Equine anatomy encompasses the gross and microscopic anatomy of horses, ponies and other equids, including donkeys, mules and zebras. While all anatomical features of equids are described in the same terms as for other animals by the International Committee on Veterinary Gross Anatomical Nomenclature in the book Nomina Anatomica Veterinaria, there are many horse-specific colloquial terms used by equestrians.
External anatomy Back: the area where the saddle sits, beginning at the end of the withers, extending to the last thoracic vertebrae (colloquially includes the loin or "coupling", though technically incorrect usage) Barrel: the body of the horse, enclosing the rib cage and the major internal organs Buttock: the part of the hindquarters behind the thighs and below the root of the tail Cannon or cannon bone: the area between the knee or hock and the fetlock joint, sometimes called the "shin" of the horse, though technically it is the third metacarpal Chestnut: a callosity on the inside of each leg Chin groove: the part of the horse's head behind the lower lip and chin, the area that dips down slightly on the lower jaw; area where the curb chain of certain bits is fastened Coupling: see "Loin" below Coronet or coronary band: the ring of soft tissue just above the horny hoof that blends into the skin of the leg Crest: the upper portion of the neck where the mane grows Croup: the topline of the hindquarters, beginning at the hip, extending proximate to the sacral vertebrae and stopping at the dock of the tail (where the coccygeal vertebrae begin); sometimes called "rump" Dock: the living part of the tail, consisting of the coccygeal vertebrae, muscles and ligaments. Sometimes used colloquially to refer to the root of the tail. Elbow: The joint of the front leg at the point where the belly of the horse meets the leg. Homologous to the elbow in humans Ergot: a callosity on the back of the fetlock Face: the area between the forehead and the tip of the upper lip Fetlock: sometimes called the "ankle" of the horse, though it is not the same skeletal structure as an ankle in humans; known to anatomists as the metacarpophalangeal (front) or metatarsophalangeal (hind) joint; homologous to the "ball" of the foot or the metacarpophalangeal joints of the fingers in humans Flank: where the hind legs and the barrel meet, specifically the area right behind the rib cage and in front of the stifle joint Forearm: the area of the front leg between the knee and elbow, consisting of the fused radius and ulna, and all the tissue around these bones; anatomically, the antebrachium. Forehead: the area between the poll, the eyes and the arch of the nose Forelock: the continuation of the mane, which hangs from between the ears down onto the forehead of the horse Frog: the highly elastic wedge-shaped mass on the underside of the hoof, which normally makes contact with the ground every stride, and supports both the locomotion and circulation of the horse Gaskin: the large muscle on the hind leg, just above the hock, below the stifle, homologous to the calf of a human Girth or heartgirth: the area right behind the elbow of the horse, where the girth of the saddle would go; this area should be where the barrel is at its greatest diameter in a properly-conditioned horse that is not pregnant or obese Hindquarters: the large, muscular area of the hind legs, above the stifle and behind the barrel. Can also be used to refer to the back end of a horse. Hock: the tarsus of the horse (hindlimb equivalent to the human ankle and heel), the large joint on the hind leg Hoof: the foot of the horse; the hoof wall is the tough outside covering of the hoof that comes into contact with the ground and is, in many respects, a much larger and stronger version of the human fingernail Jugular groove: the line of indentation on the lower portion of the neck, can be seen from either side, just above the windpipe; beneath this area run the jugular vein, the carotid artery and part of the sympathetic trunk Knee: the carpus of the horse (equivalent to the human wrist), the large joint in the front legs, above the cannon bone Loin: the area right behind the saddle, going from the last rib to the croup, anatomically approximate to the lumbar spine Mane: long and relatively coarse hair growing from the dorsal ridge of the neck Muzzle: the chin, mouth, and nostrils of the face Pastern: the connection between the coronet and the fetlock, made up of the middle and proximal phalanx Poll: commonly refers to the poll joint at the beginning of the neck, immediately behind the ears, a slight depression at the joint where the atlas (C1) meets the occipital crest; anatomically, the occipital crest itself is the "poll" Root of the tail or root of the dock: the point where the tail is "set on" (attached) to the rump; p.38 Sometimes also called the "dock" Shoulder: made up of the scapula and associated muscles, runs from the withers to the point of shoulder (the joint at the front of the chest, i.e. the glenoid); the angle of the shoulder has a great effect on the horse's movement and jumping ability, and is an important aspect of equine conformation Splints: bones found on each of the legs, on either side of the cannon bone (8 total); partially vestigial, these bones support the corresponding carpal bones in the forelimb, and the corresponding tarsal bones in the hindlimb; anatomically referred to as metacarpal/metatarsal II (on the medial aspect (inside)) and IV (on the lateral aspect (outside)) Stifle joint: corresponds to the knee of a human, consists of the articulation between femur and tibia, as well as the articulation between patella and femur Tail: the long hairs which grow from the dock; may also include the dock The long hairs alone are sometimes called the skirt. ThroatlatchInteractive points of the horse chart (also, throttle, throatlash, throat): the point at which the windpipe meets the head at the underside of the jaw, corresponding to where the eponymous part of a bridle goes. Withers: the highest point of the thoracic vertebrae, the point just above the tops of the shoulder blades, seen best with horse standing square and head slightly lowered; the height of the horse is measured at the withers.
Hooves
The hoof of the horse encases part of the second and all of the third phalanx of the lower limbs, analogous to the fingertip or toe tip of a human. In essence, a horse travels on its "tiptoes". The hoof wall is a much larger, thicker and stronger version of the human fingernail or toenail, made up of similar materials, primarily keratin, a very strong protein molecule. The horse's hoof contains a high proportion of sulfur-containing amino acids which contribute to its resilience and toughness. Vascular fold-like structures called laminae suspend the distal phalanx from the hoof wall.
Digestive system
Horses and other equids evolved as grazing animals, adapted to eating small amounts of the same kind of food all day long. In the wild, the horse adapted to eating prairie grasses in semi-arid regions and traveling significant distances each day in order to obtain adequate nutrition.Budiansky, Stephen. The Nature of Horses. Free Press, 1997. Therefore, the digestive system of a horse is about long, and most of this is intestines.
Mouth
Digestion begins in the mouth, which is also called the "oral cavity." It is made up of the teeth, the hard palate, the soft palate, the tongue and related muscles, the cheeks and the lips. Horses also have three pairs of salivary glands: the parotoid (largest salivary gland and located near the poll), mandibular (located in the jaw), and sublingual (located under the tongue). Horses select pieces of forage and pick up finer foods, such as grain, with their sensitive, prehensile lips. The front teeth of the horse, called incisors, clip forage, and food is then pushed back in the mouth by the tongue, and ground up for swallowing by the premolars and molars.
Teeth
A horse's teeth include incisors, premolars, molars, and sometimes canine teeth. A horse's incisors, premolars, and molars, once fully developed, continue to erupt throughout its lifetime as the grinding surface is worn down through chewing. Because of this pattern of wear, a rough estimate of a horse's age can be made from an examination of the teeth. Abnormal wear of the teeth, caused by conformational defects, abnormal behaviors, or improper diets, can cause serious health issues and can even result in the death of the horse.
Esophagus The esophagus is about in length, and carries food to the stomach. A muscular ring, called the cardiac sphincter, connects the stomach to the esophagus. This sphincter is very well developed in horses. This and the oblique angle at which the esophagus connects to the stomach explains why horses cannot vomit. The esophagus is also the area of the digestive tract where horses may suffer from choke.
Stomach
Horses have a relatively small stomach for their size, and this limits the amount of feed a horse can take in at one time. The average sized horse () has a stomach with a capacity of around , and works best when it contains about . Because the stomach empties when full, whether stomach enzymes have completed their processing of the food or not, and doing so prevents full digestion and proper utilization of feed, continuous foraging or several small feedings per day are preferable to one or two large ones. The horse stomach consists of a non-glandular proximal region (saccus cecus), divided by a distinct border, the margo plicatus, from the glandular distal stomach, which is further divided into the fundus and pylorus.
Stomach acid and the enzymes pepsin (which begins the breakdown of proteins into amino acid chains), and lipase (which begins the breakdown of fats) are produced by the glandular mucosa in the fundus as well as mucus. Gastrin is produced in the pylorus.
Small intestine The horse's small intestine is long and holds . This is the major digestive organ, and where most nutrients are absorbed."Horse Nutrition - The Horse's Digestive System." Bulletin 762-00, Ohio State University. Web site accessed 9 February 2007. It has three parts, the duodenum, jejunum and ileum. The majority of digestion occurs in the duodenum while the majority of absorption occurs in the jejunum. Bile from the liver aids in digesting fats in the duodenum combined with enzymes from the pancreas and small intestine. Horses, in common with mammals such as camels, do not have a gall bladder, meaning bile flows constantly. Simple nutrients are digested and absorbed into the bloodstream from the small intestine, including proteins, simple carbohydrate, fats, and vitamins A, D, . Any remaining liquids and roughage move into the large intestine.
Large intestine
The large intestine, also known as the hindgut, consists of the cecum, large colon, small colon and rectum, terminating in the anus.
The cecum (called the "water gut" in old textbooks) is the first section of the large intestine, analogous to the appendix in humans. It is a cul-de-sac pouch, about long that holds . It contains bacteria and other microbes that break down cellulose and other indigestible plant fiber through fermentation into volatile fatty acids. These microbes feed upon the portion of chyme not absorbed by the small intestine, and produce vitamin K and B complex vitamins.