304 DR. A. KEITH ON THE CHIMPAXZEES, [Mar. 7, 



it is only by dealing with a large number of the two races that 

 their essential characteristics can be arrived at. The state- 

 ments made here, concerning the arrangement of muscles, are 

 founded on accounts more or less complete of the dissections of 

 13 Gorillas and 30 Chimpanzees. When the osteologieal and 

 myological differences that separate the Chimpanzee and Gorilla 

 are analyzed it is found that they all centre round the adaptation 

 of the Chimpanzee for a life almost completely arboreal, while in 

 the Gorilla they indicate an adaptation for spending a life in the 

 open as well as on trees. In short, the body of the Gorilla is more 

 adapted for the human manner of progression than that of the 

 Chimpanzee. 



The approach to plantigrade progression is seen in the develop- 

 ment of the heel and calf-muscles of the Gorilla. The os calcis 

 projects behind the astragalus, to serve as a lever for the soleus 

 and gastrocnemius, twice as far in the Gorilla as m the Chimpanzee. 

 The projection in the Chimpanzee is always less than 1-5 cm. ; 

 it is never less than 3-5 cm. in the adult Gorilla. The soleus, too, 

 shows a much greater tendency in the Gorilla than in the Chim- 

 panzee to assume the form found in Man. It had acquired an 

 origin from the tibia in 3 out of 8 Gorillas and in only 2 out of 

 12 Chimpanzees, while in the Gorilla the soleus resembles to 

 some extent the human arrangement by being more closely fused 

 with the tendon of the gastrocnemius. 



As a grasping-organ, made up of two limbs, a hallucial limb on 

 the one side and a digital limb on the other, the foot of the 

 Gorilla does not differ materially from that of the Chimpanzee. 

 The proportional length of these limbs to each other and to the 

 louer extremity, as seen in the skeleton, are ahke in both. The 

 muscles that act on them, except iia minor details, are almost alike. 

 The foot of the Gorilla is the more bulky, broader, and the two 

 proximal phalanges of the toes lie within the plantar web. 



The muscles that flex and adduct the great toe show the same 

 arrangement and same variations in both, and in the extensor 

 muscles of that digit only the tibiahs anticus is different, making 

 an approach to the human form in the Gorilla. Of 7 Gorillas, 

 <mly the tendon was divided in 5 ; the division extended deeply 

 into the muscle in 2 : in the Chimpanzee, on the other hand, 

 resembling the lower Primates, the muscle and tendon were divided 

 in 16, the tendon only in 3. This, again, is a point in which the 

 Gorilla shows an adaptation to plantigrade progression. 



When the digital litub of the foot is examined, the Chimpanzee 

 shows the greater number of primitive features. The contrahentes 

 muscles, either as fibrous bands or as iibro-muscular slips, are always 

 more evident in the Chimpanzee than in the Gorilla. The inter- 

 osseous muscles in the foot of the Chimpanzee are arranged as in 

 all the lower Primates, the third digit receiving the insertion of the 

 2nd and 3rd dorsal interossei muscles ; but in 3 out of 7 Gorillas 

 the second digit, as is the case in Man, received the insertion of 

 the Ist and 2nd dorsal interossei muscles. In this feature also 

 the Gorilla shows an approach to an adaptation for plantigrade 



