THE PECTORAL LIMB 



terrestrial uses are very incidental, save in the elephant seal ( Aiiro/niga), 

 in which the manus helps support the weight. It is usually kept folded 

 back against the body with the segments markedly flexed so that all of 

 it save the manus is contained within the body contour. Extension is 

 possible, however, and I have seen an animal on the steep margin of its 

 pool stretch forth the manus so that as well as I could judge the arm was 

 extruded from the body contour as far as the elbow. 



The normal or static position of the humerus in respect to the scapula, 

 determined to the best of my ability during dissection, differs only 

 slightly in sea-lions and seals. The angle formed by the humeral axis 

 with the spine seems to be slightly less in the latter, showing a greater 

 degree of flexure, but the difference is not sufficient to be of much signifi- 

 cance. The position' of the humeral head in relation to the shaft is 

 slightly more mediad in the sea-lion. This, I judge, is less marked than 

 one might expect, in consideration of the fact that the most efficient 

 method of swimming is by abduction-adduction movements, but this 

 development may be so recent that it has not had sufficient time to have 

 had marked osteological effect, and after all many other motions of the 

 humerus are of great importance. In phocids the head is more posterior 

 to the axis of the shaft, showing that extension-flexion motions are of 

 more pronounced import, and its conformation is such as to indicate 

 that possible flexion of the humerus in relation to the scapula is more 

 extreme in the seal. 



In the sea-lion the greater tuberosity is markedly higher than the head 

 and than the lesser, while in the seal it is much lower, the lesser tuber- 

 osity having greater elevation. The reason for the height of the greater 

 tuberosity in the sea-lion seems solely attributable to the supraspinatus 

 muscle, acting chiefly as an extensor, and the strength of this muscle 

 is shown in the scapula also by the great extent of the supraspinous 

 fossa. The infraspinatus, chiefly a rotator, also inserts upon this process, 

 but more proximad, and its position in Phoca is quite comparable, but in 

 this animal the supraspinatus is evidently not required to act so strongly 

 as an extensor, for not only is the supraspinous fossa of the scapula much 

 smaller, but the greater tuberosity, upon which it inserts, is hardly higher 

 than the head and very much lower than in the sea-lion, giving this mus- 

 cle reduced leverage. 



The conformation of the lesser tuberosity in the two pinnipeds con- 

 sidered is not altogether what one" would expect. In the sea-lion, al- 

 though robust and subtended by a heavy ridge, it is rather low and not 

 nearly as high as the head; consequently much lower than the greater 



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