THE ARM. 263 



jmrpose of a capsular ligament, covering and adhering to the pastern joints. Its office 

 is to extend the foot and pasterns, and, at the same time, to assist in the extension of 

 the knee. 



At u, page 259, is the tendon of another extensor muscle, and at z a curious oblique 

 one, passing over the tendon of x, confining it in its situation, and likewise assisting 

 in extending or straightening the leg. 



The muscles employed in bending the leg are both numerous and powerful. Two 

 of the superficial ones are given in the cut, page 260. The first is at t, page 259 ; 

 it is also seen at 6, page 259. It is called the Jlexor medius metacarpi, because its 

 office is to bend tlie leg. The other is seen at v, page 259. It is called the jlexor 

 ■metacarpi externus, and is also designed to flex the leg. 



The internal flexor is seen at e. Its office is also to bend the leg. 



A portion of one of the most powerful of the flexor muscles, and powerful indeed 

 they must be, is delineated at c, page 259. It is the Jlexor brachii. It rises from the 

 extremity of the ridge of the shoulder-blade in the form of a large and round tendon, 

 which runs between two prominences in the upper 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 secre- 

 tion of oily fluid, so that the tendon may play freely in the pully without friction. 

 Having escaped from this pully, and passed the head of the lower bone of the shoul- 

 der, 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 GOlbs., it must act with a 

 force of thirty times sixty, or ISOOlbs. 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 coadju- 

 tors, act with a force often times 1800, or 18,000lbs. 



Why this almost incredible expenditure of muscular power] That the beauty of 

 the limb might be preserved, and the joint be conipact. If the tendon had been 

 inserted half-way down the arm, the elbow-joint v.ould 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 proportion 

 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 disadvantage; and it is applied close to the joint to give 

 an extent and rapidity of motion which could not 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 710/ be omitted. Only a 

 small portion of one of them can be seen in our cut, p. 259, at 1. 



The first is the Jlexor pedis perforatus. It is deeply seated in the posterior part of 

 the arm, where, with the perforans, it forms a thick fleshy mass, the tendons issu 

 ing 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 sud- 

 den 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 



