258 STRUCTURE OF THE FOOT. 



the coffin-joint. It is united to both ; the tendon and 

 the bone being joined to it by ligaments : and below 

 the navicular-bone presents a free polished surface, 

 under which the tendon plays. The navicular-bone 

 partially yields with every motion of the tendon ; thus 

 preventing a great deal of that concussion which must 

 otherwise have taken place, had the tendon sharply 

 turned over a fixed bone, in order to be inserted into 

 the coffin-bone. 



The navicular-bone, when the pastern presses upon 

 it, descends; the tendon descends with it, and there 

 is much weight pressing upon both. Then there is 

 interposed between the bottom of the foot another 

 highly elastic substance, which is destined to receive 

 this pressure and yielding as it receives it, obviates 

 dangerous concussion : — the internal or elastic frog — 

 upon the cushion on which the tendon and the navi- 

 cular-bone rest. 



This is the important function of the internal frog ; 

 but there is another quite as valuable. The horny 

 covering, which envelopes the foot of the horse, from 

 its very nature may occasionally subject the foot to 

 considerable pain and inconvenience. It contracts 

 when exposed to dryness or heat. The feet of our 

 stabled horses are too liable to injury from this source ; 

 and there is the fetter of the shoe, which still more 

 disposes the horn to contraction. But above all is the 

 stagnation to which our stabled horses are doomed. 

 Throw a part out of use, and it will very speedily be- 

 come absorbed. 



The elastic frog yields to the pressure of the de- 

 scending tendon and navicular-bone ; how does it 

 yield — can it be squeezed into a smaller compass? 



