Equine anatomy refers to the
grossGross 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 must be studied with the aid of a microscope.-Techniques of study:...
and microscopic
anatomyAnatomy 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
equidsEquidae 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...
. 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 VeterinariaThe 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
horseThe horse is a hoofed mammal, a subspecies of one of seven extant species of the 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
colloquialA colloquialism is an expression not used in formal speech, writing or paralinguistics. Colloquialisms are also sometimes referred to collectively as "colloquial language". Colloquialisms or colloquial language is considered to be characteristic of or only appropriate for casual, ordinary,...
terms used by equestrians.
External anatomy
- back
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 goes, begins at the end of the withersThe 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...
, extends to the last thoracic vertebrae. (Colloquially includes the loin or "coupling," though technically incorrect usage)
- barrel: the main body area of the horse, enclosing the rib cage and the major internal organs.
- cannon 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: 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
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
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 horse's 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 point where the tail connects to the rear of the horse.
- elbow: The joint of the front leg at the point where the belly of the horse meets the leg. Homologous
In evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology...
to the elbow in humans.
- ergot
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
- fetlock
Fetlock is the common name for the metacarpophalangeal and metatarsophalangeal joints of the horse. It is formed by the junction of the third metacarpal or metatarsal bones proximad and the proximal phalanx distad...
: Sometimes called the "ankleIn human anatomy, 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 of the horse meet, specifically the area right behind the rib cage and in front of the stifle joint.
- forearm: the area of the front leg between the knee and elbow. Consists of the fused 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....
and ulnaThe ulna is a long bone, prismatic in form. In anatomical position the ulna is placed at the medial side of the forearm closest to the body, parallel with the radius on both arms.-Articulations:The ulna articulates with:...
, and all the tissue around these bones. Anatomically the antebrachium.
- forelock: the continuation of the mane, which hangs from between the ears down onto the forehead of the horse.
- frog
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 hoofA 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, 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
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 of the horse.
- hock
The hock, or gambrel, is the tarsal joint of a digitigrade quadruped, such as a horse, cat, or dog. It is the anatomical homologue of the ankle of the human foot.-Horse:...
: The tarsusIn tetrapods, the tarsus are the cluster of bones in the foot between the tibia and fibula and the metatarsus. The bones of the tarsus do not belong to individual toes, whereas those of the metatarsus do...
of the horse (hindlimb equivalent to the human ankle and heel), the large joint on the hind leg.
- 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. The hoof wall is, in many respects, a much larger and stronger version of the human fingernail.
- jugular groove: the line of indentation on the lower portion of the neck, can be seen from either side, just above the windpipe. Beneath this area run the 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 and part of the sympathetic trunkThe sympathetic trunks are a paired bundle of nerve fibers that run from the base of the skull to the coccyx.-Structure:...
.
- knee: the carpus
In tetrapods, the carpals is the sole cluster of the 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 of the horse to the croup. Anatomically approximate to the lumbar spine.
- mane
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, lying on either the left or right side of the neck).
- muzzle: the chin, mouth, and nostrils of the horse's face.
- pastern: The connection between the coronet and the fetlock. Made up of the middle and proximal phalanx.
- poll
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 horse's neck, immediately behind the ears, a slight depression at the joint where the atlas (C1) meets the occipital crest-Clinical significance:Trauma to the occiput can cause a basilar skull fracture.Prominent occiput is a feature of Trisomy 18 , along with small jaw, low-set ears, and mental retardation. It is also a feature of Trisomy 9....
. Anatomically, the occipital crest itself is the "poll."
- splints: bones found on each of the legs, on either side of the cannon bone (8 total). 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)).
- shoulder: made up of the 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 affect on the horse's movement and jumping ability, and is an important aspect of equine conformationEquine 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...
.
- stifle: Corresponds to the knee of a human, consists of the articulation between femur and tibia, as well as the articulation between patella and femur.
- tail: consists of both the living part of the tail (which consists of the coccygeal vertebrae, muscules, and ligaments), as well as the long hairs which grow from the living part
- throatlatch: The point at which the windpipe meets the head at the underside of the jaw.
- withers
The 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 in "hands."
Digestive system
Horses and other Equids evolved as
grazingGrazing generally describes a type of predation in which an 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
prairiePrairies 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 long, and most of this is intestines.
The mouth
Digestion begins in the
mouthThe mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva...
, 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
forageForage 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
grainGRAIN is an international non-governmental organization based in Barcelona, Spain, which works toward sustainable agriculture. It was formed upon the realization that the genetic diversity of the world's food crops are being drastically eliminated...
, with their sensitive, prehensile
lipLips 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, as an erogenous organ used in kissing and other acts of intimacy, as a tactile sensory organ, and in the articulation of speech.-Anatomical basics of the human...
s. The front
teethHorses' 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
incisorIncisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s, clip
forageForage 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
tongueThe tongue is a muscle on the floor of the mouth that manipulates food for chewing and swallowing . It is the primary organ of taste, as much of the upper surface of the tongue is covered in papillae and taste buds. A secondary function of the tongue is speech, in which the organ assists...
, and ground up for swallowing by the
premolarThe 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
esophagusThe esophagus or oesophagus , sometimes known as the gullet, is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. The word esophagus is derived from the Latin œsophagus, which derives from the Greek word oisophagos , lit...
is about 4-5 feet in length, and carries food to the stomach. A muscular ring, called the cardiac sphincter, connects the stomach to the esophagus. This sphincter is very well developed in horses. This and the oblique angle at which the esophagus connects to the stomach explains why horses cannot vomit. The esophagus is also the area of the digestive tract where horses may suffer from
chokeChoke 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
stomachIn most mammals, the stomach is a hollow, muscular organ of the gastrointestinal tract , between the esophagus and the small intestine. It is involved in the second phase of digestion, following mastication . The word stomach is derived from the Latin stomachus, which derives from the Greek word...
for their size, and this limits the amount of feed a horse can take in at one time. The average sized horse (800 to 1200 lb) has a stomach with a capacity of only four gallons, and works best when it contains about two gallons. Because the stomach empties when 2/3 full, whether stomach enzymes have completed their processing of the food or not, and doing so prevents full digestion and proper utilization of feed, continuous foraging or several small feedings per day are preferable to one or two large ones. The horse stomach consists of a non-glandular proximal region (saccus cecus), divided by a distinct border, the margo plicata, from the glandular distal stomach
[ Andrews FM, Buchanan BR, Elliot SB, Clariday NA and Edwards LH (2005) "Gastric ulcers in horses" J. Anim. Sci. 83:E18-E21].
In the stomach, assorted
acidsGastric juice is a strong acidic liquid, pH 1 to 3 in humans, which is close to being colourless. The hormone gastrin is released into the bloodstream when peptides are detected in the stomach. This causes gastric glands in the lining of the stomach to secrete gastric juice...
and the enzyme
pepsinPepsin is an enzyme that is released by the chief cells in the stomach and that degrades food proteins into peptides. Pepsin was discovered in 1836 by Theodor Schwann who also coined this enzyme's name from the Greek word pepsis, meaning digestion...
break down food. Pepsin allows for the further breakdown of proteins into
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
chains. Other enzymes include resin and
lipaseA lipase is a soluble enzyme that catalyzes the hydrolysis of ester bonds in water–insoluble, lipid substrates. Lipases thus comprise a subclass of the esterases....
. Additionally, the stomach absorbs some water, as well as ions and lipid soluble compounds. The end product is food broken down into
chymeChyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum. In other words, chyme is half-digested food.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 intestineIn vertebrates, the small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where the vast majority of digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large...
is 50 to long and holds 10 to 12 gallons. 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.
BileBile or gall is a bitter yellowish, blue and green fluid secreted by hepatocytes from the liver of most vertebrates. In many species, bile is stored in the gallbladder between meals and upon eating is discharged into the duodenum where the bile aids the process of digestion of lipids by...
from the liver aids in digesting fats in the duodenum combined with
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
s from the
pancreasThe 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 an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to...
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, and E. Any remaining liquids and roughage move into the large intestine.
Cecum
The
cecumThe 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...
is the first section of the
large intestineThe large intestine is the second to last part of the digestive system—the final stage of the alimentary canal is the anus —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 long that holds 7 to 8 gallons. It contains
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that digest
celluloseCellulose 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
fiberFiber, also spelled fibre, 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. Human uses for fibers are diverse...
through
fermentationFermentation is the process of deriving energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. This is in contrast to cellular respiration, where electrons are donated to an exogenous electron acceptor,...
. 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
colicColic 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
colonThe colon is the last portion of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body....
, small colon, and
rectumThe 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 intestineThe large intestine is the second to last part of the digestive system—the final stage of the alimentary canal is the anus —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 10-12 feet long and holds up to 20 gallons of semi-liquid matter. It is made up of the right lower (ventral) colon, the left lower (ventral) colon, the left upper (dorsal) colon, the right upper (dorsal) colon, and the transverse colon, in that order. Three flexures are also named; the sternal flexure, between right and left ventral colon; the pelvic flexure, between left dorsal and left ventral colon; the diaphragmatic flexure, between left dorsal and right dorsal colon. The main purpose of the large colon is to absorb carbohydrates, which were broken down from cellulose in the cecum. Due to its many twists and turns, it is a common place for a type of
horse colicColic 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 10-12 feet in length and holds only 5 gallons of material. It is the area where the majority of water in the horse's diet is absorbed, and is the place where fecal balls are formed. The
rectumThe 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 one foot long, and acts as a holding chamber for waste matter, which is then expelled from the body via the
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel 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, such as bones; food...
.
Mare
The reproductive system of the mare is responsible for controlling gestation, birth, lactation, as well as the estrous cycle and mating behavior of the mare. It lies ventral to the 4th or 5th lumbar vertebrae, although its position within the mare can vary depending on the movement of the intestines and distention of the bladder.
The mare has two ovaries, usually 7-8 cm in length and 3-4 cm thick, that generally tend to decrease in size as the mare ages. The ovaries connect to the fallopian tubes (oviducts), which serve to move the ovum from the ovary to the uterus. To do so, the oviducts are lined with a layer of cilia, which produce a current that flows toward the uterus. Each oviduct attaches to one of the two horns of the uterus, which are approximately 20-25 cm in length. These horns attach to the body of the uterus (18-20 cm long). Caudal to the uterus is the cervix, about 5-7 cm long, which enters the vagina. Usually 3.5-4 cm in diameter, it can expand to allow the passage of the foal. The vagina of the mare is 15-20 cm long, and is quite elastic, allowing it to expand. The vulva is the external opening of the vagina, and consists of the clitoris and two labia. It lies ventral to the rectum. The mare has two mammary glands, which are smaller in virgin mares. They have two ducts each, which open externally.
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
In some male mammals the scrotum is a protuberance of skin and muscle containing the testicles. It is an extension of the abdomen, and is located between the penis and anus. In humans and some other mammals, the base of the scrotum becomes covered with curly pubic hairs at puberty...
. The testes of an average stallion are ovoids 8 to 12 cm long;
- the penis
The penis is an external sexual organ of certain biologically male organisms, in both vertebrates and invertebrates....
, within the prepuceThe prepuce is a retractable piece of skin which covers part of the genitals of primates and other mammals.* On a male, it surrounds and protects the head of the penis . See foreskin....
, also known as the "sheath." When not erect, the penis is housed within the prepuce, 50 cm long and 2.5 to 6 cm in diameter with the distal end 15 to 20 cm. When erect, the penis doubles in length and thickness and the glans increases by 3 to 4 times. The urethraIn anatomy, the urethra is a tube which connects the urinary bladder to the outside of the body. The urethra has an excretory function in both sexes to pass urine to the outside, and also a reproductive function in the male, as a passage for semen.The external urethral sphincter is a striated...
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
The vesicular gland is part of the male reproductive system. The vesicular gland contributes fluids, energy substrates, and buffers to semen.The "vesicular gland" can be found near the "Coagulating gland", which also belongs to the reproductive system....
;
- prostate gland; and
- bulbourethral glands.
These contribute fluid to the
semenSemen is an organic fluid, also known as seminal fluid, that usually contains spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...
at
ejaculationEjaculation is the ejecting of semen from a penis, and is usually accompanied by orgasm. It is usually the final stage and natural objective of sexual stimulation, and an essential component of natural conception. In rare cases ejaculation occurs because of prostatic disease. Ejaculation may also...
, but are not strictly necessary for fertility
Teeth
A horse's teeth include
incisorIncisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s,
premolarThe 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,
molarMolars 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
hoofA 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
keratinKeratins are a family of fibrous structural proteins; tough and insoluble, they form the hard but un-mineralized structures found in reptiles, birds, amphibians, and mammals...
, a very strong
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
molecule. The horse's hoof contains a high proportion of
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
-containing amino acids which contribute to its resilience and toughness.
Skeletal system
The
skeletonIn biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa. Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the...
of the horse has three major functions in the body. It protects vital organs, provides framework, and supports soft parts of the body. Horses have 205 bones, which are divided into the appendicular skeleton (the legs) and the axial skeleton (the
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
,
vertebral columnIn human anatomy, the vertebral column is a column usually consisting of 33 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsal aspect of the torso, separated by spinal discs...
,
sternumThe sternum is a long flat bone shaped like a capital 'T' located in the center of the thorax...
, and ribs). Both pelvic and thoracic limbs contain the same number of bones, 20 bones per limb. Bones are connected to muscles via tendons and other bones via ligaments. Bones are also used to store minerals, and are the site of
red blood cellRed blood cells are the most common type of blood cell and the vertebrate body's principal means of delivering oxygen to the body tissues via the blood. They take up oxygen in the lungs or gills and release it while squeezing through the body's capillaries. The cells are filled with hemoglobin, a...
formation.
Ligaments
LigamentIn anatomy, the term ligament is used to denote three different types of structures:# Fibrous tissue that connects bones to other bones. They are sometimes called "articular ligaments", "fibrous ligaments", or "true ligaments"....
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
TendonA 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 fascia as they are all made of collagen except that ligaments join one bone to another bone, and fascia 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
fetlockFetlock is the common name for the metacarpophalangeal and metatarsophalangeal joints of the horse. 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
collagenCollagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. It is naturally found exclusively in metazoa, including sponges. In muscle tissue it serves as a major component of endomysium...
, 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 sheathA 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
bursaA 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 any 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
myofibrilMyofibrils are cylindrical organelles. They are found within muscle cells. They are bundles of actomyosin filaments that run from one end of the cell to the other and are attached to the cell surface membrane at each end.Actomyosin motors are important in muscle contraction as well as other...
s, which are able to contract due to
actinActin is a globular, roughly 42-kDa 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
myosinMyosins are a large family of motor proteins found in eukaryotic tissues. They are responsible for actin-based motility.Following the discovery by Pollard and Korn of enzymes with myosin-like function in Acanthamoeba castellanii, a large number of divergent myosin genes have been discovered...
. A muscle together with its tendon and bony attachments form an extensor or flexor unit.
Respiratory system and smell
The horse's respiratory system consists of the nostrils, pharynx, larynx, trachea, diaphragm, and lungs. Additionally, the nasolacrimal duct and sinuses are connected to the nasal passage. The horse's respiratory system not only allows the animal to breathe, but also is important in the horse's sense of smell (olfactory ability) as well as in communicating.
Circulatory system
The horse's circulatory system includes the four-chambered heart, averaging in weight, as well as the blood and blood vessels. Its main purpose is to circulate blood throughout the body to deliver oxygen and nutrients to tissues, and to remove waste from these tissues.
The eye
The horse has the largest eye of all land mammals, and is designed to help the horse as a prey animal. It provides the horse with a wide field of
monocular visionMonocular 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 and some ability to see color. Because the horse's vision is closely tied to his behavior, the horse's visual abilities are often taken into account when handling and training the animal.
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.