The horse’s skeleton is an incredibly strong structure. It provides support for the muscles, protection for internal organs, and possesses the flexibility of parts to allow the animal to move at various speeds, lie down or graze. The skeletal system is held together by ligaments, tendons and synovial membranes. Bones are covered with a tough protective layer called periosteum. Bones are also connected to each other by joints with articular surfaces. During the formative years, good nutrition is essential to ensure proper development and prevent stunted growth that can lead to bone damage and stiffness.
There are 205 bones in the equine skeleton. The pelvic girdle is comprised of the os coxae (ossa coxae on both sides), the sacrum and the first three or more caudal vertebrae. The os coxae are the largest flat bones in the body. They have three surfaces and three borders, and are wider dorsolaterally than palmarly. The sacrum is formed by five fused bodies that extend from the back of the axial skeleton to the hind limbs. The spinal cord is a central longitudinal cylinder, and the vertebrae have spines that are shorter from front to back.
The appendicular skeleton includes the forelimb and hind limb bones. The hind limb is supported by the pelvic girdle and attached to the axial skeleton by the femur. The femur is a long, slender bone with a well-developed shaft and a short humerus. The humerus is the uppermost bone in the shoulder joint, and moves the elbow.
A horse’s forelimb is made up of the humerus, ulna, scapula and coracoid. Unlike other limbs, the forelimb of a horse does not attach directly to the spine, but is suspended in place by tendons and muscles.
There are seven or eight carpal bones in the proximal row of the horse skeleton. The radial carpal is narrower and more compressed transversely than dorsally. The ulnar carpal is slightly wedge-shaped and wider dorsally than palmarly. The accessory carpal is shaped like a nut and positioned palmarly between the ulnar and radial carpals. The distal row of carpals is broader and more irregular than the proximal. The digits of the hind limb are each made up of three phalanges and two sesamoid bones.
The horse’s digits are smaller than those of other mammals. The proximal digital sesamoids are called the digit phalanx, and the distal digital sesamoid is known as the navicular bone. The digits have a strong lever effect that allows for great mobility.