238 THE HORSE. 



foot. It therefore extends all these bones ; and as it passes over the shank, 

 being- tied down to it in every part of its course, it likewise serves to extend 

 that bone. 



At ?/, page 233, is the tendon of another extensor muscle, and at 2 a 

 curious oblique one, passing over the tendon of >r, confining it in its situa- 

 tion, and likewise itself 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 233. 

 The first is at t, page 233 ; it is also seen at 6, page 235. It is 

 called the middle flexor, or bending muscle of the shank-bone, because it 

 lies precisely on the middle of the back part of the arm. It arises from 

 the inner head of the lower bone of the shoulder, and is inserted into one 

 of the bones on the inner side of the knee. The other is seen at v, page 

 233. It is called the external flexor of the leg, because it lies on the outer 

 side of the arm, towards the back. It arises from the outer head of the 

 lower bone of the shoulder ; advancing towards the knee, it is tendinous, 

 and the tendon divides into two portions, one of which is inserted into the 

 same bone of the knee, and the other into the outer small bone of the 

 leg. The internal flexor is seen at e, page 235. It proceeds from the 

 inner head of the lower bone of the shoulder, and is inserted into the head 

 of the inner splint bone, and its office is to bend the leg, and very slightly 

 turn it. A portion of one of the most powerful of the flexor muscles, 

 and powerful indeed they must be, is delineated at c, page 235. It is 

 the flexor of the arm. 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, 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 an 

 oily fluid, so that the tendon plays 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 con- 

 taining many tendinous fibres. Deeply seated, it contributes materially 

 to the bulk of the front of the arm ; and, having reached the arm, it 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 main muscle by which, 

 almost alone, the whole of the leg below the arm is bent. 



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 lbs., it must act with a force of 

 thirty times sixty, or 1800 lbs. 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 disadvan- 

 tage is almost incalculably great. We will take it at only ten times more ; 

 then this muscle, and its feeble coadjutors, act with a Ibrce of ten times 

 1800 or 18,000 lbs. 



Why this almost incredible expenditure of muscular power? — First, that 

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

 If the tendon had been inserted halfway down the arm, the elbow-joint 

 would have offered a very unsightly appearance. 



Beauty of form, however, is the least result of this conformation ; with- 

 out it the animal would not have been fitted for the purposes to which we 

 devote him. It is a law of mechanics, that what is lost in power is gained 



