170 ANTHROPOID APES. 



must be remembered that tbe comparatively short 

 thumbs of aDthropoids have not to be employed in 

 so many different ways as the human thumb, and 

 that we cannot therefore be surprised that the de- 

 velopment of the short extensor is less complete. 

 A special extensor muscle of the index finger is 

 either altogether absent in the gorilla or very 

 slightly developed, while it is very apparent in 

 Hylohates alhimanus (c, Fig. 53). In the chimpanzee 



Fig. 53.— Muscular system of the back of a gibbon's hand. 1, The extensor carpi 

 radialis lonpior and brevior. 2, Abductor longus pollicis. 3, Extensor primi in- 

 ternodii pollicis. 4, Extensor secundi inteinodii pollicis. 5, E.xtensor communis 

 digitorum. 6, Extensor indicis. 7, Extensor minimi digiti. 8, Extensor carpi 

 ulnaris. 9. First dorsal inter-osseous muscle. 10, Continu.<\tion of the same 

 to index finger. 11, 12, The other inter-osseous muscles of this region. A, The 

 posterior annular ligament. 



this muscle sends a tendon to the middle finger. 

 In the orang there is one extensor common to the 

 four fingers. In the gibbon's hand, this, as well as 

 the other extensor and flexor muscles, is remark- 

 able for its excessive slenderness. The manifold 

 connections of the extensor tendons with each other 

 are an interesting peculiarity (Fig. 53). 



In the chimpanzee I observed a superficial flexor, 



