188 



STRUCTURE OF THE VERTEBRATES 



a single muscle. The dorsal muscle group is a large undivided 

 muscle in the lower forms. It becomes variously divided, the 

 larger median part becoming the triceps of the mammal. The 

 forearm of the urodele has the extensor, flexor, and pronator 

 muscles developed. The pronator muscle remains relatively un- 

 changed. The first two groups, however, become greatly divided, 

 accounting for the highly developed use of the hands and fingers. 

 The opposability of the thumb, which was supposed to be lim- 

 ited to man, has been shown to be a gradual evolution within 



I 



Fig. 107. Arm and Shoulder of Man, Ventral View. The ventral adductor 



and flexor muscles of the shoulder and upper arm are in normal position. 



The forearm is pronated to show the dorsal extensors and the ventral 



flexors of the hand. 



the primates, and correlated with structural changes. In the 

 lower primates the long adductors and abductors are tendinous 

 slips from the extensor and flexor series. In the higher primates 

 there is a progressive splitting off of the thumb muscles until in 

 man there is complete independence of the thumb from the 

 other digits. The short digital muscles are too complicated for 

 an elementary discussion. 



2. Pelvic limb. The extrinsic muscles of the posterior ap- 

 pendage, like the skeleton, are usually more modified in the 

 land animals than are those of the anterior limb. Beginning with 



