The skeletal system has three main functions: it protects vital organs, provides framework, and supports soft parts of the body. A horse has 205 bones. Long bones aid in locomotion, store minerals, and act as levers. Short bones absorb concussion. They are found in joints such as the knee, hock, and fetlock. Flat bones enclose bodies cavities that contain organs, and the ribs are examples of flat bones. Irregular bones protect the central nervous system. Bones are connected to muscles by ligaments and tendons, and are covered by a tough membrane called periosteum.
A healthy skeleton is essential for the health and performance of horses. Proper nutrition during gestation and the early formative years of a foal help to ensure a strong bone structure. An overabundance of certain nutrients can stunt a foal’s growth, and an underabundance of others may lead to osteoporosis in adults.
Bones are the foundation of the skeletal system and are the first to experience wear and tear as an animal moves. Muscles then contract and expand to change the shape of the bones as they move. The interplay of contraction and decontraction of muscles, bones, and tendons enables movement.
The bones of the hoof are also important for the foot’s ability to bear weight and support a horse’s movements. The coffin bone — a ring-shaped bone at the bottom of the hoof — is the first supporting bone to become worn from bearing weight, and it needs to be well-formed to allow a horse to walk and run comfortably.
As a result of its great strength and speed, the horse has evolved to be a fast runner. The articular surface of the hind feet, which bear most of the horse’s weight, has developed an angled structure to distribute weight more evenly. This structure allows for the equinus to extend its legs when running and changing the direction of its head.
A skeleton from the Late Miocene era of North America is a good example of how horses evolved to become today’s animals. This equine, named Lexington and displayed at the Smithsonian National Museum of Natural History’s Bone Hall, resembled modern equines. It had long, curved toes and a large center toe that bore most of the horse’s weight. The skull was smaller, and the cheek teeth had become more molarlike, but the horse still had four premolars and three molars in each jaw. The foot remained three-toed, but the middle and then the distal toes were much larger than those of modern horses. The fetlock joint was less complex.