THE CARTILAGES OF THE FOOl. 30 1 



a joint with both bones, and answers a very important office in 

 streuirthening the union between these parts, in receiving a por- 

 tion of the weight which is thrown on the lower pastern and in 

 enabhng the flexor tendon to act with more advantage. Suppos- 

 ing that this tendon were inserted into the coffin-bone without the 

 intervention of the navicular bone, it Avould act at great mechan- 

 ical disadvantage in bending the pastern, for it is inserted near 

 the end of the coffin-bone, and the weight, concentrated about 

 the middle of the bone, is far ofi^, and requires a great power to 

 raise it ; but when the navicular-bone is interposed, the centre of 

 motion becomes the posterior edge of that bone, where it is in 

 contact with the tendon, and then it will be seen that the distance 

 of the power from the centre of motion is nearly or quite the 

 same as the weight, and very great expenditure of muscular 

 power will be saved. In the one case, the power must be at least 

 double the weight, in the other they will be nearly equal ; and 

 also the angle at which the tendon is inserted, is considerably 

 more advantageous. Perhaps this is the principal use of the na 

 vicular-bone ; yet at the same time we are aware of the benefit 

 which accrues (see Fig. 37) i'rom a portion of the weight bemg 

 taken from the coffin-bone, and thrown on the navicular-bone, 

 and from it on the tendon, and the tendon resting on the elastic 

 frog underneath. 



THE CARTILAGES OF THE FOOT. 



There is a groove extending along the upper part of the coffin- 

 bone and on either side, except at the protuberance Avhich re- 

 ceives the extensor tendon e, Fig. 37, occupied by cartilage, wliich, 

 like the crust, is convex outwards and concave inwards. It ex- 

 tends to the veiy posterior part of the foot, rising about the quar- 

 ters half an inch or more above the hoof, and diminishing in 

 height forward and backward. These cartilages occupy a greater 

 portion of the foot than does the coffin-bone, as will be seen in 

 Fig. 40, where they are represented as extending far behind the 

 coffin-bone. They are held in their situation not merely by this 

 groove, but by other connections with the coffin-bone, the navi- 

 cular bone, and the flexor tendon, and are thus perfectly secured. 



Below are other cartilages connected with the under edges of 

 the former, and on either side of the frog. 



Between these cartilages is the sensible frog, filling up the 

 whole of the space, and answering several important purposes. 



