THE PECTORAL LIMB 



acteristic of ichthyosaurs, chiefly for the reason that all but the manus 

 is contained within the body contour, the function of the long bones 

 appearing to be chiefly that of a scaffold upon which are hung some of 

 the muscles of importance to swimming, rather than an integral part of 

 a flipper. 



The part which the antibrachium plays in the economy of aquatic 

 mammals diflPers considerably in various sorts. It is difficult to know 

 in just what light it should be considered in the Phocidae. In the 

 latter the position of the external axillary region is opposite the wrist, 

 which is capable of extreme mobility, and hence the antibrachium cannot 

 be considered as functioning with the manus. And yet the humerus 

 acts as independently of the forearm as the exigencies of the position 

 of both within the body contour will admit. Although in consideration 

 of the functional differences the osteological similarity of the forearm, 

 consisting of a broadening of the bones, in seals and sea-lions is really 

 phenomenal, the two should be considered separately. In both of them 

 the elbow joint is synovial but flexion of the forearm is apparently re- 

 duced, especially in the sea-lion. 



In the seal (Phoca bispida) which I have dissected there is a remark- 

 able broadening of the distal radius and proximal or olecranol part of 

 the ulna, which does not affect the opposite ends of these bones. No 

 useful reason for this specialization is positively known. As the anterior 

 limb is used for neither propulsion nor as a specialized rudder, as shown 

 by form of the manus, there could not be this stimulus for a broadened 

 forearm, and besides, the recession of the segment within the body 

 contour renders any such mechanical adjustment useless. Although 

 there may be perfectly good obscure reasons for this condition there is 

 only one apparent. The divisions of the phocid triceps muscle are truly 

 enormous and in addition have phenomenal leverage by virtue of their 

 attachments. Thus the longest division extends from the vertebral bor- 

 der of the scapula to beyond the middle of the fore arm. In action it 

 not only extends the antibrachium but sweeps the whole arm to the rear ; 

 and the large, broad olecranon gives it just so much more leverage. The 

 origin of the two extensores pollicis muscles from the lateral olecranon 

 well away from the joint give to them a marked ability to supinate the 

 manus. Upon the medial side of the bone an unusually large area is 

 provided for the broad and powerful flexor digit, communis, with con- 

 sequent power of digital flexion, and the broad ulna provides a better 

 lever arm for the adductor action of the peculiarly developed abductor 

 digiti quinti longus. I can see no definite muscular advantage in the 



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