AQUATIC MAMMALS 



by individual shifting of the items. In the whalebone whales the hu- 

 meral head may be said to occupy a caudo-lateral position in respect to 

 the axis of the shaft indicating that there is not such definite or else 

 such habitual flexion of this segment as in most mammals, and that it is 

 held somewhat more abducted. The tuberosities have not altered their 

 positions. The lesser, situated mediad, is practically undeveloped as a 

 process and is indicated merely by a pronounced rugosity and slight 

 elevation of the bone. The determinant in developing this region into 

 a true process is the subscapularis, although for Balaeiioptera borealis 

 Schulte (1916) showed the coracobrachialis and mastohumeralis as also 

 attached upon this area. This lack of development of the lesser tuber- 

 osity indicates that either the subscapularis is unusually weak, which its 

 extent belies, that it normally operates when the arm is much adducted, 

 or else that it has a somewhat altered function, for instance to effect ro- 

 tation of the humerus when the arm is considerably flexed. I regard 

 the latter as the most likely of these three possibilities, although there is 

 nothing else to indicate that this is the case. 



In the Mysticeti the greater tuberosity is fairly well developed and 

 may be practically as high as the head. According to Schulte's figures 

 the reason for this is partly, though perhaps unimportantly, the infras- 

 pinatus attachment, and (chiefly) the deltoid, which here inserts. We 

 must presume for the present that this muscular condition is also found 

 in other mysticetes. It will thus be seen that although this process ap- 

 pears from an exclusively osteological viewpoint to be homologous with 

 a greater tuberosity it is hardly so except in position, and is actually a 

 deltoid process. The inference then, according to Schulte's figures, is 

 that in at least one mysticete the deltoid has gradually been altered so 

 that its origin occupies the scapular area normally held by the supraspina- 

 tus, and that its insertion enables it to function in the same way, except 

 that the latter also stretches far distad and onto the forearm in a way 

 that no supraspinatus ever does, thus not only effecting moderate exten- 

 sion from a more flexed posture of the arm (shown by the height of the 

 process) but also effectively aiding to maintain with a minimum of effort 

 such moderate extension as far as an angle of 90 degrees with the body. 



In odontocetes the details of the proximal humerus are quite dif- 

 ferent. Instead of the head being. located somewhat toward the rear of 

 the shaft it is usually situated to the side, and the lateral side at that. 

 Conformation of the tuberosities is essentially variable, undoubtedly re- 

 flecting important differences in the muscular equipment. On the whole 

 the lesser tuberosity may be said to occupy its normal position, at least 



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