10 PROCEEDINGS OF THE NATIONAL MUSEUM vol.80 



The brachial flexor group as it now occurs in mammals comprises 

 the so-called biceps brachii, coracobrachialis, and brachialis. As they 

 are all served by the musculocutaneous nerve, they should be regarded 

 as originally a single genetic complex, which later differentiated into 

 the existing elements, and it is more difficult to assign to each its 

 precise phylogenetic position than would be the case if the group were 

 innervated by two or more nerve components. 



It seems likely that the ideally primitive differentiation of this 

 complex into separate elements was upon the plan of a single, long, 

 or two-joint flexor from the shoulder to the antibrachium (coraco- 

 antibrachialis), and two short or one joint flexors, a proximal one 

 from the shoulder to the humerus (coracobrachialis) and a distal one 

 from the humerus to the antibrachium (brachio-antibrachialis). As 

 there is still controversy over the question of whether the present 

 mammalian coracoid actually represents the primitive coracoid or the 

 procoracoid, it seems futile to speculate seriously as to the exact point 

 upon the primitive shoulder girdle from which arose the two ideally 

 primitive flexors of the brachium that were attached thereto. 



The long flexor and the more proximal of the short flexors above 

 referred to were the precursors of the biceps and coracobrachialis. 

 That the short biceps, from the coracoid, represents a more primitive 

 arrangement than the long head, with its origin from the bicipital 

 tubercle upon the glenoid border of the scapula, is indicated by the 

 fact that the long head, as such, is apparently found only in those 

 vertebrates higher than the reptiles. 



Whatever was precisely the original arrangement of these two 

 flexors, there accordingly seems to have been a later stage during 

 which they were confined to an exclusive origin, both of them from 

 the coracoid process of the scapula. At that time, therefore, they 

 probably had much in common, and it is likely that the part repre- 

 senting the biceps inserted upon both radius and ulna, while that 

 representing the coracobrachialis inserted upon the humeral shaft. 

 Because of their community of origin, however, it is possible that 

 some interchange of fibers took place as specialization occurred. In 

 other words, some of the true biceps fibers, partially fused with the 

 coracobrachialis, may secondarily have developed attachment to the 

 humerus, thereby increasing the area of coracobrachialis insertion, 

 while some of the true coracobrachialis fibers could conceivably have 

 separated and taken on an elongated form that would later give 

 them considerable resemblance to a biceps division. And their inner- 

 vation would not help us to segregate them according to their actual 

 derivation. 



At least the assumption may be granted, however, that the brachial 

 flexor now passing from the coracoid to the forearm (caput breve) 



