MUSCULAR SYSTEM 119 



nected with one another, but also with neighbouring muscles, 

 such as the pronator teres, palmaris longus, flexor carpi radialis 

 and ulnaris. The two flexors originally formed (as in the 

 lower Mammals) one mass ; and in the human embryo they 

 still arise as a single blastema, which is only at a later stage 

 of development split up by ingrowing partition walls of con- 

 nective tissue. 



In Anthropoids these muscles are throughout life connected 

 by anastomosing strands, which clearly indicate their former 

 union, and to this cause, and the lack of a distinct flexor 

 pollicis proprius, is due the less marked specialisation of the 

 Anthropoid hand as compared with that of Man. In Man, the 

 flexores digitorum communes, superficial and deep, are, as a rule, 

 distinct ; but the more or less complete fusion often found 

 between them points to the fact that their separation is (geo- 

 logically speaking) not of long standing, and has not yet become 

 stereotyped. 



The same is the case with the not infrequent fusions 

 involving the two radial extensors of the hand, which must also 

 be regarded as reversionary. Indeed, these two muscles may fuse 

 completely, and, in such a case, we have a realisation of that 

 lower condition in which only one single extensor carpi radialis 

 externus is present. 



A further instance of progressive development in muscles is 

 exemplified by the glutei. These, including the adductors of the 

 thigh, show their original unity by frequent blending ; and 

 very often a more or less complete fusion takes place between 

 them and the pyriformis, or between the latter and the gemellus 

 superior. Further, the frequent absence of the gemellus superior 

 in Man deserves mention, because this muscle is also often 

 wanting in the Anthropoids. 



The special development of the gluteus maximus is a charac- 

 teristic peculiarity of Man. This muscle has a humble origin 

 among the lower Vertebrates, and even in the Anthropoids there 

 is nothing comparable in size and strength with its excessive 

 development in Man, which is a direct accompaniment of the 

 upright gait. The muscle fixes and steadies the pelvis, or rather, 

 the whole trunk, on the heads of the femora, and through them 

 on the lower limbs, as on a support or stand. 



Closely connected with the assumption of the upright gait 

 by Man, which involves the transformation of the former pre- 

 hensile feet into ambulatory and supporting organs, is the 



