The equine skeletal system

Whether a horse will be fast or sound depends a great deal upon the skeletal structure he was born with(Editor's Note: This is the first in a series of monthly articles written by Douglass Hutchins, DVM, which will concentrate on topics pertaining to training racehorses. The series will include such subjects as equine anatomy, physiology, lameness, medications, and shoeing. Each monthly column will include a test. The answers will be presented in the following week's issue of Thoroughbred Times.)

A KNOWLEDGE of the basic anatomy of the horse is essential to understanding how this incredible animal functions. This knowledge should aid the individual horseman in developing proper husbandry techniques and more effective training methods, and in diagnosing and treating the disease conditions to which the racehorse is prone.
The normal adult skeleton of the horse consists of 205 bones, as shown in the following table:

• Vertebral column 54
  
• Ribs 36
  
• Sternum 1
  
• Skull 34
  
• Thoracic limbs 40
  
• Pelvic limbs 40
  
• TOTAL 205
  

The skeletal system can be further divided into the axial and appendicular sections. The axial system (vertebral column) is the fundamental part of the skeleton and consists of a chain of irregular-type bones that connect from the skull to the end of the tail. Certain sections of adult horses' vertebrae become fused or "fixed" to give the horse added strength, and those vertebrae that remain "movable" give the horse his necessary flexibility.
The vertebral column is subdivided into five regions according to the sections of the body to which they are situated. These regions are designated as cervical, thoracic, lumbar, sacral, and coccygeal. The number of vertebrae in a given horse is fairly constant in each region except for the tail section. This gives the horse a strong framework in which to support the skull, muscle mass, limbs, and internal organs.
The equine skull is an elongated, four-sided pyramid-type structure built for maximum aerodynamics. The wide nasal openings and sinus cavities provide efficient airflow into the lungs and the rapid expiration of metabolized carbon dioxide. The eye placement gives the horse the ability to see not only at a distance, but also peripherally. (This accounts for the accuracy of those well-placed hind feet when the horse decides to kick.)
The eyes are also well protected by the arched orbital bones, and the nerve supply is cleverly arranged on the opposite side of the brain. This contra-lateral type innervation is nature's way of protecting the delicate eye structures. Thus, injury to the right side of the horse's skull could damage the physical aspects of the right eye, but the innervation to that eye is maintained, being located on the left side of the brain and well protected from this injury.
The horse's ears are located at the top of the skull and funnel-shaped for maximum air flow and sound detection. They are well innervated and, when erect and moving side-to-side or front-to-back, indicate an animal that is acutely aware of his surroundings and possible impending danger.
The appendages of the horse are well muscled at the shoulders and pelvic areas and then become streamlined for agility, speed, and endurance. The shoulder and pelvic areas contain strong muscles that boast very efficient blood and nerve supplies. These muscles are complemented by strong tendinous insertions to critical areas of the skeletal structures, allowing for rapid and efficient limb placement. The horse also has a unique combination of ligamentous structures which provide additional support to the skeletal system.
It is important to understand the differences between tendons and ligaments. Tendons are the cord-like insertions of muscles to bones. As muscles become fatigued and weakened, the elasticity and flexibility of the limb becomes compromised and accounts for the tearing and bowing of these structures. Ligaments are bands of fibrous tissues that connect bones and cartilages, thus serving to strengthen and support the joints and skeletal system. Certain parts of the ligamentous structures have combined to form the so-called "stay apparatus," allowing the horse to stand and support himself for long periods of time while exerting minimal energy.
Tendons and ligaments have limited blood supplies and therefore require long periods of rest for proper healing. Ligaments tend to have excellent innervation, and because of this, injuries to them are extremely painful-and in some circumstances, almost impossible to heal. Hence the old saying that sometimes a sprain can be worse than a bone fracture.
When one observes the horse from the knees and hocks down, his fore and hind limbs are similar-each possessing a cannon bone, two splint bones, the ankle or fetlock joint with the two sesamoid bones, the first and second phalanges, and the third pedal bone and navicular bone encompassed by the hoof (Figure 1). Compare this to your hand, with the main digit of the horse being your middle finger, the splint bones being the fingers to each side, and your thumb being the rudimentary chestnut. This single digit provides the horse with extreme speed and agility. However, supporting this half-ton frame on these single bony structures-especially during periods of extreme speed-can be overpowering, and accounts for the horse's high incidence of injury. The additional weight of the rider and the horse's center of gravity being located just behind the shoulder adds additional stress and weight to the front limbs, accounting for their higher incidence of lameness and injury.
The skin of the horse is an efficient covering that provides for maximum strength and protection while allowing for sleekness and minimal air resistance during running. The skin varies in thickness from one-to-five millimeters and is thickest at the attachment of the mane and dorsum of the tail. The sebaceous and sweat glands are more numerous in the horse than in most other domesticated animals. This allows the horse to sweat profusely during exercise, and as this sweat evaporates, provides for rapid cooling of the horse's body.
When trying to assess the conformation of a specific horse, stand back and observe the animal from a distance. Imagine the horse in a picture frame, and observe the whole animal. Examine for a well-positioned, symmetrical-type head, and a balanced body with a deep chest, and well-proportioned legs. Compare the two front limbs to each other and the same for the rear. Uneven conformation or an enlargement of one joint in comparison to the same joint on the opposite leg could indicate pathology, and one foot larger than another could indicate previous lameness. Generally, for a horse to have good foot conformation, he must have reasonably good limb conformation. Remember the adage: "No frog, no foot-no foot, no horse."
It is important to remember that the dynamics of the horse in motion are influenced by a number of factors, and the horse's conformation is the one aspect that cannot be changed to a great degree, since it is inherited. It has been said that if a horse has speed, but has bad conformation, he is apt to be a short-term racehorse. However, if a horse has speed and good conformation, he is more apt to be a long-term racehorse.
One practice I have always employed is to go to the track and observe the best horses on the grounds. Most of the time, these animals will not only possess superior breeding, but superior physical and skeletal characteristics as well. While you may not be able to afford that royally bred animal, you should look for something in your price range that posesses similar physical characteristics.
One day while I was at the track, an owner came over and began discussing his horse with a trainer. He explained his frustration over the fact that the horse was performing poorly, and that the veterinarians who had examined him could not find anything wrong. At about that time the owner yelled, "There goes my horse now!" The trainer looked at the horse going for his workout and replied, "I can tell you what's wrong with your horse."
"You can? What?" said the astonished owner.
The trainer replied, "It's right in his feet. He can't move them fast enough!"

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Douglass B. Hutchins, MS, DVM, is a veterinarian in the Professional Services Department of Fort Dodge Laboratories.