288 THE HORSE. 



THE NAVICULAR-BONE. 



This navicular bone is placed behind and below the lower paslern-bone 

 and behind and above the heel ol" the coffin-bone, e, p. 249, so that it forms 

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

 ing the union between these parts; in receiving a portion of tlie 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 

 in a very disadvantageous way, 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 power to raise it proportionate 

 to the distance between the weight and the power, from tlie centre of 

 motion, which is here the place where the tendon passes over tlie end of 

 the coffin-bone : 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, like the angle pro- 

 duced by the introduction of the knee-bone, considerably more advantageous. 

 We are inclined to believe that this is the principal ir,e of the navicular- 

 bone; but at the same lime we are aware of the benefit which accrues (see 

 page 249) 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 ten- 

 don resting on the elastic frog underneath. The navicular-bone is some- 

 limes, 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: but 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 249), 

 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 ten- 

 don d, p. 253, occupied by cartilage, which, like the crust, is convex out- 

 wards and concave inwards, and which 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 cut, page 254, 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 navicular-bcne, and the 

 flexor tendon, and are thus perfectly secured. 



