Technique of dehydration applied in a foal

Equine anatomy encompasses the gross and

microscopic The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded as the scale betwe ...

anatomy Anatomy () is the branch of morphology concerned with the study of the internal structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old scien ...

of horses, ponies and other equids, including

donkey The donkey or ass is a domesticated equine. It derives from the African wild ass, ''Equus africanus'', and may be classified either as a subspecies thereof, ''Equus africanus asinus'', or as a separate species, ''Equus asinus''. It was domes ...

mule The mule is a domestic equine hybrid between a donkey, and a horse. It is the offspring of a male donkey (a jack) and a female horse (a mare). The horse and the donkey are different species, with different numbers of chromosomes; of the two ...

s and

zebra Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped coats. There are three living species: Grévy's zebra (''Equus grevyi''), the plains zebra (''E. quagga''), and the mountain zebra (''E. ...

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'', there are many horse-specific

colloquial Colloquialism (also called ''colloquial language'', ''colloquial speech'', ''everyday language'', or ''general parlance'') is the linguistic style used for casual and informal communication. It is the most common form of speech in conversation amo ...

terms used by

equestrians Equestrianism (from Latin , , , 'horseman', 'horse'), commonly known as horse riding ( Commonwealth English) or horseback riding (American English), includes the disciplines of riding, driving, and vaulting. This broad description includes the ...

External anatomy

Back The human back, also called the dorsum (: dorsa), is the large posterior area of the human body, rising from the top of the buttocks to the back of the neck. It is the surface of the body opposite from the chest and the abdomen. The vertebral c ...

: the area where the saddle sits, beginning at the end of the

withers Withers are the ridge between the shoulder blades of an animal, typically a quadruped. In many species, this ridge is the tallest point of the body. In horses and dogs, it is the standard place to measure the animal's height. In contrast, catt ...

, extending to the last

thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae of intermediate size between the ce ...

(colloquially includes the loin or "coupling", though technically incorrect usage) * Barrel: the body of the horse, enclosing the

rib cage The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great ve ...

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 The third metacarpal bone (metacarpal bone of the middle finger) is a little smaller than the second. The dorsal aspect of its base presents on its radial side a pyramidal eminence, the Third metacarpal styloid process, styloid process, which ext ...

Chestnut The chestnuts are the deciduous trees and shrubs in the genus ''Castanea'', in the beech family Fagaceae. The name also refers to the edible nuts they produce. They are native to temperate regions of the Northern Hemisphere. Description ...

: a

callosity A callosity is a type of callus, a piece of skin that has become thickened as a result of repeated contact and friction. Primates All Old World monkeys, gibbons, and some chimpanzees have pads on their rears known as ''ischium, ischial callos ...

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 In British heraldry, a coronet is a type of crown that is a mark of rank of non-reigning members of the royal family and peers. In other languages, this distinction is not made, and usually the same word for ''crown'' is used irrespective of ra ...

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 Croup ( ), also known as croupy cough, is a type of respiratory infection that is usually caused by a virus. The infection leads to swelling inside the trachea, which interferes with normal breathing and produces the classic symptoms of "bar ...

: 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 The coccyx (: coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all apes, and analogous structures in certain other mammals such as horses. In tailless primates (e.g. humans and other ...

begin); sometimes called "rump" *

Dock The word dock () in American English refers to one or a group of human-made structures that are involved in the handling of boats or ships (usually on or near a shore). In British English, the term is not used the same way as in American Engl ...

: the living part of the tail, consisting of the coccygeal vertebrae,

muscles Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

and

ligament A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...

s. 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 Ergot ( ) or ergot fungi refers to a group of fungi of the genus ''Claviceps''. The most prominent member of this group is '' Claviceps purpurea'' ("rye ergot fungus"). This fungus grows on rye and related plants, and produces alkaloids that c ...

: 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 The metacarpophalangeal joints (MCP) are situated between the metacarpal bones and the proximal phalanges of the fingers. These joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow ...

(front) or

metatarsophalangeal The metatarsophalangeal joints (MTP joints) are the joints between the metatarsal bones of the foot and the proximal bones (proximal phalanges) of the toes. They are analogous to the knuckles of the hand, and are consequently known as toe knuck ...

(hind) joint; homologous to the "

ball A ball is a round object (usually spherical, but sometimes ovoid) with several uses. It is used in ball games, where the play of the game follows the state of the ball as it is hit, kicked or thrown by players. Balls can also be used for s ...

" 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 In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

and

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

, 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 forelock or foretop is a part of a horse's mane, that grows from the animal's poll and falls forward between the ears and onto the forehead. Some breeds, particularly pony breeds, have a naturally thick forelock, while other breeds, such as ...

: the continuation of the mane, which hangs from between the ears down onto the forehead of the horse *

Frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

: 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 Girth may refer to: Mathematics * Girth (functional analysis), the length of the shortest centrally symmetric simple closed curve on the unit sphere of a Banach space * Girth (geometry), the perimeter of a parallel projection of a shape * 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 hoof (: hooves) is the tip of a toe of an ungulate mammal, which is covered and strengthened with a thick and horny keratin covering. Artiodactyls are even-toed ungulates, species whose feet have an even number of digits; the ruminants with ...

: 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 Carotid artery may refer to: * Common carotid artery, often "carotids" or "carotid", an artery on each side of the neck which divides into the external carotid artery and internal carotid artery * External carotid artery, an artery on each side of ...

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 The lumbar vertebrae are located between the thoracic vertebrae and pelvis. They form the lower part of the back in humans, and the tail end of the back in quadrupeds. In humans, there are five lumbar vertebrae. The term is used to describe t ...

* 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 pastern is a part of the leg of a horse between the fetlock and the top of the hoof. It incorporates the Equine_forelimb_anatomy#Metacarpal_bones, long pastern bone (proximal phalanx) and the Equine_forelimb_anatomy#Middle_phalanx, short past ...

: the connection between the coronet and the fetlock, made up of the middle and proximal phalanx *

Poll Poll, polled, or polling may refer to: Forms of voting and counting * Poll, a formal election ** Election verification exit poll, a survey taken to verify election counts ** Polling, voting to make decisions or determine opinions ** Polling pla ...

: 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 The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

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 evaluates 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 faults with univers ...

* Splints: bones found on each of the legs, on either side of the cannon bone (8 total); partially

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

, these bones support the corresponding carpal bones in the forelimb, and the corresponding tarsal bones in the hindlimb; anatomically referred to as

metacarpal In human anatomy, the metacarpal bones or metacarpus, also known as the "palm bones", are the appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones ( wrist bones), which articulate ...

metatarsal The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...

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 tail is the elongated section at the rear end of a bilaterian animal's body; in general, the term refers to a distinct, flexible appendage extending backwards from the midline of the torso. In vertebrate animals that evolution, evolved to los ...

: the long hairs which grow from the dock; may also include the dock The long hairs alone are sometimes called the ''skirt''. * 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 A bridle is a piece of equipment used to direct a horse. As defined in the ''Oxford English Dictionary'', the "bridle" includes both the that holds a bit that goes in the mouth of a horse, and the reins that are attached to the bit. It prov ...

goes. *

Withers Withers are the ridge between the shoulder blades of an animal, typically a quadruped. In many species, this ridge is the tallest point of the body. In horses and dogs, it is the standard place to measure the animal's height. In contrast, catt ...

: 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 The hoof (: hooves) is the tip of a toe of an ungulate mammal, which is covered and strengthened with a thick and horny keratin covering. Artiodactyls are even-toed ungulates, species whose feet have an even number of digits; the ruminants with ...

of the horse encases part of the second and all of the third

phalanx The phalanx (: phalanxes or phalanges) was a rectangular mass military formation, usually composed entirely of heavy infantry armed with spears, pikes, sarissas, or similar polearms tightly packed together. The term is particularly used t ...

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 () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

, a very strong

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

molecule. The horse's hoof contains a high proportion of

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

-containing

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

which contribute to its resilience and toughness. Vascular fold-like structures called laminae suspend the distal phalanx from the hoof wall.

Digestive system

Topography of Viscera of Horse Left Deep View

Horses and other equids evolved as

grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to free range (roam around) and consume wild vegetations in order to feed conversion ratio, convert the otherwise indigestible (by human diges ...

animals, adapted to eating small amounts of the same kind of food all day long. In the wild, the horse adapted to eating

prairie Prairies are ecosystems considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and a composition of grasses, herbs, and shrubs, rather than trees, as the ...

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 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 Forage is a plant material (mainly plant leaves and stems) 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 m ...

and pick up finer foods, such as grain, with their sensitive,

prehensile Prehensility is the quality of an appendage or organ that has adapted for grasping or holding. The word is derived from the Latin term ''prehendere'', meaning "to grasp". The ability to grasp is likely derived from a number of different origin ...

lip The lips are a horizontal pair of soft appendages attached to the jaws and are the most visible part of the mouth of many animals, including humans. Mammal lips are soft, movable and serve to facilitate the ingestion of food (e.g. sucklin ...

s. The front

teeth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...

of the horse, called

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

s, clip

, and food is then pushed back in the mouth by the

tongue The tongue is a Muscle, muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for chewing and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper s ...

, and ground up for swallowing by the

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

s and

molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...

Teeth

A horse's teeth include

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. 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 The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...

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 Vomiting (also known as emesis, puking and throwing up) is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose. Vomiting can be the result of ailments like food poisoning, gastroenteritis, pregna ...

. The esophagus is also the area of the digestive tract where horses may suffer from choke.

Stomach

Horses have a relatively small

stomach The stomach is a muscular, hollow organ in the upper gastrointestinal tract of Human, humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical t ...

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 The pylorus ( or ) connects the stomach to the duodenum. The pylorus is considered as having two parts, the ''pyloric antrum'' (opening to the body of the stomach) and the ''pyloric canal'' (opening to the duodenum). The ''pyloric canal'' ends a ...

Stomach acid Gastric acid or stomach acid is the acidic component – hydrochloric acid – of gastric juice, produced by parietal cells in the gastric glands of the stomach lining. In humans, the pH is between one and three, much lower than most other an ...

and the enzymes

pepsin Pepsin is an endopeptidase that breaks down proteins into smaller peptides and amino acids. It is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food. Pe ...

(which begins the breakdown of proteins into

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

chains), and

lipase In biochemistry, lipase ( ) refers to a class of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; howe ...

(which begins the breakdown of fats) are produced by the glandular mucosa in the fundus as well as mucus.

Gastrin Gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the pyloric antrum of the stomach, duodenum, and the pancreas. ...

is produced in the pylorus.

Small intestine

The horse's

small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...

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 The duodenum is the first section of the small intestine in most vertebrates, including mammals, reptiles, and birds. In mammals, it may be the principal site for iron absorption. The duodenum precedes the jejunum and ileum and is the shortest p ...

jejunum The jejunum is the second part of the small intestine in humans and most higher vertebrates, including mammals, reptiles, and birds. Its lining is specialized for the absorption by enterocytes of small nutrient molecules which have been pr ...

and

ileum The ileum () is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may ...

. The majority of digestion occurs in the duodenum while the majority of absorption occurs in the jejunum.

Bile Bile (from Latin ''bilis''), also known as gall, is a yellow-green/misty green fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is primarily composed of water, is pro ...

from the liver aids in digesting fats in the duodenum combined with

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s from the

pancreas The pancreas (plural pancreases, or pancreata) is an Organ (anatomy), organ of the Digestion, digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a ...

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 The hindgut (or epigaster) is the posterior ( caudal) part of the alimentary canal. In mammals, it includes the distal one third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and up to the ano-rectal junct ...

, consists of the

cecum The cecum ( caecum, ; plural ceca or caeca, ) is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix (a ...

, large colon, small colon and

rectum The rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...

, terminating in the

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

. 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 Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

and other indigestible plant

fiber Fiber (spelled fibre in British English; from ) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often inco ...

through

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

into volatile fatty acids. These microbes feed upon the portion of

chyme Chyme or chymus (; ) is the semi-fluid mass of partly digested food that is expelled by the stomach, through the pyloric valve, into the duodenum (the beginning of the small intestine). Chyme results from the mechanical and chemical breakdown ...

not absorbed by the small intestine, and produce

vitamin K Vitamin K is a family of structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-translational modification, post-synthesis modification of certain proteins ...

and B complex vitamins. The large colon is long and holds up to of semi-liquid matter. It is made up of the right

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

(lower) colon, the left ventral colon, the left

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...

(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; and the diaphragmatic flexure, between left dorsal and right dorsal colon. Besides the

transverse colon In human anatomy, the transverse colon is the longest and most movable part of the Large intestine#Structure, colon. Anatomical position It crosses the abdomen from the ascending colon at the right colic flexure (hepatic flexure) with a downward ...

, these sections are all analogous to the

ascending colon In the anatomy of humans and homologous primates, the ascending colon is the part of the colon located between the cecum and the transverse colon. Characteristics and structure The ascending colon is smaller in calibre than the cecum from wh ...

in humans. The large colon continues the fermentation process, and absorbs volatile fatty acids as an energy source. Due to its many twists and turns, the large colon is a common place for certain forms of colic including impaction, displacement and volvulus. The small colon is in length and holds only 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. This section is analogous to the descending colon in humans. The rectum is about long, and acts as a holding chamber for waste matter, which is then expelled from the body via the anus.

Reproductive system

A stallion's reproductive system is responsible for his

sexual behavior Human sexual activity, human sexual practice or human sexual behaviour is the manner in which humans experience and express their sexuality. People engage in a variety of sexual acts, ranging from activities done alone (e.g., masturbation) t ...

and secondary sex characteristics (such as a large crest). The external genitalia include the

urethra The urethra (: urethras or urethrae) is the tube that connects the urinary bladder to the urinary meatus, through which Placentalia, placental mammals Urination, urinate and Ejaculation, ejaculate. The external urethral sphincter is a striated ...

; the

testes A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone. The ...

, which average long; the

penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate. The term ''pen ...

, which, when housed within the prepuce, is long and in diameter with the distal end and when erect, increases by 3 to . The internal genitalia accessory sex glands are the vesicular glands,

prostate gland The prostate is an male accessory gland, accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found in all male mammals. It differs between species anatomically, chemica ...

, and bulbourethral glands, which contribute fluid to the

semen Semen, also known as seminal fluid, is a bodily fluid that contains spermatozoon, spermatozoa which is secreted by the male gonads (sexual glands) and other sexual organs of male or hermaphrodite, hermaphroditic animals. In humans and placen ...

ejaculation Ejaculation is the discharge of semen (the ''ejaculate''; normally containing sperm) from the penis through the urethra. It is the final stage and natural objective of male sexual stimulation, and an essential component of natural conception. ...

, but are not strictly necessary for fertility. A

mare A mare is an adult female horse or other equidae, equine. In most cases, a mare is a female horse over the age of three, and a filly is a female horse three and younger. In Thoroughbred horse racing, a mare is defined as a female horse more th ...

's reproductive system is responsible for controlling gestation, birth, and lactation, as well as her estrous cycle and mating behavior. 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. A mare has two ovaries, usually in length and thick, that generally tend to decrease in size as the mare ages. In equine ovaries, unlike in humans, the vascular tissue is cortical to follicular tissue, so ovulation can only occur at an ovulation fossa near the infundibulum. The ovaries connect to the fallopian tubes (

oviduct The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will dege ...

s), which serve to move the ovum from the ovary to the uterus. To do so, the oviducts are lined with a layer of

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

, which produce a current that flows toward the uterus. Each oviduct attaches to one of the two horns of the uterus, which are approximately in length. These horns attach to the body of the uterus ( long). The equine uterus is bipartite, meaning the two

uterine horns The uterine horns (cornua of uterus) are the points in the upper uterus where the fallopian tubes or oviducts exit to meet the ovaries. They are one of the points of attachment for the round ligament of uterus (the other being the mons pubis). T ...

fuse into a relatively large uterine body (resembling a shortened bicornuate uterus or a stretched

simplex In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension. ...

uterus). Caudal to the uterus is the cervix, about long, which separates the uterus from the vagina. Usually in diameter with longitudinal folds on the interior surface, it can expand to allow the passage of the foal. The vagina of the mare is long, and is quite elastic, allowing it to expand. The

vulva In mammals, the vulva (: vulvas or vulvae) comprises mostly external, visible structures of the female sex organ, genitalia leading into the interior of the female reproductive tract. For humans, it includes the mons pubis, labia majora, lab ...

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 A mammary gland is an exocrine gland that produces milk in humans and other mammals. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primates (for example, human ...

, which are smaller in maiden mares. They have two ducts each, which open externally.

Skeletal system

The

skeleton A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fra ...

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 The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

, and

ribs The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great vessels ...

). 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 cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...

formation. * The Appendicular system includes the

limbs of the horse The limbs of the horse are structures made of dozens of bones, joints, muscles, tendons, and ligaments that support the weight of the equine body. They include three apparatuses: the suspensory apparatus, which carries much of the weight, preve ...

; * The Axial system is composed of the spine, ribs, sternum and skull; The bones of the horse are the same as those of other domestic species, but the third metacarpal and metatarsal are much more developed and the second and fourth are underdeveloped, lacking associated digts. The first and fifth metacarpals and metatarsals as well as their associated digits are absent.Riegal, Ronald J. DVM, and Susan E. Hakola RN. Illustrated Atlas of Clinical Equine Anatomy and Common Disorders of the Horse Vol. II. Equistar Publication, Limited. Marysville, OH. Copyright 2000.

Ligaments and tendons

Ligament A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...

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.

Tendon A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...

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, 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 () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

, 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 A tendon sheath is a layer of synovial membrane around a tendon. It permits the tendon to stretch and not adhere to the surrounding fascia. It contains a lubricating fluid (synovial fluid) that allows for smooth motions of the tendon during muscl ...

or a sac called a

bursa Bursa () is a city in northwestern Turkey and the administrative center of Bursa Province. The fourth-most populous city in Turkey and second-most populous in the Marmara Region, Bursa is one of the industrial centers of the country. Most of ...

. Tendons are easily damaged if placed under too much strain, which can result in a painful, and possibly career-ending, injury.

Tendinitis Tendinopathy is a type of tendon disorder that results in pain, swelling, and impaired function. The pain is typically worse with movement. It most commonly occurs around the shoulder ( rotator cuff tendinitis, biceps tendinitis), elbow ( tenn ...

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 A myofibril (also known as a muscle fibril or sarcostyle) is a basic rod-like organelle of a muscle cell. Skeletal muscles are composed of long, tubular cells known as Skeletal muscle#Skeletal muscle cells, muscle fibers, and these cells contain ...

s, which are able to contract due to

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

and

myosin Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...

. 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 The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

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, also called obligate nasal breathing. For this same reason, horses also cannot pant as a method of

thermoregulation Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...

. The genus '' Equus'' also has a unique part of the respiratory system called the guttural pouch, which is thought to equalize air pressure on the

tympanic membrane In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pressur ...

. Located between the

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

s but below the

occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lobes of the ...

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

Circulatory system

The horse's circulatory system includes the four-chambered heart, averaging 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 may be lost when a horse is shod (eliminating the expansion and contraction of the hoof wall and raising the frog higher from the ground).

Vision

The horse has one of the largest eyes of all land mammals. Eye size in mammals is significantly correlated to maximum running speed as well as to body size, in accordance with Leuckart's law; animals capable of fast locomotion require large eyes. The eye of the horse is set to the side of its skull, consistent with that of a prey animal. The horse has a wide field of monocular vision, 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.Myers ''Horse Safe'' p.7 Often, the eye of the horse is looking in the same direction as the ear is directed.

References

External links

Veterinary Anatomical lllustrations, University of Wisconsin Digital Collections
{{Horse topics *