364 AGRICULTURAL REPORT. 
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, &c., has been stated 
to be as follows: first, the solar border 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 com- 
pound arrangement—first, the elasticity of the sensitive lamine; sec- 
ond, 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 mai. 
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 jlevor pedis perforatus and flexor 
pedis perforans ; also familiarly designated as the conmon 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 liga- 
ment 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 init 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 poste- 
rior 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 antag- 
onists to these, there are three extensor muscles situated upon the front 
of the leg, corresponding to the extensor communis digitorum and exten- 
sor 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 
I 
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