THE SENSIBLE FROG THE NAVICULAR BONE, &c JOI 



{/foceeds over the bars, and there is covered by some laminae, to unite with those 

 that have been described, page 295, as found in the bars. It is here likewise thicker 

 and more elastic, and by its elasticity is evidently assisting in obviating concussion^ 

 It is supplied with nervous fibres, and is highly sensible, as the slightest experience 

 horses will evince. The lameness which ensues from the pressure of a stont 01 

 of the shoe on the sole is caused by inflammation of the sensible sole. Corns result 

 from bruise and inflammation of the sensible sole, between the crust and the bar. 



THE SENSIBLE FROG. 



The coffin-bone does not occupy more than one-half of the hoof. The postenoi 

 part is filled by a soft mass, partly ligamentous, and partly tendinous (o, p. 272). 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 w'aole 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 p3rforans flexor tendon, which 

 passes over the navicular bone, e, p. 272, 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-hone, e, p. 272, 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 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 

 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 mus- 

 cular 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 navicular bone ; yet at the same time we are aware of the benefit which accrues 

 (see page 272) 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, how- 

 ever, 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 272), 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 eithei 

 side, except at the protuberance which receives the extensor tendon e, pa<re 272, occu- 

 pied 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 quarteis ha.lf an inch or 

 more above the hoof, and diminishing in height forward and backward. These car- 

 d'ages occupy a greater portion of the foot than does the coffin-bone, as will be seen 

 in the lowest cut, page 276, where they are represented as extending fa r behind the 

 coffin-bone. They are held in their situation not merely by this groove, but by othei 

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