AQUATIC MAMMALS 



tage to its well-being, but there appears to be a rather uniform plan of 

 scapular suspension employed in this order and it is likely that very 

 few, or very circumscribed, movements of the shoulder are indulged in. 

 There is a tendency apparent for reduction of the width of the shoulder 

 attachments, which are more segregated into three areas than is the usual 

 case: below and slightly to the rear there is a narrowed pectoralis (either 

 single or double) ; above and slightly to the rear a broad rhomboid, 

 which may be continuous along its border with a narrowed latissimus 

 dorsi ; and anteriorly there is anchorage to the atlas and mastoid region 

 by two or three narrow muscles (atlanto- and mastoscapular, and masto- 

 humeral). The serratus magnus may be either very narrow, or very 

 broad as as to function as a very efficient antagonist to the extensive 

 rhomboid. 



In pinnipeds the suspension of the shoulder is according to a different 

 principle. The dorsal, ventral and cranial anchors are all spread to a re-, 

 markable degree, allowing powerful movement in any and all directions. 

 There are three widely-spreading trapezius divisions, the anterior rhom- 

 boid reaches the head, and other shoulder muscles are specialized ac- 

 cordingly. 



There is no movement of the shoulder involved in the act of swim- 

 ming by the true seals, and yet it is inconceivable that such specialized 

 shoulder musculature could constitute merely a phylogenetic inheritance 

 from a more specialized ancestor. If we look upon it as having de- 

 veloped for the purpose of lending all possible assistance to the act of 

 swimming, then is it more understandable. The action is entirely too 

 complex for simple analysis, but after much study of the question I must 

 believe that the end toward which the shoulder muscles of these animals 

 have striven is for the purpose of accentuating the lateral movements 

 of the hinder end of the body in one direction, and the forward end in 

 the other. The muscles anterior to the shoulder would in this case act 

 largely as antagonists to those posterior thereto, and the shoulder is thus 

 comparable functionally to a sort of raphe between the two groups. The 

 chief muscles concerned are the phenomenal pectoralis and latissimus 

 dorsi on the one hand, and the cephalohumeral and humerotrapezius on 

 the other. Movement of the shoulder muscles other than those of im- 

 portance to swimming may be relatively inddental in this animal. 



In the Otariidae or sea-lions the functions of the shoulder muscles are 



very different from those in the seal. The shoulder proper does not play 



a largely passive part during swimming, but an essentially active one. 



Some of the extrinsic muscles (chiefly the pectoralis, but the cephalohum- 



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