An Anatomy Lesson


From large, discerning eyes capable of a near-complete field of vision, to the tiny bones suspended at the tip of his toe, the horse is an elaborately designed animal as well as a picture of beauty. As your horse’s caretaker and companion, it helps to know the intricacies of his anatomy and how he has adapted over the years to become the capable domesticated athlete he is today. 

In this article we’ll guide you through the equine body, from head to foot.

Head and Neck

The horse’s eyes are the largest of any land mammal. Protruding out of a bony orbit on each side of the head, they have adapted to provide him with a panoramic 330-degree field of vision. His ears are similarly adapted and can swivel to locate the sources of different sounds.

Sensitive prehensile lips, adept at selecting and grasping food, overlay his 12 incisor teeth (three in each of four quadrants) that sever grass and his 40 hypsodont teeth that grind forage (42 if the first premolars or wolf teeth are present). Each hypsodont tooth’s enamel-covered crown extends far below the gumline and erupts continuously as the grinding surface wears, on average, 2-3 mm annually.

Behind the teeth lies the pharynx (throat). This is where the passageways for respiration (nasopharynx), eating (oropharynx), and vocals (laryngopharynx) meet. A fleshy partition between the nasopharynx and oropharynx, called the soft palate, forms the back of the roof of the mouth. Food entering the oropharynx stimulates the swallowing reflex, which inhibits breathing and closes alternate passages: The root of the tongue moves backward and the soft palate and epiglottis (flap of cartilage attached to the entrance of the larynx) elevate, sealing off the nasal cavity entrance and closing the larynx, respectively. As the pharyngeal muscles contract, the chewed mass of food (the bolus) enters the esophagus, where peristalsis—passage made possible by waves of contraction--moves it to the stomach. 

Air, of course, takes a different path, entering through the nostrils and passing on through the nasal passageways and into the nasopharynx. As the horse breathes normally, the soft palate extends just underneath the base of the epiglottis, which is one of the reasons horses are obligate nasal breathers (unable to breathe through the mouth) and also a reason horses are unable to vomit (other reasons pertain to stomach anatomy). Air then passes through the open larynx (flanked by the laryngeal cartilages) into the trachea and lungs.

Beyond the horse’s nasopharynx, fish-gill-like flaps cover the guttural pouch openings. The guttural pouches, a pair of air sacs of the auditory tubes that can hold 300-500 mL of air, is unique to the horse and a few other species, such as the desert hyrax, and is a vestigial structure (having lost much of its evolutionary function) thought to help cool blood flowing to the brain during periods of heat or stress. A horse’s guttural pouches are closely associated with vital structures that pass through the back of the skull, including the internal and external carotid arteries and five cranial nerves, and they lie directly over the temporohyoid joint located between the larynx and the base of the ear.


The horse’s 18 pairs of ribs (17 in Arabians), spinal column, and sternum form his thorax, or chest cavity. This structure contains the heart, major blood vessels, caudal end of the esophagus, and lungs. The horse’s cone-shaped heart weighs, on average, 10 pounds. It makes direct contact with the ribs through the lungs’ cardiac notches from the third to fifth intercostal (between ribs) spaces on the left and the third to fourth space on the right, which are where you would listen for the heart with a stethoscope. The horse’s lungs are elongated and shallow, with the larger right lung being comprised of three lobes compared to the left lung’s two. 


Like many other herbivores, the horse has a large gastrointestinal (GI) tract folded many times within the cavernous abdomen. Food passes through the esophagus into the horse’s stomach via the cardia, the exceptionally muscular sphincter at the stomach entrance, which together with the earlier-described esophageal design is largely responsible for the horse’s inability to vomit. The stomach has a small capacity of five to 15 liters and makes up only 9% of the equine GI tract, as compared to 17% of the human GI tract. A folded edge of the stomach’s mucous membrane lining, called the margo plicatus, divides the large nonglandular region from the glandular region, and is a feature unique to the horse. Ulcers tend to develop along the margo plicatus on the nonglandular side, says Alfredo Romero, DVM, Dipl. ACVS, co-owner and surgeon at Syracuse Equine Veterinary Specialists PLLC, in New York. 

Most colic issues arise from the large intestine and involve simple gas colic or impactions.

Dr. Karen Blake

Next, food passes through the pylorus, which is the sphincter at the exit of the stomach, and into the small intestine. The small intestine occupies most of the abdomen (capable of holding 30% of the GI tract’s capacity), and its ability to shift and move within this space can cause epiploic entrapments, mesenteric rent entrapments, and intussusceptions, which are all common colic causes. A horse digests food much like other single-stomached (monogastric) animals: His stomach and small intestine break down protein, fat, and soluble carbohydrates, while plant materials such as cellulose advance toward the hindgut for microbial fermentation (hence, why a horse is referred to as a hindgut fermenter). The horse’s more than 80 feet of small intestine empty into the cecum via the ileocecal orifice. 

The hindgut, or large intestine, consists of all of the digestive organs behind the small intestine. The comma-shaped cecum is about three feet long with a 35-liter capacity, occupying 16% of the equine digestive tract. It lies in the right upper part of the abdomen against the flank, and its work digesting cellulose from the horse’s high-fiber diet into usable volatile fatty acids and amino acids can be heard using a stethoscope. The horse and the microbes in his cecum have a symbiotic (mutually beneficial) relationship: The bacteria break down otherwise indigestible feed for the horse, and in return the horse provides them a safe environment in which to live.

From the cecum, ingesta passes into the colon, which consists of the ascending, transverse, and descending colon. Together, the ascending and transverse colon are referred to as the “large colon,” and the descending colon is referred to as the “small colon.” The large colon is responsible for reabsorbing water and fermenting ingesta, and it occupies 45% of the equine digestive tract as compared to 17% of the human GI tract. Ingesta travels down the colon in this sequence: right ventral colon, ventral diaphragmatic flexure, left ventral colon, pelvic flexure, left dorsal colon, dorsal diaphragmatic flexure, right dorsal colon, transverse colon, and finally into the descending colon and rectum. 

“While the GI tract in horses is long and undulating, (and one might expect twists and torsions to be our biggest issues), most colic issues arise from the large intestine and involve simple gas colic or impactions,” says Karen Blake, DVM, Dipl. ACVS, owner of Elite Veterinary Services, in Park City, Utah. The three main sections of this seven-meter-long stretch of large intestine where these digestive snafus occur are the junction of right dorsal colon and transverse colon where the lumen (inner cavity) narrows dramatically; the pelvic flexure where there is a similar narrowing of the lumen; and the cecum, which is subject to motility disorders. Further, similar to the plight of the small intestine, the ascending colon is largely free within the abdomen and is subject to displacements and twists, which can lead to severe colic.

Musculoskeletal System

Another intricate equine anatomical feature is the musculoskeletal system that allows horses to conserve energy at rest and expend it during recreation. 

The horse’s forelimbs carry 55-60% of his weight and act as shock absorbers during movement. From top to bottom, the forelimb bones are the scapula, humerus, radius/ulna (which make up the forearm), carpus (knee), metacarpal bones (cannon bone and splint bones), and the phalangeal bones of the foot. The flexible carpus, which is the equine equivalent of the human wrist, contains two rows of bones and three levels of articulation. The uppermost radiocarpal joint allows 90-100° of flexion and lies between the radius and carpal bones. The middle carpal joint allows 45° of flexion between the two rows of carpal bones, and the bottom carpometacarpal joint, between the second row of carpal bones and the cannon bone, provides no significant movement. 

Given the tremendous amount of compressive force placed on the foot, it is no wonder that 80-90% of lamenesses are linked to damage in the foot.

Dr. Alfredo Romero

The hind limbs are adapted to providing propulsive thrust for locomotion. From top to bottom the skeletal structures include the femur, patella, tibia/fibula, tarsus (hock), metatarsal bones (cannon bone and splint bones), and the phalangeal bones of the foot. The stifle joint, where the femur and tibia/fibula meet, allows the hind limb to lock and is a major component of the passive stay apparatus that helps horses sleep while standing. It is also the hind limb’s main weight-supporting joint. The stifle joint is actually comprised of three joints: the femorotibial, femoropatellar, and tibiofibular joints. 

The femur contains two large trochlear ridges over which the diamond-shaped knee cap (patella—yes, this part of the stifle is equivalent to the human knee) glides. When the horse is stationary, he can lock his stifle and rest while standing; the patella glides over the ridges and onto a flattened shelf on top of the medial femoral trochlear ridge. Some horses are periodically unable to release a patella from the resting position, leaving them to drag the unflexed limb. This temporary condition is called upward fixation of the patella. 

The hock joint contains three rows of bones and four joints and is the equine equivalent of the human ankle. The uppermost tarsocrural joint is responsible for most of the hock’s hinge-type motion. The trochlear ridges of the largest hock bone, known as the talus, angle outward so the hind limbs pass outside the forelimbs when the horse is galloping. 

The two principal sets of muscles and tendons along the equine limb are the craniolateral group (along the front and outside of the limb) and the caudal group (along the back of the limb). The craniolateral muscles and extensor tendons extend the knee and forelimb digit and flex the hock and extend the hind digit. The caudal group is responsible for flexing the knee and forelimb digit and extending the hock and flexing the hind digit. 

The horse’s flexor tendons have evolved to absorb tremendous loads during exercise, yet the horse damages these structures, particularly the superficial digital flexor tendon (SDFT), more frequently than any other muscles, tendons, and ligaments. The superficial digital flexor tendon lies just under the skin along the back of the leg, with the deep digital flexor tendon and the suspensory ligament beneath. 

The lower limb’s bony structure includes the proximal phalanx (P1, long pastern), middle phalanx (P2, short pastern), and distal phalanx (P3, coffin bone). The equine foot is unique in that it essentially distributes the horse’s 1,000-pound weight on two fingernails and two toenails (see page 37 for an article about the hoof’s internal structures). The laminar dermis, consisting of about 600 primary dermal laminae that each contain 100-150 secondary dermal laminae, connect P3 to the hard epidermal hoof wall. Laminitis, defined as the inflammation of these laminae, compromise this strong attachment, causing P3 to pull or rotate away from the hoof wall. Given the tremendous amount of compressive force placed on the foot, it is no wonder that 80-90% of lamenesses are linked to damage in the foot, says Romero.

Take-Home Message

From teeth that erupt continuously for forage grinding to hind limbs that allow for standing sleep, the horse is a complex product of evolution. The keen visual and auditory senses, forage-adapted digestive system, and intricate musculoskeletal system that enabled them to survive as prey animals before domestication—and still do for some herds—now allow us to enjoy our horses as both companions and athletes.

About the Author

Jean-Yin, DVM, Dipl. ACVIM

Jean-Yin Tan, DVM, Dipl. ACVIM, practices with Syracuse Equine Veterinary Specialists, in Manlius, N.Y. Her professional interests include neonatology, respiratory disease, and gastroenterology.

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