122 PROCEEDINGS OF THE NATIONAL MUSEUM vol.73 



strength; but there are other muscles controlling the lumbar flexi- 

 bility. These comprise chiefly the panniculus, latissimus dorsi, the 

 posterior part of the pectoralis, rectus abdominis, and to some slight 

 extent the other abdominal muscles. In the water, with hind quar- 

 ters extended, the first three of these act as strong extensors of the 

 humerus during propulsive action of the arm. Upon the land all 

 of these muscles operate largely from the opposite end, assisting to 

 the limit of their capabilities in flexing the lumbar region and mak- 

 ing of this and the sacrum a sort of substitute femur and shank 

 which the animal can use more expeditiously than it can the two 

 upper segments of the limb proper. This flexure of the posterior 

 vertebrae is used in a variety of ways besides in swimming, as in 

 scratching, of which one can assure himself by watching the contor- 

 tions of a young otariid at play. 



The function of these muscles in the Phoca is very different. The 

 panniculus, with fibers running rather evenly cranio- ventrad rather 

 than converging to the axilla, is undoubtedly of assistance in the 

 caterpillarlike mode of terrestrial progression, but it is difficult to 

 see how the other muscles mentioned above can be of great use in 

 such movements. In fact, an analysis of the manner in which the 

 seal travels upon land is rather puzzling, but it is likely that the long 

 back musculature and the psoas complex furnish most of the motive 

 power. The rhythmic lateral movements of the hind flippers em- 

 ployed by these animals as the primary, and indeed sole means of 

 propulsion in swimming have their inception in the middle thorax, 

 as they do in the Cetacea and most fish, while the anterior thorax 

 and the neck in large degree act as a fulcrum. The muscles of the 

 posterior thorax and the lumbar region that are employed in these 

 movements are primarily the enormous sacrospinal muscles, which 

 have become massive in size but of simpler design, and accessory to 

 these the hypaxial musculature or psoas complex. Of secondary but 

 still of definite importance in this connection are also the latissimus 

 dorsi and the posterior half of the pectoralis. They help to pull the 

 posterior end of the body from side to side and operate chiefly from 

 the deltoid crest .of the humerus as a fulcrum, while the atlanto- 

 scapularis inferior, humerotrapezius, and perhaps other muscles in 

 conjunction, act as antagonists to prevent humeral movement. As 

 in fish, the apaxial and hypaxial muscles of a single side act as a 

 unit, although there is theoretically nothing to prevent them from 

 operating with equal effectiveness in the sagittal plane as is the case 

 in the Cetacea. The hypaxial muscles act upon the ilium and the 

 leg, while the sacrospinalis or apaxial mass acts chiefly upon the 

 whole pelvis, through its extensive insertion upon the " medial " face 

 of the ilium, turned sharply lateral for just this purpose. 



