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Chapter 5 Functional anatomy.

A basic understanding of the functional anatomy of the horse is essential before discussing growth, aging, movement, selection, nutrition, health, breeding, behavior, management, and even facilities. An understanding of the functional anatomy provides a basis for the "why" of everything else involved in modern scientific equine production and equitation.


After completing this chapter, you should be able to:

* List the nine systems of animals and the major organs that make up each system

* Explain the functions of the skeletal, muscular, digestive, urinary, respiratory, circulatory, nervous, reproductive, and endocrine systems

* Identify the components of the skeletal, muscular, digestive, urinary, respiratory, circulatory, nervous, reproductive, and endocrine systems

* List the five divisions of the vertebral column

* Name the bones in the foreleg and hind leg

* Describe three types of joints

* Identify three types of muscles and their locations in the body

* Trace the circulation of blood through the body

* Identify the endocrine glands and the hormones they secrete





















gastric juice









joint capsule










spinal cord









Any discussion of the structure and function of animals begins with an understanding of the terms dorsal, ventral, cranial or anterior, and caudal or posterior. Dorsal pertains to the upper surface of an animal. Ventral relates to the lower or abdominal surface. Anterior or cranial applies to the front or head of an animal. Posterior or caudal pertains to the tail or rear of an animal (Figure 5-1).


Physiology, or the life functions of horses, occurs in body systems. Nine body systems are found in animals, including horses. These systems are:

1. Skeletal

2. Muscular

3. Digestive

4. Urinary

5. Respiratory

6. Circulatory

7. Nervous

8. Reproductive

9. Endocrine


The skeletal system is the rigid framework giving the body shape and protecting the internal organs. It is composed of bone and cartilage. Bones are composed of about one part organic matter and two parts inorganic matter. The inorganic matter is mineral, mainly lime salts. The surface of each bone is covered by a dense connective tissue called the periosteum. A union of two bones is called an articulation or joint. Ligament, tendons, and a tough, fibrous capsule provide stability or tightness to the joint. Tissues and organs attach to the skeleton.

The bones and joints together compose a complex system of levers and pulleys that, combined with the muscular system, give the body the power of motion. The skeleton also stores up needed minerals, mainly calcium and phosphorus; acts as a factory for the manufacture of blood cells; and, in the adult animal, stores fat in the limb bones. The relative size and position of the bones determine the form (or conformation) of the horse and its efficiency for any particular work. Bones are classified by their shape as long, short, flat, and irregular.

* Long bones are found in the limbs. They support the body weight and act as the levers of propulsion.

* Short bones occur chiefly in the knee and hock and aid in the dissipation of concussion (the shock of impact).

* Flat bones, such as the ribs, scapula, and some of the bones of the skull, help to enclose cavities containing vital organs.

* Irregular bones are unpaired bones, such as the vertebrae and some of the bones of the skull.

All bones, except at their points of articulation, are covered with a thin, tough, adherent membrane called the periosteum, which protects the bone and partially influences its growth. This latter function is of particular interest since we know that an injury to this membrane often results in an abnormal bony growth, called exostosis, at the point of injury. Other bony growths, such as splints, spavins, and ringbone, are often the result of some injury to the periosteum. The bone is nourished partially through blood vessels in the periosteum, which also contains many nerve endings.

The articular or joint surfaces of bones are covered with a dense, smooth, bluish-colored substance known as cartilage. The cartilage diminishes the effects of concussion and provides a smooth joint surface that minimizes frictional resistance to movement.

The two main divisions of the skeleton are the trunk, or axial skeleton, and the limbs, or appendicular skeleton (Figure 5-2).

Axial Skeleton

The axial skeleton consists of the skull, spine (or vertebral column), ribs and breastbone, pelvis, and tail (Figure 5-3).

Bones of the Skull. The skull is divided into two parts--the cranium surrounding the brain, and the face enclosing the entrances to the digestive and respiratory systems. The skull is attached to the first vertebra of the spine and has a large opening, the foramen magnum, through which the spinal cord passes.



The bones of the cranium are flat or irregular bones surrounding the cranial cavity, which houses the brain. These bones join each other at immovable joints. The bone forming the poll (head area) has an articulating surface where the head joins the vertebral or spinal column. Together with the bones of the face, the cranial bones form the orbital (eye) and nasal cavities (Figure 5-4).

The bones of the face form the framework of the mouth and nasal cavities. They include the more important bones of the upper and lower jaws, known as the maxillae and mandible, respectively. Each maxilla has six irregular cavities for the cheek or molar teeth. From the maxillae forward, the face becomes narrower and terminates in the premaxilla, which contains cavities for the six upper incisor teeth. Enclosed in each maxilla is a cavity known as the maxillary sinus, which opens into the nasal passages. This sinus contains the roots of the three back molar teeth and may become infected from diseased teeth.

The mandible, or lower jaw, is hinged to the cranium on either side by a freely movable joint in front of and below the base of the ear. The mandible has cavities for the six lower incisors. Behind the incisors and ahead of the six lower molars in each branch of the mandible is a space known as the interdental space. In this space, injuries to the periosteum or possible fracture of the mandible may occur from rough usage of a bit (Figure 5-4).

Vertebral or Spinal Column. The spine is a flexible column of small bones called vertebrae. The vertebral column may be thought of as the basis of the skeleton from which all of the internal organs and passageways are suspended. It is composed of irregularly shaped bones bound together with ligaments and cartilage that form a column of bones similar to an elastic suspension bridge. An elastic pad or cushion separates each vertebra along the length of the column, from the base of the skull to the tip of the tail. Through the length of this column runs an elongated cavity or passageway, called the neural canal or spinal canal, that contains the main trunk line of nerves to the brain--the spinal cord. The bones of the vertebral column (Figure 5-5) are divided into five groups:

1. Cervical: 7 vertebrae

2. Thoracic: 18 vertebrae

3. Lumbar: 6 vertebrae (sometimes 5)

4. Sacral: 5 vertebrae (fused together to form the sacrum)

5. Coccygeal or tail: 15 to 21 vertebrae

The hip bones are two large, flat, paired bones that form the pelvis or pelvic girdle. Each hip bone is firmly attached to the spine at the sacrum and circles around to meet at the midline below the sacrum and enclose the pelvic cavity. Each hip bone contains a cavity on its outside where the femur, or first bone of the hind leg, forms a joint. The upper front angle, together with the sacrum, forms the point summit of the croup. The back angle of the hip bone is the point of the rump.



The chest is the large cavity formed by the thoracic vertebrae, the ribs on the sides, and the sternum, or breastbone, on the bottom or floor. This cavity contains the heart, lungs, large blood vessels and nerves, and part of the trachea and esophagus. The 18 pairs of ribs, all jointed to the thoracic vertebrae at their upper ends, determine the contour of the chest by their shape and length.

The sternum is a canoe-shaped prominence in the midline of the breast consisting of 7 or 8 bony segments connected by cartilage. The sternum forms the floor of the thorax.

Appendicular Skeleton

The appendicular skeleton of the horse consists of the forelegs and hind legs. It is used for locomotion, grooming, and to some extent for defense and feeding. The forelimbs have no skeletal attachment to the axial skeleton, or trunk, of the horse. The connection is made only by muscles.

The bones of the foreleg of the horse (Figure 5-6), named from the top downward, include:

* Scapula

* Humerus

* Ulna and radius

* Carpal bones

* Splint bones

* Cannon

* Sesamoids

* First phalanx

* Second phalanx

* Coffin bone

Bones of the hind leg (Figure 5-7), named from the top downward, include:

* Femur

* Patella

* Tibia and fibula

* Tarsals

* Splint bones

* Cannon

* Sesamoids

* First phalanx

* Second phalanx

* Coffin bone

The hind limbs are attached to the bony pelvis at the hip joint, unlike the forelimbs, which have no bony connection to the trunk.

Functional anatomy of the hoof is discussed in more detail in Chapter 17.



Joints or Articulations

A joint or articulation is the union of two or more bones or cartilages. Joints are classified into three types according to their structure and movability--immovable, slightly movable, and freely movable.

Immovable joints are those in which the opposed surfaces of bone are directly united by connective tissue or fused bone and permit no movement, such as the bones of the cranium.

Slightly movable joints are those where a pad of cartilage, adhering to both bones, allows only slight movement due to the elasticity of the interposed cartilage. Many joints of the vertebrae are of this nature.

Freely movable joints are those where a joint cavity exists between the two opposed surfaces, such as the joints of the legs. The freely movable joints are the truest examples of joints. The ends of the bones entering into a freely movable joint are held in opposition by strong bands of tissue, called ligaments, that pass from one bone to the other. Ligaments possess only a slight degree of elasticity and a limited supply of blood, which explains why they heal slowly and often imperfectly following injury.

In freely movable joints, the ends of the bones are covered with a smooth cartilage (articular hyaline cartilage) that absorbs concussion and provides a smooth bearing surface. The entire joint is enclosed in a fibrous sac known as a joint capsule that assists the ligament in holding the bones in position (Figure 5-8). The inner surface of this sac is lined with a thin membrane, called the synovial membrane, that secretes a fluid known as synovia or joint water. Synovia is a clear, slightly yellowish fluid with the appearance and consistency of the white of a watery egg. It lubricates the joint in the same way that oil lubricates a mechanical bearing.


The joints of the foreleg, in order from the top downward, include:

* Shoulder, formed by the scapula and humerus

* Elbow, formed by the humerus, radius, and ulna

* Knee, formed by the radius, carpal bones, and three metacarpal bones--splints and cannon bones

* Fetlock, formed by the cannon, two sesamoid bones, and the first phalanx, or long pastern

* Pastern, formed by the first and second phalanges or long and short pasterns

* Coffin, formed by the second phalange or coffin bone and the navicular bone

The joints of the hind leg, named in order from the top downward, are:

* Hip, formed by the hip bone and the femur

* Stifle, formed by the femur, patella, and tibia

* Hock, formed by the tibia, tarsal or hock bones, and the metatarsal bones--splint and cannon bone

* Fetlock, pastern, and coffin joints are named and formed the same as the corresponding joints of the foreleg

In addition to the ligaments that form a part of the joints, there are certain other important suspensory and check ligaments. The suspensory ligament of the foreleg is a very strong, flat ligament running from the back of the knee and upper end of the cannon bone down the back of the leg in a groove between the splint bones. Just above the fetlock, this ligament divides into two diverging, rounded branches that are attached to the upper and outer part of the corresponding sesamoid bone. The branches pass downward and around to the front of the long pastern bone to connect with the extensor tendon attached to the front of the coffin bone. From the lower part of the sesamoids, bands of ligaments pass downward and attach to the backs of the long and short pastern bones.

The check ligament is a short, strong ligament running from the back side of the upper end of the suspensory ligament at a point just below the knee downward and backward for a short distance to attach to the deep flexor tendon, which in turn passes down the back of the leg to attach on the undersurface of the coffin bone. When the suspensory ligament is relaxed, the check ligament converts the tendon below it into a functional ligament to assist the general action of the suspensory ligament. The suspensory ligament is considerably more elastic than the binding ligaments of the joints. Its supporting, springlike action absorbs a great deal of concussion. This ligament is the most frequently injured in horses that do a great deal of their work at the gallop. In the hind leg, this suspensory ligament is very similar to that in the foreleg, but the check ligament in the hind leg is less perfectly developed.

The plantar ligament is a strong band of ligamentous tissue on the back of the hock bones. It extends from the point of the hock to the upper end of the metatarsus or cannon bone and, because of its strong attachment to the small hock bones, braces the hock against the strong pull of the Achilles tendon.

The ligamentum nuchae or ligament of the neck is a fan-shaped ligament of elastic tissue extending from the poll and upper surfaces of the cervical vertebrae downward and backward to attach to the longest spines of the thoracic vertebrae or withers. It assists the muscles of the neck in holding the head and neck in position.
Bad Timing for a Broken Leg

In May 2006, at Preakness Stakes, favored horse Barbaro broke his
right hind leg shortly after leaving the starting gate. The leg
broke in three places: a cannon bone above the ankle, a sesamoid
bone behind the ankle, and a long pastern bone below the ankle.
After a successful 5-hour surgery using almost 24 screws and
titanium plate metal implants to stabilize his bones, Barbaro had
only a 50 percent chance of surviving. Here's why:

* With little blood circulation and little muscle in the lower part
of a horse's leg, a break below the knee could easily destroy these
fragile vessels and deprive the animal of its full immune response at
the site of the injury. Any soft-tissue damage at all can cut off blood
flow and create a haven for bacteria. If infection occurs, it is
not easy to treat a horse with antibiotics. Using the large amount
of antibiotics required can destroy the natural flora of the
horse's intestinal tract and lead to life-threatening, infectious

* Even if it manages to avoid early infection, the
animal might not make it through the recovery. The large animal
must wake up from anesthesia without reinjuring itself. (Barbaro
was revived by being suspended in a warm swimming pool in a quiet
room and then kept there for as long as possible.) Then not all
horses are willing to hang in a sling, and the antsy ones can
thrash about and break their limbs all over again.

* If a horse starts favoring his wounded leg post-surgery, he may
overload his other legs, causing a condition known as laminitis. If
that happens, the hooves on the other legs will start to separate from
the bone, and his weight will be driven into the soft flesh of the

* A horse may develop life-threatening constipation as a
side effect of the anesthetic.

Veterinarians will often put down a horse that develops a nasty
infection, reinjures its broken leg, or develops laminitis in its other

Barbaro is one of the lucky ones. By August 2006, Barbaro's broken
right leg had fused to the point where they would have replaced the
cast with a brace if his left hind leg had not been affected by
laminitis. Still, the coronary band on his left leg appeared healthy,
and all signs were encouraging. As long as Barbaro was not suffering,
his owners continued with the decision of aggressive treatment for what
could be a lengthy convalescence. On November 6, 2006, the cast was
removed from Barbaro's leg and he continued to strengthen his leg.
Unfortunately after an eight month struggle to recover and heroic
efforts by veterinarians, Barbaro was euthanized on January 29,
2007. He succumbed to laminitis that had developed in three legs
and the recent surgery on his right hind leg for an abscess that
left Barbaro without a healthy leg to stand on.

The racing industry reacted to the death of Barbaro with the creation
of the Barbaro Memorial Fund, an initiative to raise money for research
into laminitis and other equine health and safety issues. The
National Thoroughbred Racing Association will take the lead by
organizing fundraisers.


The muscular system provides movement both internally and externally. Muscles, the active organs of motion, are characterized by their property of contracting or changing shape when stimulated. Each muscle is supplied by one or more nerves that not only bring commands from the brain to make it contract but also carry back to the brain impulses that tell of the degree of contraction. This correlation results in smooth, even movements instead of jerky or staggering movements. Muscles are red flesh or lean meat and compose about 50 percent of the total body weight.

The muscular system is made up of three types of muscles:

1. Smooth or involuntary muscle

2. Cardiac or involuntary striated muscle

3. Striated or skeletal muscle

Smooth Muscle

Smooth muscle is sometime called visceral muscle. It is found in the digestive system and in the uterus of females. Smooth muscles are capable of prolonged periods of activity before becoming fatigued. The visceral muscles of the digestive system perform wavelike contractions called peristalsis for many successive hours. Contraction of smooth muscle is involuntary (Figure 5-9).

Cardiac Muscle

Cardiac muscle is found only in the heart. Contraction of the cardiac muscle is inherent and rhythmic, requiring no nerve stimulus. The rate of contraction is controlled by the autonomic nervous system and requires no conscious control. The heart must continue ceaselessly contracting throughout the horse's lifetime with only split-second intervals of rest.

Striated or Skeletal Muscle

Skeletal muscles are usually attached to the bony levers of the skeleton and move the body voluntarily, under the direct control of the will. Skeletal muscle may attach its fleshy fibers directly to a bone; but usually the main part, or belly, of the muscle terminates at either or both ends in a strong, cordlike structure called a tendon that transmits the pull of the muscle as it contracts. This tendon arrangement avoids inefficient and bulky thickenings at knees, hocks, and fetlocks and permits several large muscles to be attached on one small area of bone.

Skeletal muscles are generally arranged in opposing sets--one set bends the limb or body, the other set straightens it. Usually both sets are active at the same time but to different degrees, one acting as a brake on the other. Voluntary muscles can contract for only a short time before becoming fatigued and requiring rest.


The contractile portion, or belly, of voluntary muscles consists of many elongated muscle cells side by side and lengthwise of the muscle. When stimulated, this portion becomes shorter and thicker. The tendon of a muscle is quite similar to a ligament in structure and transmits the power of the muscle to some definite point of movement. The contractile portion of a muscle has a large supply of blood, but the supply to the denser tendons is rather limited. The body of most muscles is attached to some bone at a point called the origin. The tendon of the muscle may pass one or more joints and attach (or insert) to some other bone.

The extensor and flexor muscles of the legs are an example of muscles in sets, one group having a certain general action and the other group the exact opposite action. A muscle is an extensor when its action is to extend a joint and bring the bones into alignment. A muscle is a flexor when its action is to bend the joint. Some muscles, if their points of origin and insertion are separated by two or more joints, may act as a flexor of one joint and an extensor of another joint. Except to establish fixation and rigidity of a part, such opposed muscles do not act simultaneously in opposition to each other, but act successively (Figure 5-10).

Fueling the Muscles. Muscle contraction occurs as an all-or-nothing reaction in response to nerve stimulation, and it requires energy in the form of adenosine triphosphate (ATP). All forms of energy must first be converted to ATP before contraction can occur. It is the only form of energy acceptable to muscle. Three basic fuel systems provide material to produce ATP for muscle contraction:

1. Phosphagen system

2. Glycogen or lactate system

3. Citric acid or Krebs cycle

The phosphagen system is a rapidly available source of energy, stored in muscle cells, with the ability to support anaerobic work for approximately 30 seconds at maximum output. This process is quick and efficient and does not need oxygen. Any event done at maximum effort and lasting less than 30 seconds is supported almost entirely by this fuel system.


The glycogen or lactate system can produce energy for up to 5 minutes from glycogen stored in the muscle. The glycogen system takes over ATP production at about when the phosphagen system is depleted. The stored glycogen, a polymer of glucose, is mobilized from its storage site in the muscle and converted to ATP through a process called glycolysis. This process does not require oxygen.

The citric acid or Krebs cycle requires oxygen and produces the largest amount of ATP. It takes over where the glycogen system ends and has the potential to last for hours, assuming that oxygen can be promptly and efficiently delivered to the muscle. The amount of oxygen that can be delivered to the tissue and used is called the VO2 max. The higher the VO2 max, the more endurance a horse has. The fuel for this system can be either pyruvic acid generated from glycolysis or fatty acids mobilized from adipose tissue or absorbed from the diet. The end products of this system are carbon dioxide, which is eliminated through the lungs, and water, which can be eliminated through either sweat or urine.

Tendons, Sheaths, and Bursae

Many muscles, especially those of the legs, have long tendons that pass one or more joints and undergo changes of direction or pass over bony prominences before reaching their point of insertion. Tendon sheaths and tendon bursae at various points of friction along the length of the tendon eliminate undue friction to allow the muscle to act more efficiently. A tendon sheath is a synovial sac through which a tendon passes. This sheath secretes synovia to lubricate the tendon. A tendon bursa is a synovial sac located between the tendon and the surface over which it passes in a change of direction. It serves the same purpose as a tendon sheath but differs from a sheath in that the tendon is not surrounded by the synovial sac. Tendon sheaths and bursae are found chiefly near joints. The synovial membrane and the synovia secreted in these sacs are the same as those found in the joints.


The digestive system converts feed into a form that can be used by the body for maintenance, growth, and reproduction. It consists of all the parts of an organism involved in taking food into the body and preparing it for assimilation--incorporation into the body. In its simplest form, the digestive system is a tube extending from the mouth to the anus with associated organs. This includes the mouth, esophagus, stomach, intestines, anus, and other associated organs like the liver, teeth, pancreas, and salivary glands. Digestive systems vary according to whether the animals are herbivores (eating only plants), carnivores (eating only animals), or omnivores (eating plants and animals). Horses are herbivores.

The entire digestive tract of a mature light horse is approximately 100 feet long. This length is coiled and looped many times but is usually very small in diameter and has a capacity of about 40 to 50 gallons.

The stomach of the adult horse makes up less than 10 percent of the total capacity of the digestive tract; the small intestine, the site of most nutrient absorption, makes up only 30 percent. About 65 percent of the capacity of the digestive system is in the cecum and colon, which digest the forages consumed by the horse (Figure 5-11).

Feed passes rapidly through the stomach and small intestine. Particle size affects the rate of passage; grinding or chopping increases the rate of passage and decreases absorption of nutrients by the horse. Any feed not digested and absorbed in the small intestine is passed on to the cecum and colon within 2 to 4 hours. Because of this relatively low volume capacity and rapid rate of passage through the upper gut, it is easy to overwhelm the digestive capacity of the horse's stomach and small intestine.

The horse's cecum and colon contain large microbial populations allowing for digestion of fibrous feeds. If large amounts of concentrates reach the cecum, they will quickly become fermented and may produce excessive gas or lactic acid and cause colic or founder.



The mouth extends from the lips to the pharynx and is bounded on the sides by the cheeks, above by the hard palate, and below by the tongue. Separating the mouth from the pharynx is the soft palate, a fleshy curtain suspended from the back part of the hard palate, which permits the passage of food and water from the mouth to the pharynx but prevents passage in the opposite direction. The lips pick up loose feed, which is passed into the mouth by the action of the tongue. When horses graze, they grasp food with their incisor teeth. The food is masticated, or ground up, between the molar or cheek teeth and mixed with saliva. The saliva is secreted into the mouth by the salivary glands, the largest of which is the parotid lying below the ear and back of the jaw.

Saliva moistens and lubricates the mass of food for swallowing and, as a digestive juice, acts on the starches and sugars in feed. The ball of masticated food is forced past the soft palate into the pharynx by the base of the tongue.

Horses drink by drawing the tongue backward in the mouth and using it like the piston of a suction pump. A horse usually swallows slightly less than a half-pint at each gulp. The ears are drawn forward at each swallow and drop back between swallows.


The pharynx is a short, somewhat funnel-shaped, muscular tube between the mouth and the esophagus. It also serves as an air passage between the nasal cavities and the larynx. The muscular action of the pharynx forces food into the esophagus. Food or water, after entering the pharynx, cannot return to the mouth because of the traplike action of the soft palate. (For the same reason, a horse cannot breathe through its mouth.) Food or water returned from the pharynx passes out through the nostrils.


The esophagus is a muscular tube extending from the pharynx down the left side of the neck and through the thoracic cavity and diaphragm to the stomach. Food and water are forced down the esophagus to the stomach by a progressive wave of constriction of the circular muscles of the organ. The return of food or water through the nostrils is an almost certain indication that the horse has choked because the esophagus has been blocked by a mass of food or a foreign object. The esophagus enters the stomach through an oblique angle, making regurgitation impossible.


The stomach is a U-shaped, muscular sac in the front part of the abdominal cavity close to the diaphragm. Food entering the stomach is arranged in layers, with the end next to the small intestine filling up first. The contents of the stomach are squeezed and pressed by the muscular activity of the organ. The digestive juice secreted by the walls of the stomach is known as gastric juice.

Small Intestine

Extending from the stomach to the cecum, the small intestine is a tube about 2 inches in diameter and 70 feet long. It holds about 12 gallons and is composed of three parts: the duodenum, the jejunum, and the ileum. After leaving the stomach, the small intestine is arranged in a distinct U-shaped curve. It lies in folds and coils near the left flank, being suspended from the region of the loin by an extensive fan-shaped membrane called the mesentery.

Large Intestine

The large intestine is divided into the cecum, large colon, small colon, rectum, and anus. The horse, unlike humans or dogs, consumes large quantities of cellulose in its diet. The usual digestive enzymes are not effective against cellulose, so the horse must rely upon bacteria to break down the cellulose into substances it can absorb into its body. To give the bacteria time to act on the cellulose, the cecum and the large colon in the horse have been greatly enlarged so that the food moves slowly through this part of the digestive tract.

The cecum is an elongated sac extending from high in the right flank downward and forward to the region of the diaphragm. The openings from the small intestine and to the large colon are close together in the upper end of this organ. The contents of the cecum are always liquid. The cecum is about 4 feet long, with a capacity of about 8 gallons.

The large colon is about 12 feet long, has a diameter of 10 or 12 inches, and holds about 20 gallons. It extends from the cecum to the small colon and is usually distended with food. Bacterial action and some digestion of food takes place here also.

The small colon is about 10 feet long and 4 inches in diameter. It extends from the large colon to the rectum. The contents of the small colon are usually solid; here the balls of dung are formed. Most of the moisture in the food is reabsorbed in this portion of the large intestine.

The rectum is about 12 inches long. It is the part of the digestive tract that extends from the small colon through the pelvic cavity to the anus, where the digestive tract ends.


Life processes produce waste products. The urinary system is composed of the kidneys, ureters, bladder, and urethra. The kidneys are paired organs located on each side of the backbone opposite the 18th ribs. The chief function of the kidneys is to maintain water and mineral balance and excrete the wastes of metabolism. The urinary bladder holds the wastes until they are excreted.

The kidneys are from 6 to 7 inches long, 4 to 6 inches wide, and about 2 inches thick. The right kidney is roughly triangular with rounded corners, but the left is more bean-shaped and longer and narrower. In the course of a day, all the blood in the body of the horse passes through the two kidneys more than 400 times and is filtered of nitrogenous wastes each time (Figure 5-12).

Nephrons, the tiny functional units of the kidneys, filter the blood received by the kidneys (Figure 5-13). The outer portion or cortex of each kidney has several million tiny nephrons that filter approximately 200 gallons of liquid a day, rejecting blood cells and proteins but permitting fluid salts and other chemicals, including nitrogenous wastes, to pass through them. The kidneys also act in reverse and return to the bloodstream such valuable substances as the salts, sugars, and most of the fluids--all but about 2 gallons of the 200 gallons of fluid are returned to the blood.



Urine, containing the nitrogenous waste and any excess salts or sugars not required by the body, is collected in the inner portion of the kidney, the renal pelvis, and then drained from the kidney drop by drop through the ureter to the bladder. The bladder is a sort of muscular balloon, which in the horse can expand greatly without bursting. As urine collects in the bladder, nerve endings signal that the bladder needs to be emptied. Then the urine flows from the bladder to the outside environment through the urethra. In mares the urethra is short and wide. In males the urethra is long and narrow since it travels the length of the penis.


The respiratory system takes in oxygen from the environment and delivers it to the tissues and cells of the body; it also picks up carbon dioxide from the tissues and cells and delivers it to the environment. Organs of respiration include the nasal cavity, pharynx, larynx, trachea, bronchi, and lungs. The lungs are the essential organs of respiration; the other parts are simply passages carrying air to and from the lungs. Air is taken into the lungs, where oxygen is removed by diffusion into the blood (Figure 5-14).

The pharynx is common to both the respiratory and digestive tracts. The larynx, commonly is a short, tubelike organ between the pharynx and the trachea. Known as the voice box, commonly Stretched vertically within this cartilaginous box are the vocal cords, two folds of elastic tissue. By contracting the lungs and forcing air past these folds of tissue, the horse sets them into vibration and produces the sound known as neighing, whinnying, or nickering. The larynx also regulates the amount of air passing into or out of the lungs and aids in preventing the inhalation of foreign objects.


The trachea is a long tube connecting the larynx with the lungs and is located along the lower median border of the neck. It is composed of a series of cartilaginous rings held together by elastic, fibrous material.

Bronchi are branches of the trachea that connect the trachea with each lung. Each bronchus in turn divides into a number of minute tubes that penetrate every part of the lung tissue (Figure 5-15). The branching bronchi end in groups of minute air sacs similar to bunches of grapes, called alveoli. Here the gaseous exchange of carbon dioxide and oxygen takes place between the circulating blood and the air.

Physiology of Respiration

Respiration is the act of breathing. It consists of the exchange of oxygen in the air for carbon dioxide in the blood, and the interchange of these gases between the blood and the body tissues. The former is known as external respiration and the latter as internal respiration. External respiration consists of two movements--inspiration and expiration. Inspiration is brought about by a contraction of the diaphragm and an outward rotation of the ribs. The diaphragm bulges into the airtight thoracic cavity as a dome-shaped muscle. It works like a piston, drawing air into the lungs.

Expiration is effected by a relaxation of these muscles and a contraction of rib and abdominal muscles to force air out of the lungs. Abdominal muscles are used extensively in labored breathing. Since the diaphragm plays such an important part in respiration, it follows that the distention of the digestive tract with bulky food material interferes with normal breathing, especially when the horse is being worked at fast gaits.


The lungs of the average horse contain about 1.5 cubic feet of air. The normal horse at rest breathes at the rate of 8 to 16 times a minute and inhales at each respiration approximately 250 cubic inches of air. The amount of air required by the horse depends upon the extent of muscular work being performed.


The circulatory system distributes blood throughout the body. This system consists of the heart, veins, and arteries. Pumping action of the heart causes blood to flow through the arteries to the lungs, where it picks up oxygen and carries it to the rest of the body. Oxygen is necessary for all cells of the body. As the blood delivers oxygen to the cells of the body, it picks up carbon dioxide, a waste product, which is carried in the blood back through the veins to the heart and lungs. The lungs release the carbon dioxide to the environment and pick up more oxygen. The blood also carries food substances and waste products (Figure 5-16).


The heart is situated in the left half of the thorax, between the lungs and opposite the third to sixth ribs. In the ordinary-sized horse, the heart weighs from 7 to 8 pounds. It is enclosed in a sac called the pericardium. The heart is a muscular pump composed of four chambers:

* Right atrium

* Right ventricle

* Left atrium

* Left ventricle

Right and left sides of the heart are separated by a muscular wall. Four valves in the heart keep the blood flowing in one direction.


Blood pumped out of the left ventricle into the aorta passes through arteries of progressively smaller diameters until it reaches the capillary beds of the skin, muscles, brain, and internal organs. Here oxygen and nutrients are exchanged for carbon dioxide and water. The blood is then conducted back to the heart through veins of progressively larger diameter. Finally, the blood reaches the right atrium through the venae cavae.

Blood next passes into the right ventricle, which pumps it out to the pulmonary circulation and finally into capillaries around the air sacs in the lungs. Here the carbon dioxide is exchanged for oxygen; the blood returns to the left side of the heart by the pulmonary vein and then to the left ventricle, where the cycle begins again.

Beating of the heart is controlled internally, but the force and rate of the heartbeat are influenced by the nervous system and endocrine system. Heart rate speeds up when a horse exercises, becomes excited, runs a fever, overheats, or experiences any circumstance when its tissues need more blood.

The heart is a muscle and, as such, requires its own blood supply. The coronary vessels that provide this nourishment encircle the heart like a crown at the juncture of the atria and the ventricles, sending branches to both these structures (Figure 5-17).


The fluid tissue of the body--blood--carries food substances and oxygen to each cell of the body and takes waste products formed there away from the cells. Blood is a red, alkaline fluid composed of blood plasma and red and white blood cells. It clots almost immediately upon exposure to air. The total amount is about 7 percent of the horse's weight. The white blood cells are the active agents in combating disease germs in the body. Red blood cells originate in the bone marrow, liver, and spleen and carry oxygen from the lungs to the tissues and carbon dioxide from the tissues back to the lungs.

The blood is the body's regulator. It carries food to the tissues and waste products away from the tissues, distributes heat, assists in regulating the temperature, and neutralizes or destroys bacterial and viral invaders.


Vessels and Lymphatics

Arteries have rather thick, elastic walls and carry the blood from the heart to the tissues of the body. When the heart forces blood into the arteries, they expand and, in returning to their unexpanded state, force the blood onward. The expansion and contraction of the arteries is the pulse.

Veins have much thinner walls and, in many cases, are equipped with one-way valves at frequent intervals, opening toward the heart. The veins of the horse's legs have such valves. The veins carry the fluid from the tissues to the heart. Veins are located between muscle masses so that, as the horse moves, the veins are squeezed. The blood, having to go somewhere, is directed back to the heart by way of the venae cavae, the great veins from the front and back portions of the horse.

Capillaries are microscopic in size and function as numerous connecting tubes between the arteries carrying blood to the cells and the veins carrying blood away from the cells. Through the walls of the capillaries, exchange of food and oxygen for waste products of the body takes place.

Lymph vessels and lymph nodes, or lymphatics, consist of many well-defined groups of lymph nodes and connecting vessels. The vessels all converge to form one large duct lying parallel to the aorta, the main artery from the heart, and emptying into one of the large veins near the heart. Lymph glands are strategically located along the main vessels and act as filters for the lymph, which assists in carrying food from the digestive tract to the tissues and waste products back to the bloodstream.

Physiology of Circulation

Heart movements are controlled by an intricate group of nerves. The heartbeat is a combined cycle of contraction and relaxation of the organ. In the normal horse at rest, the heart beats from 38 to 40 times a minute. Pulse rate is determined by counting the rate of pulsation in some artery that is easily palpitated, for example, the one at the angle of the lower jaw.

The pressure and rate of flow in the veins, compared with the arteries, is very low. Movement of blood in the veins is also aided by respiration movements and muscular contraction, so good circulation is made possible by exercise. The heart, however, is the main pump of the circulatory system.


The nervous system supplies the body with information about its internal and external environment. This system conveys sensation impulses--electrical-chemical changes--back and forth between the brain or spinal cord and other parts of the body. It is a complex system consisting of the brain, spinal cord, many nerve fibers, and sensory receptors.

The nervous system is divided into two main portions: the autonomic (automatic) nervous system and the central nervous system, each controlling different functions of the body. The autonomic nervous system is concerned with control over the respiratory and digestive systems, eyes, heart and blood vessels, glandular products, and other automatic functions directed by the brain stem. The central nervous system controls the conscious or voluntary actions of the body. In general, the nervous system is the communication system of the body and consists of the brain, spinal cord, ganglia, and nerves (Figure 5-18).

The brain and spinal cord are the most important parts of the central nervous system. The brain lies in the cranial cavity of the skull. Considering the size of the horse, the brain is small when compared with the relative brain size of other animals. Brain size relative to body size cannot be considered an absolute indication of the degree of reasoning intelligence; however, there is a distinct correlation. The horse is considered to occupy the mid-position in the scale of intelligence of domesticated animals.


The brain is divided into three major portions: the brain stem, the cerebrum, and the cerebellum. The brain stem--the primitive brain--is the slightly expanded cranial end of the spinal cord. It contains the specific nerve centers absolutely essential for the life of the animal, such as the centers controlling the heartbeat, respiration, and temperature, among others. The cerebrum, what is normally thought of as the brain, performs the functions of memory, intelligence, and emotional responses. The cerebellum controls muscular coordination, balance, and equilibrium; it is smaller than the cerebrum and is situated under the caudal part of the cerebrum.

The sense organs or receptors receive stimuli and convey them via electrical impulses over sensory nerve fibers to the brain. The brain analyzes this information and sends commands back via the spinal cord, usually over the same peripheral nerve trunks along motor nerve fibers to motor or effector nerve endings, usually located in the muscles.

Ganglia--secondary nerve centers located chiefly along the spinal cord--act almost like a subexchange in a telephone system. They receive and dispatch nerve impulses that do not have to reach the brain, including such stimuli as heat, pain, excessive pressure, and others. These impulses are immediately switched over to motor filaments and cause certain muscles to react instantaneously. For example, if a horse steps on a nail, the whole leg is pulled away immediately, before the brain becomes aware of the action, in an effort known as a reflex.

Nerves are bands of white tissue emanating from the central nervous system and ganglia and extending to all parts of the body. These are the peripheral nerves, or nerve trunks, consisting of thousands of tiny filaments or wires insulated one from the other by a myelin sheath and ending in tiny specialized knobs, coils, knots, and sprays distributed widely inside the body as well as on its surface.

There are two kinds of nerves, one sending impulses to the brain over sensory fibers and the other carrying commands back from the brain over motor fibers. Those nerve endings receiving stimuli from the outside are called sense organs or receptors. General sense organs are responsive to pain, touch, and temperature. Special sense organs are concerned with smell, sight, taste, and hearing. In general, nerves follow the courses of the arteries and are similar to telephone wires: the larger nerves, like telephone cable, contain many separate lines in a bundle.


Sexual reproduction is the process of creating new organisms of the same species through the union of the male and female sex cells--sperm and eggs. Males and females exist in most species. Testes in the males produce sperm. Ovaries in the females produce eggs or ova. Fertilization occurs when the sperm unites with an egg, forming a zygote. During a period of pregnancy, the zygote develops into a fetus and eventually a new organism. An understanding of the reproductive process is important to the success of horse breeding.


The reproductive organs of the mare are shown in Figure 5-19. The ovaries produce eggs that unite with the sperm to start the new individual. They also secrete the hormone estrogen, which induces estrus, or heat, and progesterone, which conditions the reproductive tract for implantation and maintenance of the fetus.

The fallopian tubes or oviducts are the customary site of fertilization of the ovum (egg) by the sperm. They serve as a connecting link between the ovary and uterus. The uterus consists of a body, cervix, and two horns, one of which receives the fertilized ovum for development.

The vagina receives the sperm during mating and functions as a passageway during parturition, or birth.

All reproductive functions in the mare are directed by hormones produced in the glands of her endocrine system; hormonal balance controls all phases of reproductive tract stimulation and inhibition. The mare's reproductive cycle is discussed in detail in Chapter 11.


The reproductive organs of the stallion are shown in Figure 5-20. The male reproductive system consists of two testes, three accessory sex glands, and a series of tubules through which spermatozoa are transported to the female reproductive tract.



Spermatozoa are produced in the testes in small, coiled, seminiferous tubules that when extended are 400 to 500 feet long. Since developing sperm cells cannot live at body temperature, heat regulation of the testes is critical. Scrotal muscles contract and expand in the normal process of regulating the temperature of the testes. Ridgling or cryptorchid horses are those in which one or both testes have not descended into the scrotum. The testis maintained in the body cavity is sterile, but the suspended testis is fertile. This condition is hereditary and should not be propagated; castration of a cryptorchid horse is usually a serious operation.

The accessory sex glands are the seminal vesicles, prostate, and bulbourethral gland. These furnish alkaline fluid secretions to transport and neutralize the urethra. Spermatozoa are transported from the epididymis through the urethra, which terminates at the end of the penis.


The ductless glands producing internal secretions, or the endocrines, form a system that influences the vital functions of the horse from before birth until death. Endocrine secretions control the events leading up to and including conception, gestation (pregnancy), parturition (birth), digestion, metabolism, growth, puberty, aging, and many other physiologic functions. Homeostasis, or balance, in the horse is largely under the control of the endocrine system.

Secretions of the endocrines are called hormones. Hormones are secreted without a duct directly into the circulatory system, where they travel to their target organ or tissue to influence its function.

Recent discoveries in endocrinology have blurred the lines between hormones and enzymes, and the definition of a hormone is being broadened as scientists gain a better understanding of endocrinology. Major components of the endocrine system of the horse are shown in Figure 5-21.


Hormones and Their Actions

Hormones aid in the integration of body processes by stimulating or inhibiting target organs. Although the time lapse between release and effect is longer than for the nervous system, the complementary function of the two systems provides for full coordination of body responses of horses. The ultimate purpose of hormones is to provide a means of adaptation between the body and its external or internal environment.

Hormones may be classified into two categories by their chemical composition. Steroid hormones are secreted by the adrenal cortex and the gonads. Protein or protein-like hormones are secreted from the pituitary gland, the thyroid, the pancreas, and the adrenal medulla.

Hormones regulate bodily reactions through their effects on target organs. They do not cause a reaction or event that could not otherwise occur; they merely modify the rate at which target organs perform functions. Hormones function at extremely small levels in the body, and the rate of secretion varies according to the level of stimulation required.

Hormonal output is often controlled through a feedback system from the target organ. This is most evident through the interaction of the anterior pituitary gland with other endocrine glands. Hormones released by the anterior pituitary control the level of activity of several other endocrine glands (adrenal cortex, thyroid, gonads). Increased hormone production by these glands serves as a negative feedback on the pituitary, causing in it a reduced rate of secretion of the stimulatory hormone.

The pituitary and the hypothalamus work together as a functional unit to coordinate the endocrine and nervous systems in their actions. The hypothalamus is the "center" of the autonomic nervous system and "master" of the pituitary. Through direct nervous connection and the releasing of hormones (factors), the hypothalamus controls the pituitary.

Posterior Pituitary

Hormones of the posterior pituitary (neurohypophysis) differ from other pituitary hormones in that they do not originate in the pituitary, but are only stored there until needed. Two hormones, vasopressin (antidiuretic hormone, or ADH), and oxytocin (milk letdown hormone), are actually produced in the hypothalamus. Their method of transfer from the hypothalamus to the pituitary is unique because it is not through the vascular system, but along the axons of the nervous system.

ADH. Vasopressin, or antidiuretic hormone (ADH), is a small polypeptide (chain of amino acids). ADH does not always function under everyday events. Hemorrhaging, trauma, pain, anxiety, and some drugs will trigger its release, and low environmental temperatures will inhibit it. ADH exerts its effects on the distal tubules and collecting ducts of the loops of Henle of the kidney, resulting in increased water absorption.

Oxytocin. Oxytocin controls lactation and reproductive phases of the mare. A neural stimulus, such as suckling, causes the hypothalamus to stimulate the posterior pituitary into releasing oxytocin, which is circulated through the blood until it comes into contact with the myoepithelial cells surrounding the alveoli of the mammary gland. Oxytocin causes the myoepithelial cells to contract, effectively squeezing the milk out of the secreting alveoli and releasing it into the milk ducts, cistern, and teats of the mammary gland. Oxytocin also plays a role in reproductive processes. During the estrous cycle, oxytocin stimulates uterine contractions that facilitate the transport of sperm to the oviduct at estrus; during the late stages of gestation, it aids parturition.

Anterior Pituitary

Hormones of the anterior pituitary (adenohypophysis) are produced within the pituitary gland itself. They consist of the follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), and growth hormone.

FSH and LH. The two pituitary gonadotropins, FSH and LH, are necessary for the maintenance of gonadal functioning. FSH in the mare stimulates overall follicular growth. Follicle maturation is achieved through the combined actions of FSH, LH, and the female sex hormones, which are discussed in more detail later in this chapter.

The action of LH on a follicle is to increase the growth rate and stimulate the secretion of estrogen. Ovulation (the release of an egg) is triggered by this process. As a result of LH activity the follicle is converted to a corpus luteum (a gland formed on the ovary after ovulation that produces progesterone). LH controls the continued secretion of progesterone from the corpus luteum. Progesterone maintains pregnancy by keeping FSH and estrogen in check.

The actions of these hormones in stallions are analogous to those in mares. FSH in the male stimulates the formation of sperm by exerting its effect on small tubules in the testes. Full sperm production cannot be accomplished without the joint effort of LH, known as interstitial cell-stimulating hormone (ICSH) in the male, and certain levels of testosterone. ICSH facilitates the production of testosterone from specialized cells of the testes.

Prolactin. Prolactin, the lactogenic or luteotropic hormone (LTH), is vital for the proper development of lactation in horses. It cannot initiate the secretory process, and it requires estrogen and progesterone to "prime" the mammary system. Prolactin does not seem to be as necessary for the continuation of lactation as it is for its initial development and for stimulating the corpus luteum. To date, prolactin has not been demonstrated to have specific effects in male reproduction. Figure 5-22 illustrates the location in the brain of the thalamus, third ventricle, hypothalamus, pituitary gland, and infundibulum.

ACTH. Adrenocorticotropic hormone (ACTH) secreted from the anterior pituitary causes several events to occur. Of primary importance is the release of adrenocorticoid steroids from the adrenal cortex into the bloodstream. Other effects include a reduction of lipid levels from the adrenocortical cells, a lowered concentration of adrenal cholesterol

and ascorbic acid, a general increase in adrenal cell size and number, and an increase in adrenal blood flow. ACTH promotes the secretion of aldosterone, especially following body stress, such as loss of blood. (Hormones produced by the adrenal cortex are discussed later.) ACTH also influences processes not related to adrenal function, including movement of fatty acids and neutral fats from fat deposits, ketogenesis, muscle glycogen levels, hypoglycemia, and amino acid levels of the blood.


TSH. The thyroid-stimulating hormone (TSH) promotes the release of thyroxin from the thyroid gland. It also increases the rate of binding of iodine within the thyroid. The release of thyroxin serves as a general metabolic control, with higher levels of thyroxin producing an increased metabolic rate.

STH. The basic function of the growth or somatotropic hormone (STH) is to stimulate an increase in body size. Growth hormone, along with other pituitary hormones, is important in protein synthesis and provides high intracellular concentrations of amino acids. It exerts its effects on bone, muscle, kidney, liver, and adipose (fat) tissues in bones; in particular, the epiphyseal plates--long bone growth sites--are sensitive to it. Growth hormone regulates, along with the thyroid hormone, the filtration rate and blood flow through the kidney.

Growth hormone mobilizes fat from adipose tissue, resulting in increased blood levels of ketone bodies, together with stimulation of the alpha cells of the pancreatic islets, causing glucagon secretion. Growth hormone also exerts a stimulating influence on milk production in lactating mares, either partly or entirely due to an increased amount of mammary gland tissue.


The pineal gland in horses and most other mammals is responsible for melatonin synthesis. It functions on a photoreceptive basis, causing different levels of melatonin production depending on light intensity. The pineal also affects the development and function of the gonads.


The thyroid gland secretes thyroxin. This hormone controls the rate of metabolism. Another hormone, calcitonin, also produced by the thyroid, aids in the metabolism of calcium and is essential for general bone development. The thyroid is interrelated to other endocrine glands, the adrenals, and the gonads through the pituitary.

Thyroxin. The structure of the thyroid hormone, thyroxin, is unique because the element iodine is essential for biological activity and release of the hormone from the gland. Thyroxin is necessary for the maturing of animals. While growth hormone is responsible for physical growth, thyroxin is necessary for the proper differentiation of body structures. Growth and eruption of the teeth of horses is under thyroid control. Even the skin and hair are affected by thyroid changes. A lack of thyroxin will cause a thinner coat of hair, with individual hairs being more coarse and brittle.

Reproductive failures and deficiencies in both sexes may be at least partly attributed to a lack of thyroxin, causing a variety of problems from abortions and stillbirths in mares to impaired sperm production and lowered libido in stallions.

The thyroid hormone affects temperature-regulating processes. By increasing the general rate of oxygen consumption at the cellular level, heat production is increased. Thyroxin stimulates general nervous functions at all levels, decreases the threshold of sensitivity to many stimuli, shortens reflex time, and increases neuromuscular irritability.

Low levels of thyroxin during developmental stages have detrimental effects on the nervous system.

Goiter, enlargement of the thyroid area, can be brought about by either hyperthyroid or hypothyroid conditions. The most common cause in animals is a deficiency of iodine, making the animal hypothyroid. Many feedstuffs have goitrogenic (goiter-producing) effects that inhibit thyroid activity. Vegetables such as cabbage, soybeans, lentils, linseed, peas, peanuts, and all of the mustard-like plants possess goitrogens. In the thyroid, these interfere with the process of trapping iodine.


The parathyroid gland is located dorsal to the thyroid in horses and is responsible for maintaining proper calcium levels in the blood and extracellular fluids. Parathormone, the secretion of the parathyroid, increases calcium levels in the blood and affects calcium and phosphate levels of the bones and kidneys.

Thyrocalcitonin from the thyroid has the opposite effect, causing a decrease in blood serum levels of calcium during events of hypercalcemia. Parathormone affects bones directly by mobilizing calcium from the bones into the bloodstream. Parathormone also lowers the ability of the kidney to excrete calcium, thereby increasing calcium retention. Parathormone and vitamin D work together on calcium release from bone and in increased absorption of calcium from the intestine.


The pancreas is primarily an organ of digestive secretions, although mixed throughout the pancreas, there are functionally different groups of cells known as the islets of Langerhans. These cells have rich blood supplies and consist of so-called alpha and beta cells. Beta cells are the most common, and they produce the hormone insulin. Insulin lowers the blood glucose and gets glucose across the cell membrane to where it can be metabolized. Alpha cells are responsible for the production of glucagon, which increases the blood glucose.

Adrenal Cortex

The adrenal cortex is the outside layer of the adrenal glands, located near the kidneys. The adrenal cortex produces steroid hormones. These hormones bear some structural resemblance to cholesterol. Adrenal cortical hormones include glucocorticoids, mineralocorticoids, and androgens. Secretion of the glucocorticoids from the adrenal cortex is stimulated by ACTH. The glucocorticoids influence metabolic functions; the mineralocorticoids influence metabolism of minerals like sodium and potassium. Androgens are masculinizing sex hormones.

Deficiencies in glucocorticoid levels have detrimental effects on general body metabolism. A primary function of the glucocorticoids is as a catalyst in the gluconeogenic process--the formation of glucose from proteins and fats. Together with the mineralocorticoids, glucocorticoids also help regulate water metabolism.

Increase in the size of the adrenals can be observed in animals that are involved in stress situations. The stress of crowding is a major factor in adrenal enlargement, and adrenal weights of wild animals are used as a measure of population density. Overactivity of the adrenals produces androgens that inhibit the production of gonadotropins and thereby lower reproductive performance.

Other sources of steroid hormones, besides the adrenal cortex, are the ovaries, testicles, and placenta. Steroids are inactivated by their target organs as well as in the liver and kidney. These inactivated hormonal substances are water soluble and are readily eliminated through the urine.

Adrenal Medulla

The adrenal medulla, located at the center of the adrenal glands, produces two hormones-- epinephrine and norepinephrine. Epinephrine (also known as adrenaline) helps the horse adjust to stress situations and activates the fight-or-flight mechanism. Norepinephrine helps maintain the tone of the vessels in the circulatory system. Release of these two hormones is controlled by nerves that enter the adrenal medulla.


Sex hormones are primarily secreted by the ovaries and testes and, to some extent, by nongonadal organs such as the adrenals and the placenta. There are four types of hormones: androgens, estrogens, progesterone, and relaxin. The first three types are steroids, while the fourth is a protein.

The strongest and most predominant of the androgens is testosterone, which is produced by the interstitial or Leydig cells of the testicles. Testosterone and related hormones are responsible for male secondary sex characteristics of stallions, body conformation, muscular development, and libido. They are also responsible for the growth and development of secondary sex glands of the male, as well as maintaining the viability of the spermatozoa and stimulating penile growth. Testosterone is rapidly used by target organs or degraded by the liver and kidneys.

The ovaries produce two steroid hormones, estradiol and progesterone, and another protein hormone, relaxin. Estrogen comes from the Graafian (mature) follicles of the ovary. Progesterone comes from the corpus luteum on the ovary. A mature follicle ruptures at ovulation to release an egg. This ruptured follicle then develops into a second endocrine structure, the corpus luteum, and primary production shifts from estrogen to progesterone. The function of progesterone is to prepare the uterus for implantation and maintenance of pregnancy. Progesterone also suppresses the formation of new follicles and new estrus, and it prepares the mare for lactation through increased mammary development.

Relaxin is a hormone related specifically to the birth process and does not appear until late in pregnancy, just before parturition. It acts on the ligaments and musculature of the pelvis, cervix, and vagina. The precise site of formation of this hormone is not known, yet it is speculated that production may occur in the cells located in the boundary region of the cortex and medulla of the ovaries.

During pregnancy, the uterus itself takes on hormonal functions through the production of placental hormones: pregnant mare serum gonadotropin, estrogens, and progesterone. These hormones serve to maintain the uterus in a way that is favorable for the continued growth and development of the mammary gland.

Pregnant mares excrete estrogen in their urine. In 1942, a pharmaceutical company introduced estrogen extracted from the urine of pregnant mares as an estrogen replacement therapy (ERT) for human females. Estrogen from this source is still prescribed today for treatment of menopausal symptoms (see <>).

When mares are 115 to 125 days pregnant, the urine collection period begins. Estrogen production in their urine peaks between days 200 to 275 of pregnancy and then decreases as the mare approaches parturition, so the mares' urine is collected for a period of 150 to 160 days. Mares are specifically bred and housed for the purpose of collecting their urine. This has created some ethical issues with animal rights groups.

Gastrointestinal Tract

All hormones secreted by the gastrointestinal mucosa and small intestine are related to the digestive process. Five of these have been chemically identified, with the possibility of more existing, making the small intestine a major site of hormonal production, second only to the pituitary.

One hormone, secretin, is responsible for stimulating pancreatic bile and small intestine secretions. While causing an increase in fluid levels of the intestine, secretin has no effect on actual enzymatic increases. It also seems to have negative effects on the activity of the stomach.

A second hormone, enterokinin, causes an increased rate of secretion of digestive juices and enzymes of the small intestine.

Enterogastrone and cholecystokinin are two hormones related to fat levels in the diet. Enterogastrone inhibits rates of gastric secretion; in response to feed fat in the intestine, it slows down the rate of feed passage so that more time can be spent in the digestion of feed. Table 5-1 summarizes the hormones of the horse and their origin and functions.


The nine body systems of the horse are skeletal, muscular, digestive, urinary, respiratory, circulatory, nervous, reproductive, and endocrine. Proper function and control of each of these systems is essential to the survival, growth, and health of the horse. While the systems are generally discussed individually, they are interrelated and function in concert with each other.

For individuals working with horses, a basic understanding of the functional anatomy of the horse is essential before discussing growth, aging, movement, selection, nutrition, health, breeding, behavior, management, or even facilities.


Success in any career requires knowledge. Test your knowledge of this chapter by answering these questions or solving these problems.

True or False

1. The digestive system provides a large store of calcium and phosphorus.

2. The mouth is not part of the digestive system.

3. Food passes from the mouth through the trachea to the stomach.

4. Blood carries carbon dioxide and oxygen.

5. Capillaries are the largest of the blood vessels.

6. The pituitary produces the steroid hormone testosterone.

Short Answer

7. List the bones in the foreleg of the horse, from the shoulder joint down to the hoof.

8. Name six types of cells that form during morphogenesis.

9. List the nine body systems.

10. Identify the four surfaces of an animal.

11. What are the two major divisions of the skeletal system?

12. List the five divisions of the vertebral column.

13. What organ transports food from the mouth to the stomach?

14. What organ filters the waste products out of the blood and helps maintain water and mineral balance?

15. Name the two movements of external respiration.

16. What is the name for the air sacs at the end of branching bronchi in the lungs?

17. List the two main divisions of the nervous system.

18. List five reproductive organs in the mare.

19. List five reproductive organs in the stallion.

20. Name three accessory sex glands in the stallion.

21. Identify the hormones from each of the following: posterior pituitary, anterior pituitary, thyroid, pancreas, adrenal, testes, and ovaries.

Critical Thinking/Discussion

22. Identify the four classifications of bones according to their shape, and describe their location and function based on shape.

23. Describe three types of joints.

24. Explain the concept of extensor and flexor muscles.

25. Describe one cycle of external respiration.

26. Briefly outline the circulation of blood through the body of the horse, including the heart and lungs.

27. Define a hormone.

28. Describe the relationship of the anterior pituitary to the other endocrine glands.

29. Why is the nervous system like a communication system?

30. From where does energy come for muscle contraction?


1. Dissect a fresh or preserved heart. Ideally, this should be from a horse, but one from another livestock species will work. Identify all parts of the heart, and trace the flow of blood through the heart.

2. Construct a model of the visible horse. Hobby shops often sell a model called the visible horse. (Contact a hobby shop or mail-order source for a model.) This model reinforces understanding of the structure of many of the systems.

3. Find a mounted skeleton of a horse or some other species. Identify the bone shapes and joint types. Or, instead of the whole skeleton, obtain a model of the front or hind leg and carefully study the relationship of each bone and the joints formed.

4. From a biological supply company, obtain a three-dimensional model of the kidney to study for a better understanding of its function. As an alternative, dissect a fresh or preserved kidney from any of the livestock species.

5. Draw and label your own diagram of the reproductive tracts of the mare and stallion.

6. Develop a report on the senses: sight, smell, hearing, touch, and taste. Describe how these sensations are transmitted to the brain and interpreted. How is pain sensed and interpreted? In the report, draw diagrams of the various sensory receptors.

7. Create a crossword puzzle of the various hormones, using their site of origin and action as the hints.

8. Construct a model of the lungs using a bottle, some tubing, and balloons. Demonstrate how the movement of the diaphragm fills the lungs. Details can be found in a variety of old laboratory manuals.



Budras, K-D., Rock, W., Rock, S., & Sack, W. (2004). Anatomy of the horse. London: Manson Publishing.

Frandson, R. D., Fails, A. D., & Wilke, W. L. (2003). Anatomy and physiology of farm animals (6th ed.). Philadelphia: Lippincott Williams & Wilkins.

Frandson, R. D., & Spurgeon, T. L. (1992). Anatomy and physiology of farm animals (5th ed.). Philadelphia: Lea & Febiger.

Hafez, E. S. E. (2000). Reproduction in farm animals (7th ed.). Philadelphia: Lippincott Williams & Wilkins.

Kahn, C. M. (Ed.). The Merck veterinary manual (9th ed.). Whitehouse Station, NJ: Merck & Co.

McCracken, T. O., & Kainer, R. A. (1998). The coloring atlas of horse anatomy. Loveland, CO: Alpine Publications.

McKinnon, A. O., & Voss, J. L. (1993). Equine reproduction. Oxford, UK: Blackwell Publishing, Ltd.

Equipment and Supplies

Carolina Biological Supply Company, Carolina Science and Math Catalog 66, 2700 York Rd., Burlington, NC 27215-3398 <>

NASCO Agricultural Sciences, 901 Janesville Ave., Fort Atkinson, WI 53533-0901 <>

Nebraska Scientific, 3823 Leavenworth St., Omaha, NE 68105-1180 <http://www.>

Fisher Science Education, 4500 Turnberry, Hanover Park, IL 60133 <>


Internet sites represent a vast resource of information, but remember that the URLs (uniform resource locator) for World Wide Web sites can change without notice. Using one of the search engines on the Internet such as Yahoo!, Google, or, find more information by searching for these words or phrases relating to physiology or life functions of horses:










Table A-18 in the appendix also provides a listing of some useful Internet sites that can serve as a starting point for further exploration.
TABLE 5-1 Major Endocrine Glands and Hormones

Gland              Hormone                Function

Hypothalamus       Releasing hormones     Controls the pituitary gland

Posterior          Oxytocin               Stimulates uterine
pituitary                                 contractions and milk

                   Vasopressin or ADH     Increases water absorption
                                          in kidney

Anterior           Growth hormone (STH)   Promotes growth of most
pituitary                                 tissues

                   Prolactin (LTH)        Promotes lactation;
                                          stimulates corpus luteum

                   Adrenocorticotropic    Stimulates adrenal cortex
                   hormone (ACTH)

                   Thyroid-stimulating    Stimulates thyroid gland
                   hormone (TSH)

                   Follicle-stimulating   Stimulates follicle growth
                   hormone (FSH)          on the ovaries and sperm
                                          production in the male

                   Luteinizing hormone    LH stimulates ovulation,
                   (LH)/Interstitial      corpus luteum function,
                   cell-stimulating       secretion of progesterone,
                   hormone (ICSH)         and secretion of estrogen
                                          in the female; ICSH
                                          facilitates production of
                                          testosterone in the male

Pineal             Melatonin              Aids in adaptation to
                                          light-dark cycles

Thyroid            Thyroxine              Controls metabolism and
                                          affects growth,
                                          reproduction, and nutrient

                   Thyrocalcitonin        Decreases blood serum
                                          levels of calcium

Parathyroid        Parathormone           Regulates metabolism of
                                          calcium and phosphorus

Pancreas           Insulin and glucagon   Regulate glucose metabolism

Adrenal cortex     Glucocorticoids        Stimulate conversion of
                                          protein to carbohydrates
                                          for energy; decrease
                                          inflammation and immune

                   Androgens              Regulate masculine
                                          secondary sexual

                   Mineralocorticoids     Regulate sodium and
                                          potassium metabolism

Adrenal medulla    Epinephrine and        Prepare animal for
                   norepinephrine         emergencies; mobilize energy

Testes             Testosterone           Develops and maintains
                                          accessory sex glands;
                                          stimulates secondary sexual
                                          characteristics, regulates
                                          sexual behavior and sperm

Ovary              Estrogen               Promotes female sexual
                                          behavior; stimulates
                                          secondary sexual
                                          characteristics, growth of
                                          reproductive tract, mammary
                                          growth, and feedback control

                   Progesterone           Prepares uterus, maintains
                                          pregnancy and prepares
                                          mammary glands for
                                          lactation, and provides
                                          feedback control

                   Relaxin                Facilitates dilation of
                                          birth canal

Gastrointestinal   Secretin,              Control secretions and
tract              enterokinin,           motility of digestive tract
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Article Details
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Author:Parker, Rick
Publication:Equine Science, 3rd ed.
Article Type:Table
Geographic Code:1USA
Date:Jan 1, 2008
Previous Article:Chapter 4 Cells, tissues, and organs.
Next Article:Chapter 6 Biomechanics of movement.

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