Horse anatomy
Encyclopedia
Equine anatomy refers to the gross
Gross anatomy
Gross anatomy is the study of anatomy at the macroscopic level. The term gross distinguishes it from other areas of anatomical study, including microscopic anatomy, which is studied on a microscopic scale, typically with a microscope....

 and microscopic anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 of horses and other equids
Equidae
Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus...

, including donkey
Donkey
The donkey or ass, Equus africanus asinus, is a domesticated member of the Equidae or horse family. The wild ancestor of the donkey is the African Wild Ass, E...

s, and zebra
Zebra
Zebras are several species of African equids united by their distinctive black and white stripes. Their stripes come in different patterns unique to each individual. They are generally social animals that live in small harems to large herds...

s. 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
Nomina Anatomica Veterinaria
The Nomina Anatomica Veterinaria is a text prepared by the International Committee on Veterinary Gross Anatomical Nomenclature. It is used as the standard reference for anatomical terminology in the field of Veterinary Science.The text is continually under revision...

, there are many horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

-specific colloquial
Colloquialism
A colloquialism is a word or phrase that is common in everyday, unconstrained conversation rather than in formal speech, academic writing, or paralinguistics. Dictionaries often display colloquial words and phrases with the abbreviation colloq. as an identifier...

 terms used by equestrians.

External anatomy

  • back
    Back (horse)
    The back describes the area of horse anatomy where the saddle goes, and in popular usage extends to include the loin or lumbar region behind the thoracic vertebrae that also is crucial to a horse's weight-carrying ability. These two sections of the vertebral column beginning at the withers, the...

    : the area where the saddle sits, beginning at the end of the withers
    Withers
    The withers is the ridge between the shoulder blades of a four-legged animal. In many species it is the tallest point of the body, and in horses and dogs it is the standard place to measure the animal's height .-Horses:The withers in horses are formed by the dorsal spinal processes of roughly the...

    , 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 metacarpal III
  • chestnut
    Chestnut (horse anatomy)
    The chestnut, also known as a night eye, is a callousity on the body of a horse or other equine, found on the inner side of the leg above the knee on the foreleg and, if present, below the hock on the hind leg....

    : 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
    Curb chain
    A curb chain, or curb strap, is a piece of horse tack used on any type of curb bit. It is a flat linked chain or strap that runs under the chin groove of the horse, between the bit shank purchase arms. It has a buckle or hook attachment and often has a "fly link" in the middle to apply a lip strap...

     of certain bits is fastened
  • coupling: see "Loin" below
  • coronet
    Horse hoof
    A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised structures...

    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, below.
  • elbow: The joint of the front leg at the point where the belly of the horse meets the leg. Homologous
    Homology (biology)
    Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

     to the elbow in humans
  • ergot
    Ergot (horse anatomy)
    The ergot is a small callosity on the underside of the fetlock of a horse or other equine. Some equines have them on all four fetlocks; others have few or no detectable ergots. In horses, the ergot varies from very small to the size of a pea or bean, larger ergots occurring in horses with...

    : a callosity on the back of the fetlock
  • face: the area between the forehead and the tip of the upper lip
  • fetlock
    Fetlock
    Fetlock is the common name for the metacarpophalangeal and metatarsophalangeal joints of horses, large animals, and sometimes dogs. It is formed by the junction of the third metacarpal or metatarsal bones proximad and the proximal phalanx distad...

    : sometimes called the "ankle
    Ankle
    The ankle joint is formed where the foot and the leg meet. The ankle, or talocrural joint, is a synovial hinge joint that connects the distal ends of the tibia and fibula in the lower limb with the proximal end of the talus bone in the foot...

    " 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
    Radius
    In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

     and ulna
    Ulna
    The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...

    , 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
    Frog (horse)
    The frog is a part of a horse's hoof, located on the underside, which should touch the ground if the horse is standing on soft footing. The frog is triangular in shape, and extends from the heels to mid-way toward the toe, covering around 25% of the bottom of the hoof...

    : the highly elastic wedge-shaped mass on the underside of the hoof
    Horse hoof
    A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised structures...

    , 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
    Girth (tack)
    A girth, sometimes called a cinch , is a piece of equipment used to keep the saddle in place on a horse or other animal. It passes under the barrel of the equine, usually attached to the saddle on both sides by two or three leather straps called billets...

     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
  • hock
    Hock (zoology)
    The hock, or gambrel, is the joint between the tarsal bones and tibia of a digitigrade or unguligrade quadrupedal mammal, such as a horse, cat, or dog...

    : the tarsus
    Tarsus (skeleton)
    In tetrapods, the tarsus is a cluster of articulating bones in each foot situated between the lower end of tibia and fibula of the lower leg and the metatarsus. In the foot the tarsus articulates with the bones of the metatarsus, which in turn articulate with the bones of the individual toes...

     of the horse (hindlimb equivalent to the human ankle and heel), the large joint on the hind leg
  • hoof
    Horse hoof
    A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised structures...

    : 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
    Jugular vein
    The jugular veins are veins that bring deoxygenated blood from the head back to the heart via the superior vena cava.-Internal and external:There are two sets of jugular veins: external and internal....

    , the carotid artery
    Carotid artery
    Carotid artery can refer to:* Common carotid artery* External carotid artery* Internal carotid artery...

     and part of the sympathetic trunk
    Sympathetic trunk
    The sympathetic trunks are a paired bundle of nerve fibers that run from the base of the skull to the coccyx.-Structure:...

  • knee: the carpus
    Carpus
    In tetrapods, the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The bones of the carpus do not belong to individual fingers , whereas those of the metacarpus do. The corresponding part of the foot is the tarsus...

     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
    Mane (horse)
    The mane is the hair that grows from the top of the neck of a horse or other equine, reaching from the poll to the withers, and includes the forelock or foretop. It is thicker and coarser than the rest of the horse's coat, and naturally grows to roughly cover the neck...

    : 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
    Poll (horse)
    The poll is a name of the part of an animal's head, alternatively referencing a point immediately behind or right between the ears. This area of the anatomy is of particular significance for the horse....

    : 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
    Occiput
    The occiput is the anatomical term for the posterior portion of the head, in insects the posterior part of those head capsule.-Clinical significance:Trauma to the occiput can cause a basilar skull fracture....

    ; 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; Sometimes also called the "dock"
  • shoulder: made up of the scapula
    Scapula
    In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....

     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
    Equine conformation
    Equine conformation evaluates the degree of correctness of a horse's bone structure, musculature, and its body proportions in relation to each other. Undesirable conformation can limit the ability to perform a specific task. Although there are several universal "faults," a horse's conformation is...

  • 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: 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
  • throatlatch (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
    Bridle
    A bridle is a piece of equipment used to direct a horse. As defined in the Oxford English Dictionary, the "bridle" includes both the headstall that holds a bit that goes in the mouth of a horse, and the reins that are attached to the bit....

     goes.
  • withers
    Withers
    The withers is the ridge between the shoulder blades of a four-legged animal. In many species it is the tallest point of the body, and in horses and dogs it is the standard place to measure the animal's height .-Horses:The withers in horses are formed by the dorsal spinal processes of roughly the...

    : 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.

Digestive system

Horses and other Equids evolved as grazing
Grazing
Grazing generally describes a type of feeding, in which a herbivore feeds on plants , and also on other multicellular autotrophs...

 animals, adapted to eating small amounts of the same kind of food all day long. In the wild, the horse adapted to eating prairie
Prairie
Prairies are considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and grasses, herbs, and shrubs, rather than trees, as the dominant vegetation type...

 grasses in semi-arid regions and traveling significant distances each day in order to obtain adequate nutrition. Therefore, the digestive system of a horse is about 100 feet (30.5 m) long, and most of this is intestines.

The mouth

Digestion begins in the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of 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 of forage
Forage
Forage is plant material eaten by grazing livestock.Historically the term forage has meant only plants eaten by the animals directly as pasture, crop residue, or immature cereal crops, but it is also used more loosely to include similar plants cut for fodder and carried to the animals, especially...

 and pick up finer foods, such as grain
GRAIN
GRAIN is a small international non-profit organisation that works to support small farmers and social movements in their struggles for community-controlled and biodiversity-based food systems. Our support takes the form of independent research and analysis, networking at local, regional and...

, with their sensitive, prehensile lip
Lip
Lips are a visible body part at the mouth of humans and many animals. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech...

s. The front teeth
Horse teeth
Horses' teeth are often used to estimate the animal's age, hence the sayings "long in the tooth", "straight from the horse's mouth" and "Don't look a gift horse in the mouth".- Types of teeth :At five years of age a horse has between 36 and 44 teeth...

 of the horse, called incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...

s, clip forage
Forage
Forage is plant material eaten by grazing livestock.Historically the term forage has meant only plants eaten by the animals directly as pasture, crop residue, or immature cereal crops, but it is also used more loosely to include similar plants cut for fodder and carried to the animals, especially...

, and food is then pushed back in the mouth by the tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

, and ground up for swallowing by the premolar
Premolar
The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...

s and molars.

The esophagus

The esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

 is about 4 to 5 ft (1.2 to 1.5 m) 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
Choke (horse)
Choke is a condition in horses in which the esophagus is blocked, usually by food material. Although the horse is still able to breathe, it is unable to swallow, and may become severely dehydrated. A secondary condition, aspiration pneumonia, may also develop if food material and saliva accumulate...

.

The stomach

Horses have a relatively small stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

 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 (362.9 to 544.3 kg)) has a stomach with a capacity of around 4 gallons (15.1 l), and works best when it contains about 2 gallons (7.6 l). 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 plicatus, 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
Pepsin
Pepsin is an enzyme whose precursor form is released by the chief cells in the stomach and that degrades food proteins into peptides. It was discovered in 1836 by Theodor Schwann who also coined its name from the Greek word pepsis, meaning digestion...

 break down food. Pepsin allows for the further breakdown of proteins into amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 chains. Other enzymes include resin and lipase
Lipase
A lipase is an enzyme that catalyzes the formation or cleavage of fats . Lipases are a subclass of the esterases.Lipases perform essential roles in the digestion, transport and processing of dietary lipids in most, if not all, living organisms...

. Additionally, the stomach absorbs some water, as well as ions and lipid soluble compounds. The end product is food broken down into chyme
Chyme
Chyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum.Also known as chymus, it is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum...

. It then leaves the stomach through the pyloric valve, which controls the flow of food out of stomach.

The small intestine

The horse’s small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...

 is 50 to 70 ft (15.2 to 21.3 m) long and holds 10 to 12 US gal (37.9 to 45.4 L). This is the major digestive organ, and where most nutrients are absorbed. 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
Bile
Bile or gall is a bitter-tasting, dark green to yellowish brown fluid, produced by the liver of most vertebrates, that aids the process of digestion of lipids in the small intestine. In many species, bile is stored in the gallbladder and upon eating is discharged into the duodenum...

 from the liver aids in digesting fats in the duodenum combined with enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s from the pancreas
Pancreas
The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...

 and small intestine . Horses do not have a gall 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, . Any remaining liquids and roughage move into the large intestine.

Cecum

The cecum
Cecum
The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...

 is the first section of the large intestine
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...

. It is also known as the "water gut" or "hind gut." It is a cul-de-sac pouch, about 4 feet (1.2 m) long that holds 7 to 8 US gal (26.5 to 30.3 L). It contains bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 that digest cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 plant fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....

 through fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

. 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
Horse colic
Colic in horses is defined as abdominal pain, but it is a clinical sign rather than a diagnosis. The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. The most common forms of colic...

, as the new food is not properly digested.

Colon

The large colon
Colon (anatomy)
The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...

, small colon, and rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

 make up the remainder of the large intestine
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...

. The large colon is 10 to 12 ft (3 to 3.7 m) long and holds up to 20 gallons (75.7 l) of semi-liquid matter. It is made up of the right ventral (lower) colon, the left ventral colon, the left dorsal (upper) colon, the right dorsal colon, and the transverse colon, in that order. Three flexures are also named; the sternal flexure, between right and left ventral colon; the pelvic flexure, between left ventral and left dorsal 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 of horse colic
Horse colic
Colic in horses is defined as abdominal pain, but it is a clinical sign rather than a diagnosis. The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. The most common forms of colic...

 called an impaction.

The small colon is 10 to 12 ft (3 to 3.7 m) in length and holds only 5 gallons (18.9 l) 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. The rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

 is about 1 feet (30.5 cm) long, and acts as a holding chamber for waste matter, which is then expelled from the body via the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

.

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 to 8 cm (2.8 to 3.1 in) in length and 3 to 4 cm (1.2 to 1.6 in) 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 to 25 cm (7.9 to 9.8 in) in length. These horns attach to the body of the uterus (18 to 20 cm (7.1 to 7.9 in) long). Caudal to the uterus is the cervix, about 5 to 7 cm (2 to 2.8 in) long, which enters the vagina. Usually 3.5 to 4 cm (1.4 to 1.6 in) in diameter, it can expand to allow the passage of the foal. The vagina of the mare is 15 to 20 cm (5.9 to 7.9 in) 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.

Stallion

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:
  • the testes, which are suspended horizontally within the scrotum
    Scrotum
    In some male mammals the scrotum is a dual-chambered protuberance of skin and muscle containing the testicles and divided by a septum. It is an extension of the perineum, and is located between the penis and anus. In humans and some other mammals, the base of the scrotum becomes covered with curly...

    . The testes of an average stallion are ovoids 8 to 12 cm (3.1 to 4.7 in) long;
  • the penis
    Penis
    The penis is a biological feature of male animals including both vertebrates and invertebrates...

    , within the prepuce
    Prepuce
    Prepuce may refer to:* The foreskin, which surrounds and protects the head of the penis* The clitoral hood, which surrounds and protects the head of the clitoris...

    , also known as the "sheath." When not erect, the penis is housed within the prepuce, 50 centimetres (19.7 in) long and 2.5 to 6 cm (0.984251968503937 to 2.4 in) in diameter with the distal end 15 to 20 cm (5.9 to 7.9 in). When erect, the penis doubles in length and thickness and the glans increases by 3 to . The urethra
    Urethra
    In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...

     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 the semen
Semen
Semen is an organic fluid, also known as seminal fluid, that may contain spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...

 at ejaculation
Ejaculation
Ejaculation is the ejecting of semen from the male reproductory tract, and is usually accompanied by orgasm. It is usually the final stage and natural objective of male sexual stimulation, and an essential component of natural conception. In rare cases ejaculation occurs because of prostatic disease...

, but are not strictly necessary for fertility

Teeth

A horse's teeth include incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...

s, premolar
Premolar
The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...

s, molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....

s, 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
Horse hoof
A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised structures...

 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, primarily keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

, a very strong protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 molecule. The horse's hoof contains a high proportion of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

-containing amino acids which contribute to its resilience and toughness.

Skeletal system


The skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

 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 , which are divided into the appendicular skeleton (the legs) and the axial skeleton (the skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

, vertebral column
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

, sternum
Sternum
The sternum or breastbone is a long flat bony plate shaped like a capital "T" located anteriorly to the heart in the center of the thorax...

, and ribs). Both pelvic and thoracic limbs contain the same number of 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 of red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...

 formation.

Ligaments

Ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...

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

Tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...

s are 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
Fetlock
Fetlock is the common name for the metacarpophalangeal and metatarsophalangeal joints of horses, large animals, and sometimes dogs. It is formed by the junction of the third metacarpal or metatarsal bones proximad and the proximal phalanx distad...

, 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
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

, 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 a tendon sheath
Tendon sheath
A tendon sheath is a layer of membrane around a tendon. It permits the tendon to move.It has two layers:* synovial sheath* fibrous tendon sheathFibroma of the tendon sheath has been described....

 or a sac called a bursa
Bursa (anatomy)
A bursa is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of slimy fluid . It provides a cushion between bones and tendons and/or muscles around a joint. This helps to reduce friction between the bones and allows free movement...

.

Tendons are easily damaged if placed under too much strain, which can result in a painful, and possibly career-ending, injury. Tendinitis 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
Myofibril
A myofibril is a basic unit of a muscle. Muscles are composed of tubular cells called myocytes or myofibers. Myofibers are composed of tubular myofibrils. Myofibrils are composed of long proteins such as actin, myosin, and titin, and other proteins that hold them together...

s, which are able to contract due to actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 and myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

. 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. The soft palate blocks off the pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...

 from the mouth (oral cavity) of the horse, except when swallowing. This helps prevent the horse from inhaling food, but also means that a horse cannot use its mouth to breathe when in respiratory distress—a horse can only breathe through its nostrils. For this same reason, horses also cannot pant
Pant
Pant may refer to:* Pants, an article of clothing* To breathe quickly, spasmodically, or in a labored manner** Panting is used by many furry animals as a means of thermoregulation* Pant , a North Indian and Nepalese surnamePlaces...

 as a method of thermoregulation
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...

. The genus Equus
Equus (genus)
Equus is a genus of animals in the family Equidae that includes horses, donkeys, and zebras. Within Equidae, Equus is the only extant genus. Like Equidae more broadly, Equus has numerous extinct species known only from fossils. This article deals primarily with the extant species.The term equine...

also has a unique part of the respiratory system called the guttural pouch
Guttural pouch
In the equids and some rodent-like species such as the desert hyrax, the guttural pouch is a pair of air chambers in the neck just behind the skull and below the ears...

, which is thought to equalize air pressure on the tympanic membrane.Located between the mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

s but below the occiput
Occiput
The occiput is the anatomical term for the posterior portion of the head, in insects the posterior part of those head capsule.-Clinical significance:Trauma to the occiput can cause a basilar skull fracture....

, it fills with air when the horse swallows or exhales.

Circulatory system

The horse's circulatory system includes the four-chambered heart, averaging 8.5 lb (3.9 kg) 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 hoof (including the frog - the V shaped part on the bottom of the horses hoof) is a very important part of the circulatory system. As the horse puts weight onto the hoof, the hoof wall is pushed outwards and the frog compressed, driving blood out of the frog, the digital pad, and the laminae of the hoof. When weight is removed from the hoof, the release of pressure pulls blood back down into the foot again. This effectively creates an auxiliary blood-pumping system at the end of each leg. Some of this effect is lost when a horse is shod (eliminating the expansion and contraction of the hoof wall and raising the frog higher from the ground).

The eye

The horse has the largest eye of all land mammals, and is designed to help the horse as a prey animal. It provides the horse with a wide field of monocular vision
Monocular vision
Monocular vision is vision in which each eye is used separately. By using the eyes in this way, as opposed by binocular vision, the field of view is increased, while depth perception is limited. The eyes are usually positioned on opposite sides of the animal's head giving it the ability to see two...

, as well as good visual acuity. Horses have two-color, or dichromatic vision, which is somewhat like red-green color blindness  in humans. Because the horse's vision is closely tied to behavior, the horse's visual abilities are often taken into account when handling and training the animal.

Hearing

The hearing of horses is good, superior to that of humans, and the pinna of each ear can rotate up to 180°, giving the potential for 360° hearing without having to move the head. Often, the eye of the horse is looking in the same direction as the ear is directed.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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