If we examine the under surface of the foot, we find another provision against jar, for whilst the sole rests upon a bony basis, the frog does not (fig. 629). The body of the coffin-bone only extends backwards to about an inch past the point of the frog. It there divides into two processes which extend nearly to the heels, but leaving between them a large space which is filled by a pad of elastic material, over which the frog rests. This arrangement permits the frog great freedom of movement, and gives to the back portion of the hoof the special feature of elasticity so necessary to its function of breaking concussion when the foot comes to the ground during progression. The front part of the foot, by the thickness and hardness of the wall, and by the rigid basis of bone within, is specially fitted to sustain the strain which is placed upon it when the toe takes the weight of the horse, as it does in all forward movements. The back part of the foot, by its thinner and more elastic horn, by its prominent and soft frog, and by the partial substitution of cartilage for bone as its inner basis, is specially endowed for receiving its first impact with the ground during progression. That the foot may preserve its functions intact the hoof must be maintained in its best form. No parts must be defective, and all must be proportionate. A foot denuded of horn may have its sensitive portions injured, and a foot covered by an excessive or disproportionate hoof may so destroy the balance of the limb as to cause grave lesions, resulting in lameness.
Fig. 629. - Under Surface of the Coffin-Bone, showing its Position within the Hoof.
a, Os pedis. B, Sensitive and insensitive laminae. c, Wall of hoof. D, Horny frog.
Shoeing is necessary to protect the foot by preventing wear of hoof, but shoeing by preventing wear leads inevitably to excessive growth of horn. Good shoeing, then, entails regular removal of shoes and systematic reduction of the overgrowth of horn. Before a shoe can be properly placed upon a foot, the hoof must be prepared for it, and this operation requires for its skilful performance a knowledge of the normal form of a horse's foot, of the proper proportion of its various parts, and some idea of the right relative position of the foot to the limb.