THE PECTORAL LIMB 



der that a whale may elevate or depress its line of progress. It is clear 

 that small muscles inserted near the head of the humerus would be cap- 

 able of but feebly waving about a long arm with broad paddle upon its 

 end. For efficiency the arm should be shortened, as already argued, the 

 critical muscles should be strengthened, and their effective leverage in- 

 creased by a migration of their insertions distad from the head of the 

 humerus. Such alteration in muscle attachment will effect alteration in 

 the bones, and this in turn may greatly change the functions of the mus- 

 cles involved. 



As already discussed there is no longer any reason for aquatic forms to 

 hold the limbs in vertically dependent posture and the tendency, if un- 

 complicated, is probably for these members to be held at an angle of 

 about 45 degrees. We have no means of knowing the exact angle fav- 

 ored by various sorts of cetaceans, but the osteological evidence would 

 indicate either that in most porpoises the habitual posture of the flippers 

 is more abducted than in Mysticeti — which seems unlikely — or that the 

 chief u'ork performed is instigated from a position with the flippers 

 more elevated or abducted in the former cetaceans. Theoretically this 

 chief work should consist of strong downward movements of the flippers 

 after they have first been elevated, ostensibly for elevating the anterior 

 end of the animal. 



Incidentally it should be mentioned that at least most cetaceans prob- 

 ably cannot extend the flipper forward to an angle greater than 90 de- 

 grees to the body axis, if indeed even this much extension be possible. 

 The point at issue, however, is that whales abduct (elevate) the limb 

 at the shoulder joint to a considerably greater amount than does the aver- 

 age terrestrial mammal, the degree depending upon the sort of whale. 

 Thus, whereas the limb movement in most mammals is fore and aft, or 

 by extension and flexion, in whales it is rather in the transverse plane, 

 involving abduction and adduction. This change is accompanied by cer- 

 tain definite alterations in the shoulder, which are reflected in the hu- 

 merus. 



It is frequently stated that there has been rotation of the cetacean hu- 

 merus. I do not altogether approve of this term as it is somewhat mis- 

 leading, and the process has evidently been entirely different from that 

 experienced by man, in which the upper arm has been rotated by a shift 

 in the usual position of the elbow joint. In whales the elbow is in the 

 plane usual in Mammalia. In the" Mysticeti the elements of proximal 

 humerus have not undergone much alteration in position, but in the 

 Odontoceti they have, apparently not by any twisting of the bone but 



[219] 



