28 PEOCEEDINGS OF THE NATIONAL MUSEUM vou 80 : aet. l:? 



in the more primitive primates. The musculocutaneous nerve usually 

 passes between media and profunda in prosimians and monkeys, but 

 in the great anthropoids and man its customary course is one whereby 

 it pierces the media. 



Most constant of all the brachial flexors is the brachialis. In its 

 primitive condition it was evidently entirely separable into two heads, 

 and traces of this original condition are found even among the more 

 advanced primates. 



The epitrochleo-anconeus, while topographically a member of the 

 brachial flexor group, is really a portion of the flexor carpi ulnaris 

 complex. This is indicated by its constant innervation by the ulnar 

 nerve, and never by the musculocutaneous, which is the nerve of the 

 true brachial flexors. This small muscle apparently occurs hap- 

 hazardly among the various groups of primates. 



Most striking, perhaps, are the contrasting specializations of the 

 biceps muscle in the Lorisinae, among the lemurs, and in the Hylo- 

 batidae, among the catarrhines. In all other primates this muscle 

 normally is composed of the usual mammalian coracoid and long 

 heads. The lorises {Nycticehus^ Lons^ and Stenojys), however, nor- 

 mally possess but one head, of the long variety, the coracoid head 

 being absent. A quite different and most complicated arrangement 

 obtains in the gibbons (Hylobatidae). This in principle consists of 

 the normal absence of the coracoid head and its replacement by one 

 arising from the humerus. The long head is present. There are 

 exhibited most intimate connections with surrounding muscles, such 

 as the pectoralis major, dorsoepitrochlearis, and the forearm flexors. 

 This produces a mechanical arrangement which is well adapted to the 

 extreme mode of brachiation exhibited by the gibbons. This unique 

 anatomical arrangement is clearly an extreme functional adaptation 

 peculiar to the gibbon. In no sense can it be regarded as represent- 

 ing a stage in the evolution of the biceps of man and the anthropoiil 

 apes. Though this curious structure of the gibbon's biceps seems 

 undoubtedly to be adaptive, it is apparently not a necessary outcome 

 of the brachiating mode of locomotion, for such able and constant 

 brachiators as Ateles, Golohvs, Pan^ and Pongo exhibit no trace of 

 such an arrangement. 



