24 PROGEEDINGS OF THE NATIONAL MUSEUM vol. 80 



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and partly medial to the deltoid insertion. In Galago the origins 

 were similar, but the two heads were not clearlj^ separable, which 

 may have been partly attributable to the poor condition of the alco- 

 holic specimen. Broadh' speaking, this is the general condition in 

 many of the lower mammals, such as insectivores and rodents. A 

 more advanced type, as typified in anthropoid apes and some of the 

 monkeys, is either for the lateral head to originate a trifle above the 

 termination of the deltoid insertion, and for the medial head to 

 begin just below this point, or else for the origin of the two 

 to form a V partly embracing the end of the deltoid. Very often, 

 both in man and other primates, the brachialis is partly separable 

 longitudinally (for example, in Pan, Papio^ SaimiH, Aofus, and 

 Nycticebus) downward from the apex of the deltoid insertion. It is 

 clear that its logical treatment should call for the recognition of a 

 pars lateralis and a pars medialis. Each part is served by branches 

 of the musculocutaneous nerve, but frequently the lateral head re- 

 ceives in addition a fine twig of the radial nerve, indicating that 

 with this head of the brachialis there may at times be incorporated 

 some relatively slight element originally derived from the extensor 

 complex. Occasionally the muscle may be represented almost en- 

 tirely by the medial head, as in our gibbon {Hylohates). 



In some human cases the brachialis may likewise be almost com- 

 pletely separated into distinct superficial and deep layers as far as 

 the tendon. It is the lateral portion of this deep layer that is fre- 

 quently innervated by the radial nerve. At times it is separable from 

 the remainder of the muscle as a more or less distinct slip that 

 joins the main tendon just before insertion on the ulna. It is not 

 uncommon to find this lateral portion of the brachialis fused super- 

 ficially with the inner border of the brachioradialis. This union may 

 be so intimate as completely to bridge over the radial nerve. These 

 variations suggest that the fibers of the extensor element incorporated 

 with the pars lateralis of the brachialis are derived from the brachio- 

 radialis rather than from the adjacent triceps complex. This view 

 is shared by Paterson (1919). 



The only other primate in which we have encountered any radial 

 innervation of the brachialis is Pygathrix. In this animal brachialis 

 and brachioradialis were completely fused superficially, bridging 

 over the radial nerve. The lateral portion of the brachialis was in- 

 nervated by both musculocutaneous and radial twigs, but the part 

 served by the latter element w^as not differentiated as a separate slip. 



In our dissections all nerves were traced to their emergence from 

 the plexus, without, however, particular attention being paid to 

 their finer variations. A few of the more interesting details may 

 here be mentioned. 



