AQUATIC MAMMALS 



In those odontocetes dissected the extremely robust deltoid inserts 

 upon the whole lateral face of the distal humerus, acting as a powerful 

 abductor of the arm, but this insertion is thicker and stronger craniad and 

 it likely has some rotating action as well. The latter fact is not shown 

 osteologically in all toothed whales but in fully adult Tursiops, for in- 

 stance, it seems clearly indicated by a very pronounced process upon the 

 distal third of the anterior aspect of the humerus. This strongly im- 

 presses one with the fact that if the latter protuberance were shifted 

 more to the proximal end of the bone, as is largely the case in some in- 

 dividuals of Kogia, it would form precisely the same sort of "greater 

 tuberosity" as appears to have been caused in the Mysticeti by the deltoid. 

 There is a similar process, evidently attributable to the deltoid, in zeu- 

 glodonts, but located just distad to the middle of the shaft. 



The above muscles taken together seem to show that in the Cetacea 

 there is a reduced power of extension of the arm, and increased power 

 of abduction, and perhaps a slightly greater power of both adduction and 

 rotation upward of the anterior border. The latter act is also markedly 

 assisted in mysticetes by flexion of the serratus magnus. There is not 

 apparent any special provision for depression of the anterior border of 

 the arm. The general conformation perhaps makes this motion not so 

 necessary, or it may be effected by special action of the latissimus dorsi 

 or other muscles. 



The distal humerus of the Cetacea lacks the stimulus supplied by 

 functional antibrachial muscles and a synovial joint. Accordingly the 

 lateral and medial elevations of the condyles have atrophied. Even were 

 there absent some undefinable stimulus for a broadening in an antero- 

 posterior direction of all the arm bones, which indubitably exists, this en- 

 tire lack of reason for condyle definition would likely be sufficient to al- 

 low the distal humerus to broaden out in an antero-posterior direction to 

 conform to the extent in this plane of the antibrachial bones. 



It is unfortunate but nevertheless true that when I was preparing a 

 previous paper on the pinnipeds (Howell, 1929) I was unable clearly 

 to observe all the motions followed by a sea-lion when swimming at 

 speed. I gained a satisfactory understanding of movements during more 

 leisurely progress and made the mistake of supposing that these were 

 used at a faster gait, for I had not then encountered the correct condi- 

 tions of light and clarity of the water to see them dart at speed deep 

 below the surface. This course seemed entirely justified for the reason 

 that the posterior part of the pectoralis and the latissimus dorsi are so 

 well developed for executing powerful backward thrusts. The statements 



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