ART. 13 BRACHIAL FLEXOES IN PRIMATES HOWELL AND STRAUS 27 



by branches issuing directly from the median. The same arrange- 

 ment occurred in the chimi^anzees studied by Gratiolet and by Sutton 

 (cited by Bolk). In our chimpanzee the musculocutaneous nerve 

 was quite a distinct entity in each arm, and the formation and 

 proximal distribution of the entire brachial plexus agreed quite 

 closely with the conditions usually obtaining in man. According to 

 Bolk. if the musculocutaneous nerve does not pierce the coraco- 

 brachialis but lies medial to it, and if no blood vessels lie between it 

 and the median nerve, there is no factor present that may hinder 

 the ontogenetic union of the two nerve trunks. These ideal condi- 

 tions for union were realized in both his chimpanzee and his gibbon. 

 In the latter animal he regarded the union of median and ulnar 

 nerves as an expression of the result of the narrow spatial relation- 

 ships in the vessel-nerve canal on the medial side of the arm ; that is, 

 in the sulcus bicipitalis. These hypotheses are quite ingenious and 

 exceedingly plausible, but this particular subject needs further and 

 more detailed investigation. 



We have usually obtained the innervation of the epitrochleo- 

 anconeus, and this has always been by the ulnar nerve. Hence, this 

 slip has no relationship with the triceps group. It should never be 

 confused, as is usually done, with the anconeus internus sometimes 

 found (as in Didelphis). This triceps element is situated more 

 upon the extensor side of the epicondyle than is the epitrochleo- 

 anconeus, at least in primates. 



CONCLUSIONS 



Among the primates the brachial flexors as a whole exhibit no very 

 distinct phylogenetic trend. The variations are individual rather 

 than generic. 



There is to be noted, among the so-called higher primates, a tend- 

 ency for the two heads of the biceps brachii to fuse more completely 

 and more proximally. Supernumerary biceps heads should probably 

 be regarded as atavistic in some instances. In others they may rep- 

 resent nothing more than fortuitous variations. The lacertus fibrosus 

 is present more frequently in man than in other forms. Since this is 

 probably a primitive feature, man is to be regarded as relatively 

 unspecialized in respect to this character. 



The coracobrachialis, in its most complete expression, is composed 

 of three parts, which we prefer to call superficialis, media, and pro- 

 funda. The first of these is present only in very rare instances. 

 Among the gibbons, great anthropoids, and man the pars profunda 

 tends toward complete disappearance. Similarly, the pars media 

 exhibits a tendency to terminate its insertion farther proximally than 



