Functional anatomy of the horse's foot.

frog. The wall is simply that part of the hoof that is visible when the horse is standing. It covers the front and sides of the third phalanx, or coffin bone. The wall is made up of the toe (front), quarters (sides) and heel. When the foot is lifted off the ground, the sole and frog are visible, as well as the bars of the wall and the collateral grooves (see Figure 1). The wall of the hoof is composed of a horny material that is produced continuously and must be worn off or trimmed off. The hoof wall does not contain blood vessels or nerves. In the front feet, the wall is thickest at the toe; in the hind feet the hoof wall is of a more uniform thickness. The wall, bars and frog are the weight-bearing structures of the foot. Normally the sole does not contact the ground. Inside the hoof, lateral cartilages extend back and up from the inner and outer sides of the third phalanx (Figure 2a). These cartilages are flexible, but as the horse ages, they are usually ossified and replaced by bone. Between the second and third phalanges and above the deep flexor muscle tendon is a small bone called the navicular bone (Figure 2b). The navicular bone and its associated bursa — a fluid-filled sac that reduces friction between the tendon and the bone — are involved in navicular disease, which is a common cause of lameness. The digital cushion is a mass of flexible material that contributes to the formation of the heels (see Figure 3). This structure is one of the primary shock absorbers of the foot. As weight is placed on the hoof, pressure is transmitted through the phalanges to the wall and onto the