Proceedings of the general meetings for scientific business of the Zoological Society of London, 1899

Page 355

1899.] DR. A. KEITH ON THE CHIMPANZEES. 307 The latissimo-condyloideus, a muscle which has been reduced to a mere fibrous vestige in Man, is much diminished in size in both Gorilla and Chimpanzee, but it is larger and more primitive in its attachments in the Chimpanzee than in the Gorilla. The humerus is proportionally long in the Gorilla (40 per cent, or more of the limb). The arm of the Chimpanzee, considering all its characters, approaches the conditions found in the brachiating Apes and shows features adapted for climbing not shown by that of the Gorilla. A distinctive feature of the Gorilla, and one adaptative to plan­tigrade progression, is the great development of the anterior-superior part of the ilium. The breadth of the iliac fossa, measured from the posterior-superior to the anterior-superior iliac spine is never less than 17 cm. in the adult Gorilla and never more than 13 cm. in the Chimpanzee. The result of this development is that the posterior part of the external oblique muscle of the abdomen is inserted to it; the tensor vagina femoris arises from it; the iliac crest acts as a fulcrum for these muscles to balance the body on its lateral aspects. The bones and muscles of the Chimpanzee thorax resemble the arrangement found in lower Brimates more closely than those of the Gorilla. One of m y pupils, Mr. Tredgold \ has shown that the average costal development of the Chimpanzees is 13*20 ribs, for Gorillas 12*86; there are commonly 13 pairs in both, but 12 pairs occur in the Gorillas occasionally and 14 pairs not unfrequently in the Chimpanzees. The lower limbs of the Gorilla show a tendency to be fixed to a vertebra higher up than in the Chimpanzee 2. The lumbar curve is more pronounced in the Gorilla 3. .Further, in the more extensive attachment of the pectoral muscles to the chest-wall, and in the absence of a hiatus between the clavicular and sternal parts of the pectoralis major, the Chimpanzee recalls the arrangement in the lower Brimates more than is the case in the Gorilla. The secondary attachment of the pectoralis minor to the coracoid process, a constant insertion in Man, is the rule in Gorillas and the exception in Chimpanzees ; it occurred in 8 out of 9 Gorillas and 7 out of 18 Chimpanzees. That point also indicates adaptation in the arm of the Chimpanzee to brachiation. There is a very well-marked difference between the Gorilla and Chimpanzee in the attachment of the extensor muscles of the neck. The difference is seen in the Chimpanzee's more extensive cervical origin or insertion of the trapezius, rhomboideus, splenius colli, levator anguli scapulce, and omo-trachelien muscles ; they have also a more extensiA-e attachment to the dorsal vertebra? below. The wide cer­vical attachment, which was the rule for these muscles iu the Chim­panzee, was the exception in the Gorilla. These attachments are adaptive to the greater mobility of the head of the Chimpanzee, a feature in wiiich it much more resembles M a n than the Gorilla. 1 Journ. of Anat. & Physiology, vol. xxi. p. 288. 2 A. M. PATERSON. Trans. Roy. Society, Dublin, 1893, ser. 2, vol. v. pt. iii. 3 D. T. CUNNINGHAM. Cunningham Memoirs, Roy. Irish Acad. 1892, no. vii.