THE ARM. 239 



in velocity. The product of the power, and the space passed over by the 

 arm of the lever to which it is attached, must always be equal to the pro- 

 duct of the weight, and of the space passed over by the arm that supports 

 it ; and if a power, equal to thirty times the weight, is obliged to be 

 exerted at the upper part of the bone of the arm, the centre of that bone., 

 which may be considered as the centre of the weight, will pass over thirty 

 times the space, and the extremity of the limb will pass over sixty times 

 the space. The muscle will contract with a great deal of rapidity, but the 

 foot will move with sixty times that rapidity, in order to pass over sixty 

 times the space in the same time. This is precisely what we want. Exten- 

 sive and rapid motion are the excellencies of the horse. He is valuable in 

 proportion as he has them combined with stoutness; and by this confor- 

 mation of the limb alone could he obtain them. Therefore the tendon is 

 at first unusually strong ; it plays through the natural but perfect pulley 

 of the bone of the arm without friction ; the body of the muscle is mixed 

 with tendinous fibres, and the insertion into the fore-arm is very extensive, 

 lest the application of such immense force should tear it from its adhe- 

 aions. There is sufficient strength in the apparatus; the power may be 

 safely applied at this mechanical disadvantage ; and it is applied close to 

 ;he joint to give an extent and rapidity of motion which could not otherwise 

 have been obtained, and without which the horse would have been compar- 

 atively useless. 



At the back of the arm are other flexor muscles of great power, to bend 

 the lower portions of the limb. We have described two of them belonging 

 to the arm and the leg, and we must not pass over the very peculiar ones 

 acting on the feet. Only a small portion of one of them can be seen in our 

 cut, page 233, at 1. 



The first is the perforated flexor muscle : the reason of the name will 

 presently appear. It rises from the lower and back part of the inner head 

 of the lower bone of the shoulder, and intermixed with, or rather between 

 the origins of the muscle next to be described, and called the perforating 

 muscle. As it descends along the bone of the arm, it becomes tendinous ; 

 and, approaching the knee, it is bound down by arches or bands of liga- 

 ment, that it may not start in sudden and violent action. Proceeding from 

 the knee it widens, and partly wraps round the tendon of the perforating 

 muscle, and they run down together in contact, yet not adhering; freely 

 playing over each other, and mucous fluid obviating all friction. Both of 

 them are inclosed in a sheath of dense cellular substance, attached to them 

 by numerous fibrils; and they are likewise supported by various ligament- 

 ous expansions. Near the fetlock the tendon still farther expands, and 

 forms a complete ring round the tendon of the perforating muscle. This 

 is seen at J, p. 111. The use of this will be best explained when we are 

 treating of the fetlock. The perforated tendon soon afterwards divides, and 

 is inserted into the smaller and larger pastern bones, and flexes or bends them. 



The next is the perforating flexor muscle. It has nearly the same origin 

 as the other, but with somewhat distinct heads. It continues muscular 

 farther down the arm than the perforated, and lies before it. At the knee 

 it passes, like the perforated, under strong ligamentary arches, which 

 confine it in its situation. It then becomes round, and is partly wrapped 

 up in the perforated ; and at the fetlock is entirely surrounded by it. It 

 emerges from the perforated when that tendon divides, and continues its 

 progress alone after the other has inserted itself into the pasterns, and 

 passing over the navicular bone, terminates on the base of the coffin-bone, 

 or bone of the foot. 



It is sufficiently plain that the arm should be large and muscular, 



