ART. 13 BRACHIAL FLEXOES IK PRIMATES HOWELL AND STRAUS 15 



coid head was vestigial, and in H. agilis entirely absent. The 

 assumption of complete or partial migration of the coracoid 

 head in Hylohates is strengthened by the fact that in the occa- 

 sional instances in which both coracoid and humeral heads occur 

 in the same specimen of gibbon, the former, according to data col- 

 lected by Gronroos (1903), is usually weakly developed. But Gron- 

 roos did not consider the humeral head (which he termed caput 

 tuberculo-septale) of the Hylobatidae as homologous with the usual 

 coracoid head, but rather that the two are distinct structures, of 

 which as a rule only one develops; for example, in man the cora- 

 coid and in the gibbon the " tuberculoseptal " head. This assump- 

 tion is hardly justified by comparative anatomical facts, despite its 

 ingenious application by Gronroos in attempting to explain the 

 accessory biceps heads in man. 



Eeally the most interesting and peculiar thing about the condi- 

 tion in Hylobates is that the pectoralis major is inserted directly 

 upon the tendon of origin of this caput humerale of the biceps.^ 

 This is very significant. By this means flexion of the pectoralis 

 not only acts through the biceps, but continuously as far as the 

 wrist by virtue of biceps-flexor digitorum sublimis fusion. If the 

 pectoral thus acted upon a biceps with long tendon from the scapula 

 the effect might be much less efficient, because a longer tendon would 

 give more slack than one firmly anchored to the lesser tuberosity. 

 One should note that this action of the pectoral through the biceps 

 is more effective when the gibbon arm is elevated sideways, thus 

 putting the pectoral under tension, than when elevation is accom- 

 plished in the sagittal plane. 



This does not exhaust the interesting details of the gibbon biceps, 

 however. The dorsomedial border of the proximal third of the 

 fleshy portion of the humeral head was fused with the ventral 

 border of the dorsoepitrochlearis, and a slender tendon, running 

 along the dorsomedial border of this biceps head, fused with the 

 slender tendon continuing to the epicondyle from the dorsoepitroch- 

 learis. It thus appears that when the pectoralis is acting upon the 

 antibrachium through the humeral biceps, the fusion of the latter 

 with the dorsoepitrochlearis prevents the biceps from springing for- 

 ward unduly and thus weakening the action. Gronroos (1903) has 

 also discussed the action of such a muscle complex; that is, from 

 latissimus dorsi through latissimo-condyloideus (sive dorsoepitroch- 

 learis) through caput tuberculoseptale (sive caput humerale) to 



- Gronroos apparently doubted that the humeral head ever arises in Hplohates from 

 the pectoralis major tendon, as some authors have stated, and In his specimens he inter- 

 preted the situation as fusion of the two structures. In our specimen, at least, there 

 was no shadow of doubt that the pectoralis major was actually inserted upon, or into, 

 the tendon of the humeral head. 



