AQUATIC MAMMALS 



tuberosity. Its height proximad has nothing to do with the strength of 

 the attached muscles, of course, for this detail depends upon the opti- 

 mum angle of leverage. Its relatively low height, then, would indicate 

 that the muscles operating from it are chiefly flexed when the arm is in 

 a rather adducted posture. Not only is the subscapularis well placed in 

 respect to the head for leverage in adduction, but insertion of the 

 episubscapularis (absent in seals) is located still farther distad along the 

 ridge, giving added power. What one might not expect in this animal 

 is that the lesser tuberosity is not situated directly mediad of the head 

 (to be most efficient in adduction) but medio-craniad. This situation 

 might easily translate movement that might otherwise be adduction 

 to one largely of rotation. This, in effect, may be actually what occurs 

 at times; or flexion of the same muscles may be productive of pure ad- 

 duction when other muscles are used as antagonists. It seems that chief 

 among the latter might be the teres major, whose insertion is broadly 

 along the medial shaft of the humerus distad of the middle of the bone. 



In the seal these medial insertions are disposed upon the humerus for 

 different action. The elevation of the lesser tuberosity is quite surprising, 

 it being much higher than either the head or the "greater" tuberosity. 

 The attachment of the cephalohumeral thereto can hardly account for 

 its height, and the only reason apparent is that the chief work of the 

 subscapularis, and attendant subscapulo-capsularis (absent in sea-lions), 

 is performed when the arm is pronouncedly abducted or elevated. This 

 might well take the form of a strong downward heave of the manus 

 while swimming for the purpose of quickly elevating the body, or while 

 scratching holes in the ice. Other muscles of this region are less well 

 situated for adduction of the humerus than in the sea-lion. There is no 

 episubscapularis and the insertion of the teres major is farther craniad 

 in respect to the head, theoretically giving a greater rotating action to 

 this muscle. 



The deltoid ridge, comprising a proximal continuation of the greater 

 tuberosity, is phenomenally developed in the pinnipeds, and a complex 

 of muscular stimuli is brought to bear upon it. In the seal this ridge 

 with tuberosity is but little more than half the length of the bone, while 

 in the sea-lion it is almost two-thirds, and in the latter especially the 

 concerned muscles accordingly have a phenomenal leverage. While in 

 the seal the humeral attachment of the cephalohumeral is confined to 

 the greater tuberosity, in the sea-lion it extends over the entire length 

 of the deltoid crest. In the seal this is the case with the humerotrapezius, 

 while in the sea-lion the insertion of this muscle is for practically the 



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