trocarisation in acute bloat at the centre of the fossa, and is the commonest approach for caesarean section in cattle. The right flank gives access to the abomasum, duodenum and intestines and is used to correct abomasal displacement by fixing the omentum to the right body wall; a right paramedian approach with the cow on her back allows direct abomasopexy. Horses and dogs are usually operated through the ventral midline, along the strong, relatively avascular linea alba.
Flank anaesthesia in cattle blocks the thirteenth thoracic and first and second lumbar nerves. In the proximal paravertebral technique they are blocked at the intervertebral foramina, using the cranial edges of the transverse processes of L1, L2 and L3 respectively. In the distal paravertebral technique the nerve branches are blocked at the ends of the transverse processes of L1, L2 and L4. A successful proximal block warms the skin, because sympathetic fibres are blocked too, and curves the spine with its convexity toward the blocked side. An inverted-L block infiltrates along the caudal border of the last rib and below the transverse processes; a line block infiltrates only the incision.
The cornual nerve is blocked below the temporal line about halfway between the lateral canthus and the horn base. Goats also need the cornual branch of the infratrochlear nerve blocked near the dorsomedial margin of the orbit. The auriculopalpebral block stops blinking only; the eye and orbit of cattle are anaesthetised by a retrobulbar block or the Peterson block at the foramen orbitorotundum. Infraorbital and mental nerves are blocked at their foramina for lip surgery.
Caudal epidural anaesthesia in cattle is given at the sacrocaudal or first intercoccygeal space, found by pumping the tail and feeling the first movable joint behind the sacrum; low volumes anaesthetise tail, perineum, vulva and anus without affecting the hind limbs, whereas large volumes paralyse them. In the dog and small ruminants the lumbosacral space, on the midline between the tubera coxae just behind the last lumbar spine, is used. Equine diagnostic limb blocks proceed from distal to proximal: the palmar digital nerve block at the pastern desensitises the heel region including the navicular apparatus, then the abaxial sesamoid block and the low and high four-point blocks progressively more. Surgery on the bovine digit is often done with intravenous regional anaesthesia below a tourniquet.
In the cow the operator first finds the pelvic brim and floor. The cervix is a firm cylinder on or near the pelvic floor; in older cows the tract lies forward over the brim. The uterine horns curl ventrally and laterally from the cervix; the intercornual ligament is hooked to retract the uterus. The ovaries lie lateral to the horns near the brim and are examined for follicles and corpora lutea. Signs of pregnancy include horn asymmetry with fluid, slip of the fetal membranes, the amniotic vesicle, placentomes, the fetus and fremitus of the middle uterine artery. Other landmarks are the left kidney, the dorsal sac of the rumen on the left, the aorta and the internal iliac lymph nodes.
In the horse, rectal examination is used for colic and reproduction. Normally palpable are the caudal pole of the left kidney, the caudal edge of the spleen, the nephrosplenic ligament between them, the ventral band of the caecum on the right, the small colon with faecal balls, the bladder, the root of the mesentery and, in the male, the inguinal rings. The equine rectum is thin; tears are usually dorsal, and the arm must be relaxed during peristaltic waves.
Tracheotomy is made on the ventral midline at the junction of the upper and middle thirds of the neck, separating the sternothyrohyoid muscles. In the caudal neck the oesophagus lies to the left of the trachea, so oesophagotomy is done from the left. Thoracocentesis uses a lower intercostal space above the costochondral junction, with the needle along the cranial border of a rib. Abdominocentesis in the horse and dog is at the lowest point of the ventral midline; in cattle it is ventral and usually right of midline to avoid the rumen. Liver biopsy in cattle is done through a right caudal intercostal space, about the tenth or eleventh, on a line from the tuber coxae to the point of the shoulder, with slight differences between texts. The canine spay begins on the ventral midline just caudal to the umbilicus; the ovarian suspensory ligament must be stretched or broken, and the right ovary lies more cranially than the left.
Many avian bones are pneumatic; in the fowl the humerus is aerated from the clavicular air sac, so a humeral fracture can communicate with the airways. The chicken has about fourteen very mobile cervical vertebrae. Thoracic vertebrae fuse into a notarium; the last thoracic, lumbar, sacral and first caudal vertebrae fuse with the pelvis as the synsacrum; the pygostyle carries the tail feathers. Ribs have uncinate processes. The sternal keel anchors the pectoralis (wing depressor) and supracoracoideus (elevator). The coracoid braces the wing against the sternum, and fused clavicles form the furcula. Compound bones include the carpometacarpus, tibiotarsus and tarsometatarsus. Laying hens store calcium for eggshell in medullary bone.
Birds have no diaphragm. The lungs are small and fixed; air is moved by air sacs. Most birds have nine; the fowl is usually described with eight: paired cervical, unpaired clavicular, paired cranial and caudal thoracic and paired abdominal. Gas exchange occurs in parabronchi with one-way airflow. The voice organ is the syrinx at the tracheal bifurcation. Because breathing depends on sternal movement, gripping the body too tightly can suffocate a bird.
The avian right atrioventricular valve is a muscular flap, and there are two cranial venae cavae. A renal portal system carries venous blood from the legs to the kidneys, one reason injections are given into the pectoral muscles. Blood is taken from the right jugular vein (larger than the left), the cutaneous ulnar (wing) vein or the medial metatarsal vein. The chicken has no true lymph nodes; the bursa of Fabricius and thymus are its primary lymphoid organs.
Birds have a beak, no teeth and no soft palate; the choanal slit joins mouth and nasal cavity. The fowl's oesophagus widens into a crop; in pigeons the crop produces crop milk. The glandular proventriculus secretes acid and pepsin; the muscular gizzard grinds food with grit and is lined by koilin. The pancreas lies in the duodenal loop; the vitelline (Meckel's) diverticulum marks the jejunum-ileum junction; paired caeca arise where ileum meets rectum. The cloaca has a coprodeum, a urodeum receiving the ureters and genital ducts, and a proctodeum; the bursa of Fabricius lies dorsal to the proctodeum. The fowl has a gall bladder; the pigeon does not.
The kidneys have cranial, middle and caudal divisions lying in the synsacrum; most birds have no urinary bladder, and uric-acid urine enters the urodeum.
Only the hen's left ovary and oviduct develop. The oviduct comprises infundibulum (fertilisation), magnum (albumen), isthmus (shell membranes), uterus or shell gland (shell; the longest stay) and vagina; the whole passage takes a little over a day. The cock has intra-abdominal testes near the kidneys, ductus deferentes opening into the cloaca, no accessory glands and a small phallus.
The rat, mouse and golden hamster have 2(I 1/1, C 0/0, P 0/0, M 3/3) = 16 teeth; the guinea pig 2(I 1/1, C 0/0, P 1/1, M 3/3) = 20; the rabbit 2(I 2/1, C 0/0, P 3/2, M 3/3) = 28. Rabbits are lagomorphs, with small peg teeth behind the upper incisors. Rodent and lagomorph incisors grow continuously, and so do the cheek teeth of the rabbit and guinea pig; malocclusion leads to overgrowth and inability to eat.
The rat has no gall bladder, has a limiting ridge between the non-glandular and glandular stomach and cannot vomit. Its left lung is a single lobe and the right has four. Its Harderian gland secretes porphyrin that stains tears red under stress. The hamster has cheek pouches. The rabbit has a large caecum ending in a lymphoid vermiform appendix, a sacculus rotundus at the ileocaecal junction and eats its caecotrophs; its light skeleton means a badly restrained rabbit can fracture its lumbar spine by kicking; the male's inguinal canals stay open. The rabbit's marginal ear veins and central ear artery are used for sampling; in rats and mice the lateral tail veins are used, and intraperitoneal injection is given in the lower right abdomen to avoid the caecum.
Structure follows function. The ruminant is a foregut fermenter: rumen and reticulum hold ingesta at stable temperature and pH, buffered by saliva, while microbes digest fibre; rumination reduces particle size, and only small particles pass the reticulo-omasal orifice. The horse is a hindgut fermenter, digesting soluble carbohydrate in the small intestine and fermenting fibre in the caecum and colon; grain overload of the hindgut is linked to colic and laminitis. The carnivore gut is short and its teeth cut rather than grind.
The equine limb is built for economical running: muscle is concentrated proximally, the distal limb is moved by long tendons that store elastic energy, and the stay and reciprocal apparatuses support standing. The hoof wall grows down from the coronary band and is bonded to the distal phalanx by interlocking dermal and epidermal laminae, several hundred primary laminae around the hoof. In laminitis this bond fails and the distal phalanx may rotate or sink. The frog, digital cushion and collateral cartilages absorb shock and aid venous return. In cattle the lateral hind claw carries most load, which explains the frequency of sole ulcers there. The avian air-sac system, panting and sweating for heat loss, and the countercurrent cooling of the testis are further examples of anatomy matched to function.
In the first professional year you will dissect the ox or dog and then learn other species by difference. Learn landmarks you can feel on a live animal alongside every bone: tuber coxae, ischial tuber, lumbar transverse processes, last rib, scapular spine, jugular groove. Link each nerve to what an animal cannot do when it is cut, and each vessel to where you would take a pulse or draw blood. Use the formulae and counts as a scaffold, not as the goal; examiners at PG level increasingly ask which structure is at risk, or which block to use, rather than how many of something there are.
In practice the anatomy is in your hands every day: finding the sacrocaudal space on a fat cow, placing a cornual block before disbudding, choosing left or right flank for a caesarean or a displaced abomasum, locating the paralumbar landmarks for a paravertebral block, feeling the cervix and ovaries on routine fertility visits, and interpreting a mare's rectal examination in colic. Good anatomy prevents harm: injecting cattle in the neck rather than the rump, passing a thoracocentesis needle along the cranial border of a rib, and avoiding the sciatic nerve in a dog's thigh. When a block fails, return to the landmarks before adding more drug.
After many years the value is in teaching and in judgement at the margin: recognising an unusual presentation because you know what normal looks like, such as a left kidney displaced by rumen tympany or a malpositioned fetus; deciding when a standing flank approach will not give enough access; advising on injection-site policy for a dairy or feedlot client so carcasses are not downgraded; and training juniors to feel before they cut. Senior clinicians also judge where textbooks disagree, such as the exact lung field or intercostal space for a biopsy, and know that individual variation is the rule.
Research anatomists and clinical scientists use gross anatomy as the base for imaging (ultrasound, CT and MRI atlases of species that were described largely from dissection), for comparative and evolutionary studies of limb and gut design, for biomechanics of the equine limb and bovine claw, and for developing and validating new regional anaesthetic techniques by dye studies on cadavers. A PhD-level reader should know where descriptions rest on few specimens, where breed variation is large, and where standard terms are defined by the Nomina Anatomica Veterinaria, and should design studies that report individual variation rather than a single typical figure.
This manual does not cover histology, embryology or neuroanatomy at the level of tracts and nuclei (these belong to the histology-embryology subject). It gives few intercostal-space-level landmarks for heart auscultation, lung fields and biopsy sites because textbooks differ by a space or more; learn these on live animals from your clinical teachers. Muscle-by-muscle origins, insertions and actions are summarised by group, not listed individually. Camelids, buffalo, cat-specific anatomy and exotic pets beyond the rat, mouse, hamster, guinea pig and rabbit are covered only in passing. Age estimation by teeth is given only for cattle incisor eruption in broad ranges; equine dental ageing is left out because its reliability is disputed. There are no figures yet; use a dissection atlas alongside this text.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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