306 DH. A. KEITH ON THE CHIMPANZEES. [Mar. 7, 



such a slip occurred in only 1 out of over 20 Chimpanzees. On 

 the other hand, the extensor lonr/us polUcis of the Chimpanzee 

 frequently sends a slip to the proximal phalanx, an occurrence not 

 met with in Gorillas. The thumb in the Chimpanzee is on the 

 whole the more robust, but in the arrangement of the extensor 

 muscles the Grorilla approaches most nearly to Man. In proportion 

 to the length of the upper limb, the thumb of the Chimpanzee is 

 slightly the longer. 



There are certain well-marked points of distinction between the 

 palmar and digital parts of the hand of the African Anthropoids. 

 The hand of the Chimpanzee is long and narrow, a hook to cling 

 by ; the hand of the Gorilla is shorter and broader. The meta- 

 carpal and phalangeal parts of the Chimpanzee hand make up over 

 25 per cent, of the length of the upper extremity ; it seldom exceeds 

 22 per cent, of the Gorilla's arm and is frequently less. The hand 

 of the Chimpanzee is adapted for bracliiation, the hand of tl:e 

 Gorilla is not. The contrahentes muscles to the 4th and 5th digits 

 are very seldom absent in the Chimpanzee ; they are seldom present 

 in the hand of the Gorilla. The tendon of the flexor profundus 

 digitorum to the index digit commonly sends a slip to the tendon 

 of the third, a rare occurrence in the Chimpanzee. 



The arrangement of muscles on the back of the hand, as in the 

 case of those of the flexor aspect and of the thumb, is most primi- 

 tive in the Chimpanzee. In both apes the superficial extensor 

 muscle to the fifth finger is small or absent ; the extensor indicis, 

 a muscle of the deep layer of extensors, was present in all the 

 Chimpanzees examined, but only in 7 out of 8 Gorillas : the 

 deej) extensor of the 3rd digit was present in none of the Gorillas, 

 but in 5 of 12 Chimpanzees ; the corresponding tendon to the 4th 

 digit was present iu 1 of 8 Gorillas and in 4 of 12 Chimpanzees. The 

 deep extensor of the fifth digit was present with equal frequency. 



A curious transmigration in the origin of the forearm muscles, 

 resembling the change that has occurred to a greater extent in 

 Man, is seen at the elbow of the Chimpanzee. The pronator radii 

 teres has in the Chimpanzee an origin from the coronoid process of 

 the ulna in 9 animals out of 11, in only 3 out of 8 Gorillas; 

 an origin of the flexor corpi radialis from the radius is more 

 comnion amongst Chimpanzees ; the flexor sublimis digitorum had 

 a coronoid origin in 10 out of 12 Chimpanzees and in only 1 out 

 of 8 in the larger ape. 



A consideration of muscles which have become more or less 

 vestigial in Anthropoids shows how closely the Chimpanzee and 

 Gorilla are related to each other, and at the same time how they 

 differ. The following list will show this at a glance : — 



Gorilla. Ciujipanzee. 



Pi-esent. Absent. Present. Absent. 



Palmaris lonrjiis .... 4 7 9 3 



Plantaris 12 25 17 



Fenoneus quinti digiii. Equally sm.all or absent in both. 



Pi/raniidalis „ „ „ 



Psoas jjarinis „ „ „ (absent 40 p.c). 



