14 PROCEEDINGS OF THE NATIONAL MUSEUM vol.80 



any emphasis to be placed upon this point. These humeral heads, 

 although fleshy, were usually rather weakly developed. Origin was 

 always above the middle of the humerus lateral to and coextensive 

 with the insertion of the coracobrachialis medius; while insertion 

 was into the posteromedial aspect of the common biceps mass just 

 above the elbow. 



The occurrence of a humeral biceps head in Aotus, as mentioned 

 above, should not be regarded as normal for the genus, however, for 

 this extra muscular slip was not present in the other arm, nor in 

 the upper extremities of an additional specimen (Johns Hopkins 

 Anat. No. 244) that we investigated. Nor does the presence of 

 accessory biceps heads in our chimpanzee represent the normal con- 

 dition in this animal. In fact, accessory heads for the biceps would 

 seem to occur quite infrequently in primates other than man and 

 gibbons. Testut (1884) described an extra or capsular head in a 

 Cercopithecus { = Lasiopyga). Chudzinski (cited by Kohlbrtigge, 

 1897) found a humeral head in two out of five orang-utans that he 

 studied. This extremely low frequency of extra heads in monkeys 

 is worthy of note, especially in view of Gronroos's (1903) specula- 

 tions concerning the evolution of the human biceps. This theory 

 will be considered subsequently. 



In some respects these accessory biceps heads may be largely 

 fortuitous, although possibly atavistic,^ but it is at least shown that 

 this detail readily responds to variational stimuli. If it be in 

 response to some need for a humeral attachment, then a continuation 

 and strengthening of the stimulus might well result, finally, in the 

 migration of a part, or indeed the whole, of the primitive coracoid 

 head to the humerus. This, we are inclined to think, may be just 

 what has happened in the case of Hylobates. In the specimen of the 

 latter dissected by us the long head had, immediately adjoining, 

 another head, which in all respects was comparable to a caput breve 

 except for the fact that origin was strongly from the lesser tuberosity 

 of the humerus immediately adjacent to the bicipital groove and 

 not at all from the coracoid. This would appear to be the usual 

 arrangement in the Hylobatidae. Though this assumed migration 

 of the caput breve was complete in our gibbon, it appears to have 

 been incomplete in other instances. For example, the rodent Visca- 

 cha was found by Parsons (1894) to have, in addition to a normal 

 longum and breve, just such a humeral head, but from the greater 

 instead of the lesser tuberosity. Kohlbrtigge (1890) found similar 

 conditions in Hylobates leuciscus^ but in H. syndactylus the cora- 



^ The accessory biceps head in our chimpanzee, arising superficially from the capsule, 

 and the comparable conditions occurring in man might be interpreted as indicating tran- 

 sitional stages In the migration of the long head from a coracoid to a supraglenoid 

 origin. 



