ART. 18 BRACHIAL FLEXORS IN PRIMATES HOWELL AND STRAUS 21 



whom no one has had more extensive experience with the muscula- 

 ture of the lower mammals, has stated that he has never found the 

 nerve piercing the muscle in anj^thing but primates. 



Judged from our own dissections and from a consideration of the 

 data given by Kohibriigge (1897) and Bolk (1902), this feature of 

 piercing or nonpiercing is extremely variable in monkeys and lemurs. 

 It seems to vary not only within a single genus but perhaps even 

 within a species. We ourselves have found the musculocutaneous 

 nerve piercing the coracobrachialis medius only in the left arm of 

 our chimpanzee, in Pygathrix^ Lasio'pyga^ and Perodicticus. In the 

 right arm of the chimpanzee, as in both extremities of our Hylobates^ 

 the nerve passed entirely superficial to the muscle. No piercing of 

 the muscle was found in any of our other specimens of monkeys or 

 prosimians, in which the nerve passed entirely deep to (above) all 

 the fibers of the coracobrachialis medius, between this muscle and 

 the bone. This arrangement obtained regardless of the absence or 

 presence of the coracobrachialis profundus. Apparently the nerve 

 usually pierces the coracobrachialis medius in the great apes, though 

 possibly with less frequency than in man, at least in the chimpanzee. 



Many authors, however, have evidently considered that when the 

 nerve pierces the muscle in primates the portion superficial to the 

 nerve and the more distally situated is a pars longa. This treatment 

 we regard as unfortunate. The feature of piercing or nonpiercing 

 and the proportions of the muscle parts involved are too variable to 

 carry much phylogenetic or taxonomic weight, although for conven- 

 ience it is legitimate to term the part of the coracobrachialis medius 

 distal to the nerve b.y some such appellation as the distal portion, and 

 the deeper division the proximal one. The latter we regard as never 

 homologous to the coracobrachialis profundus or brevis, in spite of 

 the fact that where the latter occurs the musculocutaneous nerve usu- 

 ally, if not invariably, passes entirely deep to the pars media, for the 

 part of the latter above the nerve, where piercing occurs, is almost 

 always well segregated from the area of insertion of pars profunda. 

 That this may not invariably be the case is indicated by the mention 

 by Parsons (1898) that continuity has been observed between the two 

 in Gorilla. Also, pars media may begin to insert higher than the 

 distal bordei- of the latissimus tendon, as in the gibbon; not, we 

 found, upon the tendon itself, as stated by Kohibriigge (1890), but 

 just adjoining its termination. 



The same may be said of the part of the coracobrachialis medius 

 that lies distal to the point of piercing. This may reach barely to 

 the middle of the humerus (as in Aotus)., or it may continue well 

 down the entepicondyloid ridge, an arrangement that is often or usu- 

 ally the case in those mammals having an entepicondylar foramen 

 (as in Tarslus, Nycticebus. and Galago). There is no justification 



