In a horse, the bones provide a rigid framework for support, protection and movement. In addition, they help store minerals such as calcium and phosphorus. Bones are held together by ligaments, which connect bone to bone and muscles to bone, and tendons, which attach muscles to bone. The skeleton also contains cartilage, which reduces friction between bones and enables them to move easily. A tough membrane called periosteum covers the outside of each bone. The inside of a joint is lined with synovial fluid, which lubricates the joints.
The horse has three different types of skeletal bone – the axial skeleton, which includes the skull, spine and sternum; the appendicular skeleton, which attaches to the limbs; and the pelvis and ribs, which form the thoracic skeleton. The axial skeletal system is attached to the backbone, which in turn is attached to the ribs.
There are eight “true” pairs of ribs in the horse, and there is one false pair at the end of the sternum. The horse’s sternum is canoe-shaped, and is compressed laterally except in the caudal portion. The lateral part of the bone is thin and sharp, but the medial portion is thick and rounded.
Each digit of the hindlimb has a proximal, middle and distal phalanges. The proximal phalanx is shorter and wider proximally than the distal phalanx, and the middle phalanx is narrower than the proximal phalanx. Each digit also has two sesamoid bones, which are small, round bone-like structures that sit in the middle of the digit. The sesamoids are used by the flexor tendons to attach the muscles to the bones in the digit, and they provide a pulley through which the tendons can run.
The forelimb of the horse has a humerus, radius, ulna and metacarpals. The humerus has a cylindrical body, which is flattened laterally and larger distally in the horse than in other mammals. The lateral surface of the humerus has a large eminence on its proximal side known as the lesser trochanter. The distal surface of the humerus has trochlea and is compressed craniocaudally. It also has a well-marked radial tuberosity on its dorsal side.
In the premaxilla of the horse’s skull, there are six alveolar sockets for upper incisors. The infraorbital foramen is found midway between the facial tuberosity and the nasoincisive notch, and is covered by the levator labii superioris muscle (Fig. 18-8) – it has to be pushed aside to locate the foramen.
The horse’s fetlock has dorsal and ventral conchae, with dorsal, middle and ventral meatuses. It also has a nasal septum with both cartilaginous and bony parts, as well as a dorsal and ventral nasolacrimal sac.