- Equine anatomy
The anatomy of the horse comes with a large number of
horse specific terms.External anatomy
* back: the area where the saddle goes, begins at the end of the
withers , extends to the last thoracic vertebrae. (Colloquially includes the loin or "coupling," though technically incorrect usage)
* barrel: the main body area of the horse, enclosing the rib cage and the major internal organs.
* cannon orcannon bone : The area between the knee or hock and the fetlock joint, sometimes called the "shin" of the horse, though technically it is the metacarpal III.
* 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 thecurb 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 horse's hindquarters, beginning at the hip, extending proximate to thesacral vertebrae and stopping at the dock of the tail (where thecoccygeal vertebrae begin). Sometimes called "rump."
* dock: the point where the tail connects to the rear of the horse.
* 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.
*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 of the horse 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. Consists of the fusedradius andulna , and all the tissue around these bones. Anatomically theantebrachium .
* 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, 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 of the 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. The hoof wall 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 thejugular vein , thecarotid artery and part of thesympathetic trunk .
* knee: thecarpus 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 of the horse to the croup. Anatomically approximate to thelumbar spine .
* mane: long and relatively coarse hair growing from the dorsal ridge of the neck, lying on either the left or right side of the neck).
* muzzle: the chin, mouth, and nostrils of the horse's 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 horse's 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."
* splints: bones found on each of the legs, on either side of the cannon bone (8 total). Partiallyvestigial , these bones support the corresponding carpal bones in the forelimb, and the corresponding tarsal bones in the hindlimb. [Getty (1975)"Equine Osteology" in Sisson and Grossman's The Anatomy of the Domestic Animals Volume 1", Sunders, ISBN 0-7216-4102-4] Anatomically referred to asMetacarpal /Metatarsal II (on the medial aspect (inside)) and IV (on the lateral aspect (outside)).
* shoulder: made up of thescapula and associated muscles. Runs from the withers to the point of shoulder (the joint at the front of the chest, i.e. theglenoid ). The angle of the shoulder has a great affect on the horse's movement and jumping ability, and is an important aspect ofequine conformation .
* stifle: 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: consists of both the living part of the tail (which consists of the coccygeal vertebrae, muscules, and ligaments), as well as the long hairs which grow from the living part
* throatlatch: The point at which the windpipe meets the head at the underside of the jaw.
*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 in "hands."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 eatingprairie 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. ISBN 0-684-82768-9] Therefore, the digestive system of a horse is about convert|100|ft|m long, and most of this is intestines.The 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 parotid (largest salivary gland and located near the poll), submaxillary (located in the jaw), and sublingual (located under the tongue). Horses select pieces offorage and pick up finer foods, such asgrain , with their sensitive,prehensile lip s. The front teeth of the horse, calledincisor s, clipforage , and food is then pushed back in the mouth by thetongue , and ground up for swallowing by thepremolar s andmolars . Giffen, James M. and Tom Gore. ‘’Horse Owner’s Veterinary Handbook., 2nd ed. New York:Howell Book House, 1989, 1998. ISBN 0876056060]The esophagus
The
esophagus is about 4-5 feet 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 cannotvomit . The esophagus is also the area of the digestive tract where horses may suffer from choke.The 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 (800 to 1200 lb) has a stomach with a capacity of only four gallons, and works best when it contains about two gallons. Because the stomach empties when 2/3 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 plicata, from the glandular distal stomach Andrews FM, Buchanan BR, Elliot SB, Clariday NA and Edwards LH (2005) "Gastric ulcers in horses" "J. Anim. Sci." 83:E18-E21] .In the stomach, assorted acids and the enzyme
pepsin break down food. Pepsin allows for the further breakdown of proteins intoamino acid chains. Other enzymes include resin andlipase . Additionally, the stomach absorbs some water, as well as ions and lipid soluble compounds. The end product is food broken down intochyme .Fact|date=February 2007 It then leaves the stomach through thepyloric valve , which controls the flow of food out of stomach.Fact|date=February 2007The small intestine
The horse’s
small intestine is 50 to convert|70|ft|m long and holds 10 to 12 gallons. This is the major digestive organ, and where most nutrients are absorbed. [http://ohioline.osu.edu/b762/b762_5.html "Horse Nutrition - The Horse's Digestive System." Bulletin 762-00, Ohio State University.] Web site accessed February 9, 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 withenzyme s from thepancreas and small intestine . Horses do not have agall bladder , so bile flows constantly. Most food is digested and absorbed into the bloodstream from the small intestine, including proteins, simple carbohydrate, fats, and vitamines A, D, and E. Any remaining liquids and roughage move into the large intestine.The large intestine
Cecum
The
cecum is the first section of thelarge intestine . It is also known as the "water gut" or "hind gut." It is a cul-de-sac pouch, about convert|4|ft|m long that holds 7 to 8 gallons. It containsbacteria that digestcellulose plantfiber through fermentation. [http://www.esc.rutgers.edu/publications/factsheets_nutrition/FS038.htm Williams, Carey A.,Ph.D., Extension Specialist. "The Basics of Equine Nutrition" "from" FS #038, Equine Science Center, Rutgers University, Revised: April 2004.] Web site accessed February 9,2007] These bacteria feed upon digestive chyme, and also produce certain fat-soluble vitamins which are absorbed by the horse. The reason horses must have their diets changed slowly is so the bacteria in the cecum are able to modify and adapt to the different chemical structure of new feedstuffs. Too abrupt a change in diet can cause colic, as the new food is not properly digested.Colon
The large colon, small colon, and
rectum make up the remainder of thelarge intestine . The large colon is 10-12 feet long and holds up to 20 gallons of semi-liquid matter. It is made up of the right lower (ventral) colon, the left lower (ventral) colon, the left upper (dorsal) colon, the right upper (dorsal) colon, and the transverse colon, in that order. Threeflexures are also named; the sternal flexure, between right and left ventral colon; the pelvic flexure, between left dorsal and left ventral colon; the diaphragmatic flexure, between left dorsal and right dorsal colon. The main purpose of the large colon is to absorb carbohydrates, which were broken down from cellulose in the cecum. Due to its many twists and turns, it is a common place for a type ofhorse colic called an impaction.The small colon is 10-12 feet in length and holds only 5 gallons of material. It is the area where the majority of water in the horse's diet is absorbed, and is the place where
fecal balls are formed. Therectum is about one foot long, and acts as a holding chamber for waste matter, which is then expelled from the body via theanus .Reproductive system
Mare
The reproductive system of the mare is responsible for controlling gestation, birth, lactation, as well as the estrous cycle and mating behavior of the mare. It lies ventral to the 4th or 5th lumbar vertebrae, although its position within the mare can vary depending on the movement of the intestines and distention of the bladder.
The mare has two ovaries, usually 7-8 cm in length and 3-4 cm thick, that generally tend to decrease in size as the mare ages. The ovaries connect to the fallopian tubes (oviducts), which serve to move the ovum from the ovary to the uterus. To do so, the oviducts are lined with a layer of
cilia , which produce a current that flows toward the uterus. Each oviduct attaches to one of the two horns of the uterus, which are approximately 20-25 cm in length. These horns attach to the body of the uterus (18-20 cm long). Caudal to the uterus is the cervix, about 5-7 cm long, which enters the vagina. Usually 3.5-4 cm in diameter, it can expand to allow the passage of the foal. The vagina of the mare is 15-20 cm long, and is quite elastic, allowing it to expand. The vulva is the external opening of the vagina, and consists of the clitoris and two labia. It lies ventral to the rectum. The mare has two mammary glands, which are smaller in virgin mares. They have two ducts each, which open externally.tallion
The reproductive system of the stallion is responsible for the sexual behavior and secondary sex characteristics (such as a large crest) of the stallion.The external genitalia comprise:
* thetestes , which are suspended horizontally within thescrotum . The testes of an average stallion are ovoids 8 to 12 cm long;
* thepenis , within theprepuce , also known as the "sheath." When not erect, the penis is housed within the prepuce, 50 cm long and 2.5 to 6 cm in diameter with the distal end 15 to 20 cm. When erect, the penis doubles in length and thickness and the glans increases by 3 to 4 times. Theurethra opens within the urethral fossa, a small pouch at the distal end of the glans.The internal genitalia comprise the accessory sex glands:
*vesicular glands ;
*prostate gland ; and
*bulbourethral glands .These contribute fluid to thesemen atejaculation , but are not strictly necessary for fertility ["The Stallion: Breeding Soundness Examination & Reproductive Anatomy", University of Wisconsin-Madison. Accessed at [http://www.wisc.edu/ansci_repro/101equinelab/reproduction/stallion_exam.html] on 7th July 2007]Teeth
A horse's teeth include
incisor s,premolar s, 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.Feet/Hooves
The hoof of the horse encases the second and 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, primarilykeratin , a very strongprotein molecule. The horse's hoof contains a high proportion ofsulfur -containingamino acids which contribute to its resilience and toughness.keletal system
The
skeleton of the horse has three major functions in the body. It protects vital organs, provides framework, and supports soft parts of the body. Horses have 205 bones, which are divided into the appendicular skeleton (the legs) and the axial skeleton (theskull ,vertebral column ,sternum , andribs ). Both pelvic and thoracic limbs contain the same number of bones, 20 bones per limb . Bones are connected to muscles via tendons and other bones via ligaments. Bones are also used to store minerals, and are the site ofred blood cell formation.Ligaments and tendons
Ligaments
Ligament s attach bone to bone or bone to tendon, and are vital in stabilizing joints as well as supporting structures. They are made up of fibrous material that is generally quite strong. Due to their relatively poor blood supply, ligament injuries generally take a long time to heal.Tendons
Tendons are fibrous cords of connective tissue attaching muscle to bone, cartilage or other tendons. They are a major contributor to shock absorption, are necessary for support of the horse’s body, and translate the force generated by muscles into movement. Tendons are classified as flexors (flex a joint) or extensors (extend a joint). However, some tendons will flex multiple joints while extending another (the flexor tendons of the hind limb, for example, will flex the
fetlock , pastern, and coffin joint, but extend the hock joint). In this case, the tendons (and associated muscles) are named for their most distal action (digital flexion).Tendons form in the embryo from fibroblasts which become more tightly packed as the tendon grows. As tendons develop they lay down
collagen , which is the main structural protein of connective tissue. As tendons pass near bony prominences, they are protected by a fluid filled synovial structure, either atendon sheath or a sac called a bursa.Tendons are easily damaged if placed under too much strain, which can result in a painful, and possibly career-ending, injury.
Tendonitis is most commonly seen in high performance horses that gallop or jump. When a tendon is damaged the healing process is slow because tendons have a poor blood supply, reducing the availability of nutrients and oxygen to the tendon. Once a tendon is damaged the tendon will always be weaker, because the collagen fibres tend to line up in random arrangements instead of the stronger linear pattern. Scar tissue within the tendon decreases the overall elasticity in the damaged section of the tendon as well, causing an increase in strain on adjacent uninjured tissue.Muscular system
When a muscle contracts, it pulls a tendon, which acts on the horse's bones to move them. Muscles are commonly arranged in pairs so that they oppose each other (they are "antagonists"), with one flexing the joint (a flexor muscle) and the other extending it (extensor muscle). Therefore, one muscle of the pair must be relaxed in order for the other muscle in the pair to contract and bend the joint properly. A muscle is made up of several muscle bundles, which in turn are made up of muscle fibers. Muscle fibers have
myofibril s, which are able to contract due toactin andmyosin . A muscle together with its tendon and bony attachments form an extensor or flexor unit.Respiratory system and smell
The horse's respiratory system consists of the nostrils, pharynx, larynx, trachea, diaphragm, and lungs. Additionally, the nasolacrimal duct and sinuses are connected to the nasal passage. The horse's respiratory system not only allows the animal to breathe, but also is important in the horse's sense of smell (olfactory ability) as well as in communicating.
Circulatory system
The horse's circulatory system includes the four-chambered heart, averaging convert|8.5|lb|abbr=on in weight, as well as the blood and blood vessels. Its main purpose is to circulate blood throughout the body to deliver oxygen and nutrients to tissues, and to remove waste from these tissues.
The eye
The horse has the largest eye of all land mammals, [ [http://www.horsewyse.com.au/howhorsessee.html Horsewyse: How horses see. Date Accessed 7/11/07] ] and is designed to help the horse as a prey animal. It provides the horse with a wide field of
monocular vision , as well as good visual acuity and some ability to see color. Because the horse's vision is closely tied to his behavior, the horse's visual abilities are often taken into account when handling and training the animal.References
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