THE PECTORAL LIMB 



the sea-lion antibrachium which the former seems now to lack. In 

 otariids the normal posture places the external axillary angle at about 

 the middle of the fore arm, while in phocids it is situated opposite the 

 wrist. Hence in the former this segment is influenced by the stimulus 

 for becoming flatter and broader so as to offer less resistance, and has 

 been more completely so in the past. The development of the seal manus 

 seems to indicate that this same quality of stimulus has at no time been 

 a very important factor in its evolution. Further, in the sea-lion the 

 distal fore arm is essentially a mechanical part of the paddle (manus) , 

 as it~is not in the seal, and that the broadness of the flipper in the 

 former pinniped has had some real influence in continuing such broad- 

 ening onto the fore arm, thus affecting the radius, while the correspond- 

 ing broadening of the olecranol part of the ulna should rather be at- 

 tributed to muscular stress. In view of this evidence I am of the opinion 

 that the largely similar osteological specialization of the fore arm in 

 these two pinnipeds is likely not ascribable strictly to the same stimuli, 

 but rather to two (or more) rather diverse influences which have effected 

 similar conformation or convergence. 



That the sea-lion has need for an antibrachium broader than the 

 bones alone have been able to supply may be shown by the fact that 

 upon the radial edge there is a thickened, partly fibrous development of 

 tissue which is continuous with the pectoralis profundus; or this may 

 have been built up as a buffer by the action of water resistance. In 

 otariids the purpose for which the flipper is used necessitates that it nor- 

 mally be held more extended than the phocid finds advisable. Ob- 

 servation of the segments during dissection, correlated with the position 

 of the humeral condyles in relation with the axis of the bony shaft, 

 indicates that the brachial-antibrachial angle in static posture is about 

 125 degrees or less in Phoca, and 155 degrees in Zalophus. 



As in Phoca the lateral aspect of the olecranol surface of Zalophus 

 is occupied by the very broad origins of the extensores pollicis longus 

 and metacarpi pollicis, while the triceps longus and lateralis, for ex- 

 tension of the fore arm and flexion of the whole limb, gain added lever- 

 age by virtue of the broad olecanon. The two pollicis extensors are 

 extremely broad in their tendinous parts and are well fitted for supinat- 

 ing movements of the anterior flipper border. I believe, however, that 

 the chief stimulus for the remarkable broadening of the proximal ulna 

 of the Otariidae has been the origin of the palmaris longus (fused with 

 the smaller head of the flexor carpi ulnaris) . This is a very specialized 

 muscle, not only at origin but distad, where it extends in a tendon 25 



[235] 



