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 spht 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 eompared 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 1s 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 
