OCR Text |
Show If •:•::'." DR. A. KEITH ON THE CHIMPANZEES. 305 progression. In both, the muscles of the fifth toe show a marked tendency to become vestigial-a condition which occurs in Man. and which Mr. Herbert Spencer believes to be due to the wearing of boots ; but the retrograde changes are most marked in the Gorilla. In 4 out of 11 Chimpanzees this digit received a tendon from the extensor brevis diyitorum. a tendon found in only •rilla out of S. The flexor brevis of this digit was absent in 3 Chimpanzees and fibrous in 11 : it was absent in 3 Gorillas, fibrous in 6. and muscular in 3. The flexor aceessorius is equaUy variable in both : it was found in the feet in 6 out of 10 Gorillas and in 6 out of 11 Chimpanzees. The origin of the flexor brevis digitorum shows much variation in both animals, but the tendency for a complete transference of the origin of this muscle from the tendon of the long flexor of the foot to the tuberosity of the heel is most marked in the Chimpanzee, a character in which it more resembles M a n than its congener. The better adaptation of the lower extremity of the Chimpanzee for a climbing-organ is seen in the extensive insertion of the semi-tendinosns. gracilis, sartorius. and biceps to the fascia of the leg, in the occasional slip from the adductor magnus to the inner head of the gastrocnemius, and in the separation of the scansorius. The scansorius is a segmention from the anterior border of the deepest gluteal sheet, for the more complete flexion of the hip-joint. It existed as a separate muscle in 6 out of 11 Chimpanzees and in only 2 out of 8 Gorillas. The lower extremity is nearly equal in length (sometimes longer") to the upper extremity: in the Gorilla it is always shorter; but the proportion of the anterior and posterior limbs varies considerably. Some well-marked features, related to their methods of locomotion, distinguish the upper extremity of the Chimpanzee from the Gorilla. The arm of the Chimpanzee is that of the brachiators, anthropoids like the Orang and Gibbon, which use the arms as one of the main organs of locomotion. The arm of the Gorilla resembles more in its proportions that of the lower Apes. Both the Chimpanzee and Gorilla agree in showing many retrograde changes in the thumb. In neither is it a grasping-organ. The flexor longus pollicis is vestigial in both ; in Gorillas it was represented by a tendinous thread springing from the deep flexor of the index digit in 2 ; in the remaining 1" it was completely absent or represented by a piece of tendon in the thumb only. In 25 Chimpanzees it Avas present as a thread in 15. and in the remaining 10 it was completely absent or merely the terminal part of the tendon was present. The retrograde change has made furthest nrogress in the Gorilla. The short muscles that flex the thumb have the same arrangement in both, except that the oppc pollicis is better marked in the Gorilla. There are differences in the extensor muscles of the thumb. The tendon of the extensor ossis metacarpi is much more com-pletelv divided into a carpal and a metacarpal part in the Chini-panzee: and while this tendon sent a slip to the proximal phalanx of the thumb, as it always does in Man, in 4 out of 9 Gorillas. |