402 THE NAVICULAR BONE. 



THE SENSITIVE FROG. 



Tlie coffin-bone does not occupy more tlian one half of the hoof. The 

 posterior part is filled by a soft mass, which is fibrous and elastic in its 

 nature. Its shape below corresponds with the cavities of the horny 

 frog ; in front it is attached to the inferior part of the coffin-bone ; 

 and farther 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, 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 cofiin-bone, 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 advan- 

 tage. Supposing that tliis tendon were inserted into the coffin-bone 

 without the intervention of the navicular 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 requii-es 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 expen- 

 diture 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 advan- 

 tageous. Perhaps this is the principal use of the navicular bone ; yet at 

 the same time we are aware of the benefit which accrues from a portion 

 of the weight being taken from the coffin-bone, and thrown on the navi- 

 cular bone, and from it on the tendon, and the tendon resting on the 

 elastic frog underneath. The navicular bone is sometimes, but inaccu- 

 rately, 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, howevei', 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 extend- 

 ing 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, the navicular bone is placed 

 in that situation which enables the flexor tendon to act with greatest ad- 

 vantage 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, occupied by cartilage, which, like the crust, is convex outwards 

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

 I'ising about the quarters half an inch or more above the hoof, and dimi- 



