1899,] DR. A. KEITH ON THE CHIMPANZEES. 307 



The latissimo-condyloideus, a muscle which has heen reduced to 

 a mere fibrous vestige iu Man, is much diminished in size in both 

 Gorilla and Chimpanzee, but it is larger and more primitive in its 

 attachments in the Chimpaiizee 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 plan- 

 tigrade 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 nnterior-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 Primates more closely than those of 

 the Gorilla. 



One of my 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 '. The 

 lumbar curve is more pronounced in the Gorilla ^. 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 

 stern nl parts of the pectoralis major, the Chimpanzee recalls the 

 arrangement in the lower Primates 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 ihe 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, rliomhoideus, splenms colli, levator 

 anguli scapulce, and omo-traclielien muscles ; they have also a more 

 extensive attachment to the dorsal vertebras below. The wide cer- 

 vical attachment, which was the rule for these muscles iu the Chim- 

 panzee, was the exception in the Gorilla. These attachments are 

 adaptive to the greater mobility of the head of the Chimpanzee, 

 a feature in which it much more resembles Man than the Gorilla. 



^ Journ. of Anat. & Physiolo°ry, vol. xxi. p. 288. 



^ A. M. Paterson. Trans. Roy. Society, Dublin, 1893, ser. 2, vol. v. pt. iii. 

 ^ D. T. Cunningham. Cunningham Memoirs, Roy. Irish Acad. 1892, 

 no. vii. 



