Overview of Equine Body Systems
Dividing the parts of the horse into different systems, like any system of classification, is somewhat arbitrary. For instance, are teeth part of the skeletal system, or part of the digestive system? However, it can still sometimes be useful to consider the traditional body systems and their functions.
Horses are members of phylum Chordata, sub-phylum Vertebrata, along with fishes, amphibians, reptiles, birds and other mammals. All these animals share the characteristic that they have internal skeletons which include a backbone. The horse's "backbone" stretches along the topline from the poll in front into the tail in the rear, and is not actually a single bone, but linked vertebrae. The horse's skeleton as a whole supports the horse's body, defines the basic shape or framework, protects delicate internal organs and provides attachment points for the muscles that make movement possible. It also houses the bone marrow, which is where blood cells form. Ligaments help connect bones.
Muscles attach to the skeleton (by way of tendons), and do the work of moving parts of the animal. Muscles can work only by contracting, pulling - they cannot push. Therefore, many muscles are in pairs - one to extend (or straighten) a joint, and the other to flex (or bend) that joint. There are three main types of muscle - skeletal muscle, which is what we usually think of when we think 'muscle', and which helps the horse walk, trot, gallop, eat, or look around; smooth, or involuntary, muscle, like the muscles of the digestive system which push the food through the alimentary canal; and heart muscle. The skeletal muscle fibers may be classed as two kinds: slow twitch and fast twitch. Draft horses and sprinters tend to have more 'fast twitch' muscle fibers, which are more powerful in the short term. Endurance horses and stayers tend to have more 'slow twitch' muscle fibers, which can work longer. In order to work efficiently, muscles must be able to relax between contractions - a muscle that is constantly contracting without frequent, short breaks to relax will become tired, stiff, and more prone to injury. Muscles generally get stronger with work, but overworked muscles will not be able to repair minor damage quickly enough, and may be torn, or strained.
The respiratory system is the breathing apparatus - nostrils, larynx, trachea and lungs. Unlike humans, horses cannot easily breath through their mouths, but must move all air in and out through their nostrils. Like humans, the horse's respiratory system is vulnerable to colds, coughs, pneumonia and even "emphysema" (Chronic Obstructive Respiratory Disease - often called heaves in horses). Horses must often live in dusty environments, and eat dusty hay, stressing their lungs. As athletes, horses need healthy lungs to perform well.
We humans can speed up our breathing when we are running and get out of breath. A horse can do the same thing at the walk or trot, but not at a gallop. If you listen to a horse breathing at a gallop, you will be able to hear that the rhythm of the breathing is the same as the rhythm of the gallop. There are two main reasons for this:
1. During the suspension phase of the gallop, the horse is "flying" through the air at 15-45 miles per hour. When the hooves touch down, the horse's body is briefly slowed down. There is a law of physics that says "objects in motion tend to stay in motion". The horse's intestines (and their contents) slosh forward against the lungs, just as you would be "pushed" forward inside a car when the driver slams on the brakes. This makes it very difficult for the lungs to expand at that instant in the stride.
2. At the gallop, both hind legs come forward almost together. Some of the muscles that bring the hind legs forward are the abdominal muscles. It would also be difficult for the lungs to expand when those abs are contracting. Try breathing in while you are on the "up" phase of doing sit-ups or crunches, and you will see what I mean.
As do all mammals, horses possess a four-chambered heart which receives blood from the veins, and pumps it out into the arteries to keep the entire body supplied with oxygen and nutrients, and to move out waste products like carbon dioxide. Along with the heart and blood vessels, the circulatory system includes the blood, the lymph nodes and vessels, and lymph. The lymph system is part of the immune defences which protect the horse from disease.
Unlike humans, horses rarely experience 'heart attacks'. Heart failure is quite rare in horses.
The main components of the digestive system are the various sections of the long tube, sometimes called the alimentary canal, that starts in the mouth and leads back to the anus. Its parts, in order from front to back, are called the mouth, pharynx, esophagus, stomach, small intestine, cecum, large colon, small colon and rectum. Other organs which assist in digestion are the teeth, salivary glands, pancreas and liver.
The alimentary canal is roughly 100 feet long in an average horse, but the stomach is relatively small, holding 3-4 gallons when full. This is one reason it is better to feed horses small quantities at frequent intervals. Also, if the horse is fed large quantities at once, some of the food will be pushed out of the stomach before the digestive acids and enzymes have had enough time to properly begin the digestive process. Pressure in the stomach pushes the membrane between the esophagus and stomach closed, which is why it is extremely rare for horses to vomit. If a horse does vomit, stomach contents will come out of the nostrils, not the mouth, because the soft palate blocks off the mouth.
The small intestine, around 70 feet long, is where most of the fats, sugars and proteins are absorbed into the bloodstream. The fiber from feeds goes on into the cecum and large colon, where beneficial bacteria help break down the fiber, releasing more nutrients which the horse absorbs, along with most of the water still in the digestive tract. Some vitamins are manufactured in the intestines, and vitamins and minerals are absorbed. In the small colon, what is left of the undigested feed is formed into the little balls of manure typical of healthy horses, which is stored in the rectum until the horse defecates.
The most common problem of equine digestive systems is colic, although horse may also develop stomach ulcers.
See also 4-H Horse Science pp.23-25
The Nervous System
The nervous system consists of the central nervous system (the brain and spinal cord), and the peripheral nervous system (the nerves in the rest of the body). Through the nerves to the sensory organs (ears, eyes, nose, tongue and skin), the horse perceives information about what is going on around him. Through the nerves to all the organs and systems of the body, the horse's brain coordinates all activities, including breathing, heartbeat, movement, and digestion. Although it is somewhat of an oversimplification, you might consider the nervous system to be the communications and control department for the body.
The horse's brain is fairly well protected inside the skull, but a severe blow to the poll can kill a horse by damaging the brain. This is most likely to happen if a horse rears suddenly and strikes his head on a beam or other solid object, or if a horse panics and flips over backward on concrete or rock. The spinal cord is likewise well-protected inside the vertebrae, but a serious neck or back injury can cut or pinch the spinal cord, causing paralysis or death.
Diseases and problems that affect the nervous system include rabies, encephalomyelitis, tetanus, botulism, wobbles and various kinds of poisoning.
The Reproductive System
Horses have a typical mammalian reproductive system. An (ungelded) male has testicles and a penis, which is usually protected inside the sheath. A mare has ovaries, a uterus and vagina. During the breeding season (from early spring till early autumn) the mare becomes fertile at roughly three-week intervals. The several days when she will accept the stallion, she is said to be "in heat". Mares may be spayed, but this procedure is much less common than gelding. Fertility problems, abortions, and sexually transmitted diseases are all problems in horses. When it is time for a foal to be born, mares generally deliver the foal quickly and easily, with no help.
Other systems you may read about include the excretory system (mainly the kidneys & bladder, which get rid of waste products), the endocrine system (mainly the glands which secrete hormones), the immune system (which protects the horse's body from infection), and the integumentary system (mainly the skin). Kidney and bladder problems are quite rare in horses. Older horses sometimes suffer from Cushings disease, which is an endocrine problem. The most common skin problems are caused by fungus or external parasites (mites or lice).
See also Basic Horsemanship pp. 283-289
USPC Manual 3 pp. 223-250
UC Davis Book of Horses pp. 145-300
Equine Science pp. 96-137
EQUUS Illustrated Handbook of Equine Anatomy
Illustrated Atlas of Clinical Anatomy and Common Disorders of the Horse