AQUATIC MAMMALS 



for flexing the arm. It appears equally likely that the corresponding 

 muscle groups in the seal have their chief use in pulling the head and 

 the hinder end respectively in a lateral direction while swimming. To 

 me it seems beyond question that if these highly specialized muscles 

 extending from the phocid humerus were actually a relic from a time 

 when seals might have used the arm as a primary means of propulsion 

 the external form of the manus would now show far more indication 

 of this fact. 



In the sea-lion the lateral epicondyle of the distal humerus projects 

 scarcely laterad of the trochlea, but is much better defined in the seal, 

 while in the former the medial epicondyle is greatly developed, but 

 slightly so in the seal. This is an indication that in the sea-lion the 

 flexor muscles of the fore arm have more leverage and hence are pre- 

 sumably more efficient than the extensors, and that in the seal the re- 

 verse is the case. These muscles will be more fully discussed later. 



The musculature of the dugong is almost unknown but it is likely 

 that in brachial details it is considerably different from the manati be- 

 cause there are very important differences in the form of the humerus. 

 In addition, Murie failed to figure many important items of the manati, 

 and in consequence my discussion of the sirenian brachium should be 

 considered as only, tentative and subject to amendment. 



The humerus in the dugong and the manati show two very different 

 trends, that of the former being very pinniped-like in some general 

 respects, while in the latter this is not the case. The head in both is 

 situated fairly posterior to the shaft axis. This is a rather trustworthy 

 indication that the chief direction of movement is in the sagittal plane, 

 or at least that any other primarily important movement that might have 

 been recently adopted has not yet had time to cause appreciable alter- 

 ation osteologically. This is also borne out by the fact that the lesser 

 tuberosity is not medially situated. Rather is it continuous with the 

 greater, the two conjoined forming a broad, transverse ridge fairly 

 anterior to the head. The medial part of this ridge, homologous to the 

 lesser tuberosity, is much higher than the latter, indicating, I should 

 think, that the humerus is normally held somewhat abducted from the 

 scapular plane. There is no teres minor in this genus, so the scapular 

 muscles inserting upon this ridge, or broadened tuberosity, are the sub- 

 scapularis upon the medial part, the infraspinatus upon the outer, and 

 the supraspinatus between them. The first, operating alone, should ac- 

 complish some inner rotation with adduction; the second, slight ab- 

 duction with outer rotation, or flexion when the humerus is also strongly 



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