THE PECTORAL LIMB 



a part of the flipper proper, for which it is fitted partly by the broaden- 

 ing of the radius and partly by subcutaneous tissue deposited upon the 

 radial and ulnar borders. 



The manus of the sea-lion differs in many respects from that of the 

 seal. It is not carried in an abducted posture and there has been no 

 crowding of the carpal elements. The scapholunar is large to corre- 

 spond with the great width of the distal radius, and its proximal articu- 

 lar surface, as well as that of the ulnare and the metacarpals, extends 

 farther onto the dorsal surface than in the seal. This is so as freely 

 to aHow extension of the manus to an angle of 90 degrees with the anti- 

 brachium, which is essential for terrestrial progression. The ulnare is 

 not on a transverse line with the scapholunar but above it, to correspond 

 with the more proximal position of the distal extremity of the ulna. The 

 reason for this is obscure. The first carpale is very large to match the 

 size of the first metacarpal, and the unciform upon which the fifth meta- 

 carpal abuts, is on a line with the three carpales. The latter have no 

 tendency to adopt a conical form as in the seal and are not crowded. 

 Also there is perhaps less tendency than usual for the crowding of the 

 proximal ends of the metacarpals. These latter are on a line in the 

 first four, and somewhat more proximad in the fifth. The first digit 

 is much more robust than the others and is the longest. This is not 

 accomplished almost entirely by the elongation of the first phalanx, as 

 in the seal, but the metacarpal is also markedly lengthened. Ulnad of 

 the pollex the other digits are progressively shorter in gradual sequence. 

 The increase in the robustness of the pollex is to be expected, in order 

 that the anterior border of the flipper should be strengthened, for it is 

 this part of the manus that furnishes the chief impulse in swimming, 

 while the posterior border plays a more passive role. All the digital ele- 

 ments are much more definitely flattened than in the seal, and this is most 

 pronounced in the ungual phalanges, which are also of very irregular 

 shape, this likely having followed the atrophy of the nails, for the same 

 situation obtains in the Sirenia. 



There is a considerable deposit of fibrous tissue upon the entire 

 radial border of the flipper and even more at the ulnar border proximad 

 of the tip of the fifth digit, this helping to give a broader surface. This, 

 and the leathery covering of the manus greatly hinders adductive and 

 abductive action. After this tissue has been removed it is found that in 

 static posture the axis of the manus is abducted from that of the anti- 

 brachium by not more than 15 degrees. Thence practically no further 

 adduction is possible, but abduction is permitted to about 9'0 degrees. 



[251] 



