THE CARTILAGES OF THE FOOT 3791 



back, it adheres to the lower part of the cartilages of the heels, where they 

 begin to form the rounded protuberances that constitute the heel of the foot. 

 It occupies the whole of the back part of the foot above the horny frog and 

 between the cartilages. Running immediately above the frog, and along the 

 greater part of it, we find the perforans flexor tendon, which passes over the 

 navicular bone, e, p. 345, and is inserted into the heel of the coffin-bone. 



THE NAVICULAR BONE 

 Is placed behind and beneath the lower pastern-bone, and behind and above the 

 heel of the coffin-bone, e, p. 345, so that it forms a joint with both bones, and 

 answers a very important office in strengthening the union between these parts, 

 in receiving a portion of the weight which is thrown on the lower pastern and 

 in enabling the flexor tendon to act with more advantage. Supposing that this 

 tendon were inserted into the coffin-bone without the intervention of the navi- 

 cular bone, it would act at great mechanical 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 off, and requires a great power to raise it ; 

 out 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 j and also the angle at which the tendon 

 is inserted, is considerably more advantageous. Perhaps this is the principal 

 use of the navicular bone ; yet at the same time we are aware of the benefit 

 which accrues (see page 345) from a portion of the weight being 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 navicular bone is 

 sometimes, but inaccurately, said to descend with the motion of the foot. It 

 does not do that. It cannot ; for it is connected both with the pastern and 

 coffin-bones by inelastic ligaments. When, however, the horny bulb, with its 

 tuft of hair, at the back of an oblique fetlock, descends in the rapid gallop, and 

 almost touches the ground, the navicular bone, being as it were a part of the 

 pastern, must descend with it. With this exception, both in the extending and 

 the bending of the pastern, the navicular bone turns or rolls upon the other bones 

 rather than descends or ascends, and with this remarkable advantage, that when 

 the pastern is extended (see page 345), the navicular bone is placed in that 

 situation which enables the flexor tendon to act with greatest advantage in again 

 bending the foot. 



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 which receives the extensor tendon e, 

 page 345, occupied by cartilage, which, like the crust, is convex outwards and 

 concave inwards. It extends to the very posterior part of the foot, rising about 

 the quarters 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 the lowest cut, page 350, 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 connexions with the coffin- 

 bone, the navicular 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, 



