Fig.16 Fig. 13. Ground surface of hoof. Fig. 14. Vertical section of horse's foot. Fig. 15. Dissection of horse's foot. Fig. 16. Posterior view of the coffin bone. Fig.17. cone-like mass, as viewed on its inner upper surface, has received the name of frog stay or bolt. Looking upon both the exterior and interior surface of the frog, we see that with the bars it forms three elastic foldings, which act as springs to keep the heels apart and the foot well spread. In the natural, unshod foot, the frog, though protected to some degree by the solar border of the wall and by the sharp prominences of the bars, must still receive pressure at each step. The order of force in which the different parts of the foot press the ground in walking, running, etc., has been stated to be as follows: First, the solar body of the wall; second, the bars; and third, the frog. In the foot that has never been shod the frog has nearly if not quite as much pressure in the full step as the wall. In rapid stepping, the edge of the wall, which is nearest the point of the coffin-bone, receives the first force of the blow, while the frog which mainly rests upon the elastic heels, a much more yielding substance, receives the weight as the foot settles back to its level. The effect of pressure is to flatten the arches of the commissures and cleft, to widen the frog, throw out the heels, and keep the foot freely expanded. The elasticity of the step of the horse is the result of a highly compound arrangement first, the elasticity of the sensitive laminæ; second, the greater elasticity of the sensitive sole; third, the elasticity of the horny wall; fourth, the arch of the sole; and, fifth, the triple spring formed by the foldings of the frog and the manner of its union with the bars. The movements of the foot are produced by two sets of muscles, flexors and extensors, similar to the distribution of a single finger in man. The flexors are two in number, and are situated upon the posterior aspect of the leg. In the fore legs these muscles are the flexor sublimis perforatus, (Fig. 14, "6,") and the flexor profundus perforans, (Fig. 14, 7,") also called in works on farriery flexor pedis perforatus and ficxor pedis perforans, also familiarly designated as the common and deep flexor muscles. These muscles take their origin in common from the internal protuberance of the humerus, and are united for a considerable distance down the arm, when they separate to form two distinct tendons. Of these, that belonging to the perforatus runs beneath the annular ligament of the carpus, to be inserted into the upper and back part of the lower pastern or coronet bone. Just before reaching the pastern joint, this tendon divides, to allow the tendon of the perforans to pass through it. Each division where it plays over the joint has in it a sesamoid bone. The tendon of the perforans, lying deeper above, passes between the divided tendon of the first-named muscle, to be inserted into the posterior concavity of the coffin-bone. Attached to this tendon, as it passes over the joint formed by the coronet and coffin-bones, is the navicular bone, considered as a sesamoid bone in this tendon. Two supplementary flexors, the accessorii, are described as arising from the posterior and inferior aspect of the ulna and the radius, and uniting below with the two main flexors. These muscles flex the foot upon the leg. As antagonists to these, there are three extensor muscles situated upon the front of the leg, corresponding to the extensor communis digitorum and extensor minimi digiti of the human arm. The extensor communis, otherwise called in hippotomy extensor pedis, arises from the external condyle of the humerus, and from contiguous fascia, and from the upper and lateral part of the radius, and has a strong, fleshy belly, which terminates in a single tendon which passes down over the front of the leg to be inserted into the coronal process of the last phalanx - the coffin-bone. It unites by a slip with the tendon of the next muscle. The extensor proprius minimi digiti is represented in the horse by two muscles. One of these, called the extensor of the pastern, is inserted by a strong tendon into the side of the first phalanx, the pastern bone. The second muscle, placed between the two preceding muscles, furnishes a strong tendon which passes down in front of the carpus and becomes united with communis at an acute angle. The united tendon (Fig. 14, "5") passes behind the coronary border of the hoof to its insertion in the coffin-bone. The office of these muscles is to extend the foot upon the leg. Another muscle, the adductor longus pollicis, called in hippotomy the oblique extensor of the cannon, by its insertion into the base of the cannon-bone, acts as an extensor of the foot. A similar arrangement exists in the muscles of the hind leg. The tendon of the plantaris, of great strength, has a divided insertion corresponding to that of the flexor sublimis perforatus of the fore leg, while the flexor perforans sends its single strong tendon between the two divisions of the preceding muscle to be inserted into the terminal phalanx. These flexors of the foot are assisted by the tendon of the flexor hallucis, which unites with |