AQUATIC MAMMALS 



broadened distal part of the radius save chiefly in providing better lever- 

 age for the extensor metacarpi pollicis, and possibly to a slight extent of 

 the pronator teres and supinator brevis. So far as we know there is no 

 terrestrial nor natational use for such a specialized equipment for power- 

 ful motions in several directions of the manus, and I am accordingly 

 compelled to believe that it has other functions of exceeding speciali- 

 zation, which the very shortness of the external arm facilitates. Perhaps 

 the reported use of the manus of northern phocids for scratching holes 

 through the ice is of more importance than might appear at first glance. 

 But this function could not be of influence in the case of those tropical 

 genera in which the nails are well developed, so perhaps the manus is 

 particularly useful for some such purpose as scratching about on the bot- 

 tom for echinoderms or similar fare. 



In the Phoca hispida dissected there were very deep grooves (fig. 35) 

 upon the distal radius for the passage of the tendons of the extensores 

 metacarpi pollicis, digitorum communis and lateralis, and a shallower 

 one for the extensor pollicis longus. These grooves are almost as well 

 defined in P. fasciata but much less so in P. groenlandica, while that for 

 the extensor metacarpi pollicis is the only one at all marked in the genus 

 Monachus. Neither are they a character in Odobenus and they are 

 entirely absent in the Otariidae. These grooves, when they occur, are 

 for the purpose of fixing the position of the tendons concerned. They 

 would indicate that contraction of the respective muscles, effecting ex- 

 tension of the whole manus as well as abduction of the pollex, is often 

 repeated and rather strong; but this is perhaps incidental. Their real 

 significance is to prove that the chief action of these muscles is effected 

 at a time when the manus is already considerably extended, or, at the 

 very least, on a line with the antibrachial axis; and further, I believe 

 that the fact that these grooves are sunk into the bone not at a perfect 

 right angle but facing somewhat in the ulnar direction is an indication 

 either that the chief extensor action of the muscles is combined with 

 some definite abductive movement of the manus, or else that the manus 

 is usually definitely abducted during their action. By means of these 

 grooves and ligaments above them the tendons are prevented from 

 pulling away from the bone. This action of the wrist will be discussed 

 further. 



In the sea-lion (Zalophus calif ornianus) which I dissected there is an 

 even more marked broadening of the proximal ulna, while the breadth 

 of the distal radius is no greater than in the seal. Unlike the latter 

 animal, however, there is a purely mechanical need for a broadening of 



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