THE PECTORAL LIMB 



these are double in the three middle digits of the platypus and single 

 in the sea-lion, so although the two are analogous in some respects they 

 need not be homologous. In other words the same need has been ful- 

 filled by somewhat different structures. But the chances are that in 

 both sorts the membranous border of the manus extended well beyond 

 the digits before there was much stiffening needed, although in the 

 platypus the stiffening elements have now outstripped the intervening 

 membrane. 



Presumably there must be a contrivance in the sea-lion for over- 

 coming involuntarily the tendency for the water resistance while swim- 

 ming to force the manus to extension with the fore arm through 90 de- 

 grees ; in other words to prevent the manus from assuming its usual ter- 

 restrial posture. It is likely that this need is provided for by special 

 tonus of the palmaris longus. The latter is exceedingly broad and 

 powerful, and partly tendinous throughout a considerable portion of its 

 length, so that it could well perform such a static function. Further- 

 more its palmar tendon splits into two parts, the more robust of which 

 extends along the radial border of the hand and the other along the ul- 

 nar border. Flexion while forcing the palm against the water would 

 accordingly have a cupping action upon the palm, the force being more 

 pronounced along the anterior border. The fourth head of the flexor 

 digitorum communis and the flexor carpi radialis, in addition to the ex- 

 tension of a part of the pectoral, are also well disposed to act upon the 

 radial border of the flipper, so that the latter has an unusually powerful 

 equipment for sweeping the flipper obliquely through the water after the 

 manner that will prove most effective in propulsion. 



The nails of the manus in the sea-lion should receive further brief 

 mention. As the part of the flipper distad of the bony termination of 

 the digits cannot be folded back against the palm the nails can no 

 longer be used in scratching the body or in any other effective way. 

 Accordingly there is no stimulus for their retention and they are now 

 reduced to insignificant nodules which do not project beyond their in- 

 tegumentary pits. 



It is frequently stated that the manus of the walrus is of a character 

 intermediate between that of the seal and the sea-lion, but this is not 

 quite correct. In size this is so, but in characteristics it is merely a 

 less developed otariid flipper that is shorter and broader, but with digi- 

 tal cartilages and rudimentary nails also. 



Just as there is considerable variation in the details of the remainder 

 of the cetacean arm, so is there corresponding differences in the manus. 



[253] 



