ORGAN AND FUNCTION. 



75 



is inserted higli in the perinaoum. If one can rely on the 

 anatomical plates of Cuvier and of Laurillart, the negro has 

 the perineeal insertion of the biceps not so high as in the 

 white man, thus approximating to its position in the ape. 



Neglecting at present the question why there should be 

 this variety in the attachment which regulates the motion of 

 the biceps, let us content ourselves with considering the con- 

 sequences which this arrangement may have upon its function. 

 It is clear that during the movement of the flexion and ex- 

 tension of the knee, each portion of the bone describes around 

 this articulation an arc of a circle which is larger as it recedes 

 from the centre of motion. It is equally evident that each of 

 these points will move to a greater or less distance from the 

 femur or the ischium, according to the extent of the circular 

 movement which it executes. And as great movements should 

 correspond with long contractile fibres, we ought to find 

 inequalities in the length of the biceps in different mammals. 



This is precisely what is observed. In man, whose biceps 

 has its lower insertion very near the knee, the extent of the 

 movements of the moveable attachments is not very consider- 

 able ; so the contractile fibre will have relatively little length, 

 while the tendon will occupy a certain part of the extent of 

 the biceps. In the ape, the inferior attachment of the muscle 

 taking place lower down will consequently have greater mo- 

 bility; whence the necessity of a greater length of active 

 muscle, which is effected by the tendinous part being shorter. 

 In quadrupeds the tendon of the biceps almost entirely dis- 

 appears, and the muscle is formed of red fibre throughout 

 almost all its extent. 



The rectus internus muscle of the thigh presents the same 

 variability in its attachments and its structure. If we observe 

 its arrangement in man (fig. 16), we see at once that the 

 attachment of this muscle to the leg is very near the knee, 

 and that its tendon is very long. Let us examine the same 

 muscle in an ape (figs. 17 and 18), we find that its tibial 

 attachment is much farther from the knee, and as a conse- 

 quence of the more extended movements which this attachment 

 executes, we find that the muscular fibre gains length at the 

 expense of that of the tendon, which is extremely short. 



