THE PECTORAL LIMB 



entire length of the humerus. In both animals the pectoral is double, 

 one division being confined to the deltoid ridge and the other being al- 

 most as long as the whole humerus. In the seal the atlantoscapularis 

 inferior inserts upon most of the deltoid ridge, while in the sea-lion this 

 muscle does not reach the humerus but inserts upon the spine of the 

 scapula. Most of these insertions are by fascia and although it would 

 be utterly unjustifiable to say that the muscles mentioned have been only 

 of secondary importance in the development of such a high deltoid crest, 

 still it may be stated that all of them would appear to be equally effective 

 if operating upon a humerus without a crest. The large deltoid, how- 

 ever, which operates chiefly as a flexor by virtue of the shortness of the 

 humerus, is given the function also of a powerful rotator by the lateral 

 definition of this crest. 



Little more need be said regarding the pinniped pectoralis, for it has 

 already been considered in preceding chapters. In Phoca the main de- 

 velopment of this complex is posteriorly, for swinging the hinder end 

 sidewise, but the part medial to the arm is also very powerful, either for 

 quickly adducting the arm for steering, holding it firmly adducted while 

 the muscles anterior and posterior from the arm are used in swimming, 

 or probably both. In the sea-lion although the abdominal pectoral is 

 powerful it is far less so. It can be used to help control flexion of the 

 arm during terrestrial progression or backward sweeps of the flipper 

 when the need arises. In the seal the insertion of the pectorals upon 

 the humerus gives unusually efficient leverage, and in the sea-lion this 

 is even better developed. Not only is there double insertion over the 

 length of the humerus but extension of a superficial sheet onto the 

 forearm, giving the greatest possible leverage for strong and constantly 

 repeated adduction of the appendage. Flexion of the part inserting 

 over the deltoid crest also greatly aids the rotation of the anterior 

 flipper border that is so necessary during swimming. 



The significance of most of these extrinsic muscles of the humerus 

 has already been discussed in relation to the neck and other parts of 

 the body, but they must receive further consideration in the present 

 connection. Although these muscles have many significant similarities 

 in seals and sea-lions, yet their functions appear to be very different. 

 It seems almost certain that in the sea-lion the muscles extending from 

 the humerus to the head and neck are used chiefly for extending the 

 arm craniad, both when the need arises while in the water and during 

 progression by a lunging gallop while on land. And similarly that 

 the abdominal pectoral, latissimus, and panniculus may be likewise used 



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