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Show 304 DR. A. KEITH ON THE CHIMPANZEES. [Mar. 7, it is only by dealing AAith a large number of the tAvo races that their essential characteristics can be arrived at. The statements 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 osteological 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, Avhile in the Gorilla they indicate an adaptation for spending a life in the open as AA'ell 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 development 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, tAAice as far in the Gorilla as in 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 Chimpanzee 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 tAvo 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 lower extremity, as seen in the skeleton, are alike in both. The muscles that act on them, except in 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 tibialis anticus is different, making an approach to the human form in the Gorilla. Of 7 Gorillas, only the tendon was divided iu 5 ; the division extended deeply into the muscle in 2: in the Chimpanzee, on the other hand, resembling the lower Brimates, 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 limb of the foot is examined, the Chimpanzee shows the greater number of primitive features. The contrahentes muscles, either as fibrous bands or as fibro-muscular slips, are always more evident in the Chimpanzee than in the Gorilla. The interosseous 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 1st and 2nd dorsal interossei muscles. In this feature also the Gorilla shows an approach to an adaptation for plantigrade |