OCR Text |
Show 306 DR. 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 longus pollicis of the Chimpanzee frequently sends a slip to the proximal phalanx, an occurrence not met AAith in Gorillas. The thumb in the Chimpanzee is on the whole the more robust, but in the arrangement of the extensor muscles the Gorilla 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 Avell-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 metacarpal 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 brachiation, the hand of the 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-tiAe 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, Avas present in all the Chimpanzees examined, but only in 7 out of 8 Gorillas ; the deep 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 Avas present in 1 of 8 Gorillas and in 4 of 12 Chimpanzees. The deep extensor of the fifth digit Avas 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 common amongst Chimpanzees ; the flexor sublimis digiiorum 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 SIIOAVS 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. CHIMPANZEE. Present. Absent. Present. Absent. Palmaris longus .... 4 7 9 3 Plantaris 0 12 25 17 Penoneus quinti digiti. Equally small or absent in both. Pgramidalis ,, ,, ,, Psoas parvus „ „ „ (absent 40 p.c). |