THE ARM. 335 



part of the front of the lower bone of the shoulder, and in as perfect a groove 

 or pulley as art ever contrived. This groove is lined with smooth cartilage ; 

 and between it and the tendon there is a secretion of oily fluid, so that the 

 tendon may play freely in the pulley without friction. Having escaped from 

 this pulley, and passed the head of the lower bone of the shoulder, the cord 

 swells out into a round fleshy body, still containing many tendinous fibres. 

 Deeply seated, it contributes materially to the bulk of the front of the arm, and 

 is inserted into the head and neck of the bone of the arm, and likewise into the 

 capsular ligament of the elbow joint. It is the muscle by which, almost alone, 

 the whole of the leg below the arm is bent, and carried forward and upward. 



It acts at great disadvantage. It is inserted into the very head of the bone 

 of the arm, and expanded even upon the joint. Then the power is applied 

 almost close to the centre of motion, while the weight to be raised is far distant 

 from it. The power is thirty times nearer the centre of motion than is the 

 weight ; and, calculating, as before, the weight of the arm and the rest of the 

 limb at 60 Ibs., it must act with a force of thirty times sixty, or 1800 Ibs. In 

 addition to this, the line of the direction of the force strangely deviates from a 

 perpendicular. The direction of the muscle is nearly the same as that of the 

 limb, and the mechanical disadvantage is almost incalculably great. If it is 

 calculated at only ten times more, this muscle, and its feeble coadjutors, act 

 with a force of ten times 1800 or 18,000 Ibs. 



Why this almost incredible expenditure of muscular power? That the 

 beauty of the limb might be preserved, and the joint be compact. If the 

 tendon had been inserted half-way down the arm, the elbow-joint would have 

 offered a very unsightly appearance. 



Beauty of form, however, is the least result of this conformation. Extensive 

 and rapid motion are among the excellences of the horse. He is valuable in pro- 

 portion as he has them combined with stoutness ; and by this conformation of the 

 limb could he alone 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 adhesions. There is sufficient strength in 

 the apparatus ; the power may be safely applied at this mechanical disadvan^ 

 tage ; and it is applied close to the joint to give an extent and rapidity of motion 

 which could net otherwise have been obtained, and without which the horse 

 would have been comparatively useless. 



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

 lower portions of the limb. Two of them have been described belonging to the 

 arm and the leg, and some very peculiar ones acting on the feet must not be 

 omitted. Only a small portion of one of them can be seen in our cut, p. 330, at 1. 



The first is the flexor pedis perforates. It is deeply seated in the posterior 

 part of the arm, where, with the perforans, it forms a thick fleshy mass, the 

 tendons issuing from which are adapted to the convexity and concavity of each 

 other. 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 ligament, 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 a 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 ligamentous expansions. 



Near the fetlock the tendon still further expands, and forms a complete ring 

 round the tendon of the perforating muscle. This is seen at J, p. 158. The 

 use of this will be best explained when the fetlock is treated of. 



