m THE HORSE. 



THE NAVICULAR BONE, 



This navicular bOne is placed behind and below the lower pastern-bond 

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

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

 streni^thenin^ the union between these parts ; in receiving; a portion of the 

 wei"-ht which is thrown on the lower pastern ; and in enabling the flexor 

 tendon to act with more advantage. Supposing that this tendon were in- 

 serted 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 in- 

 serted 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 powe^, from the centre of motion, 

 which is here the place where the tendon passes over the end of the coffin- 

 bone: but when the navicular bone is interposed, the centre of motion be- 

 comes 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 produced by the introduc- 

 tion of the knee-bone, considerably more advantageous. We are inclined 

 to believe that this is the principal nse of the navicular-bone ; but at the 

 same time 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 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 liga- 

 ments. 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 advan- 

 tage, that when the pastern is extended (see page 24^^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 

 d, p. 253, occupied by cartilage, which, like the crust, is convex outwards, 

 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, ])age 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 connexions with the coffin-bone, the navicular-bone, and the 

 flexor tendon, and are thus perfectly secured. 



