AQUATIC MAMMALS 



during adduction combined with extension forward of the limb. In the 

 sea-lion this correspondingly affects the cephalohumeral, part of which 

 arises from the anterior border of the pectoral. 



In the seal the panniculus carnosus muscle does not converge mark- 

 edly to the arm pit, but the fibers have an even cranio-ventral inclination. 

 This condition may have been purely an ancestral inheritance or may be 

 useful in the wriggling motions accompanying terrestrial locomotion, 

 but could hardly be of any help in swimming. Contrasted with this is 

 the situation in the sea-lion, in which fibers of the postbrachial part of the 

 panniculus all converge strongly to the arm pit. In this animal the sheet 

 of muscle covers the knee and extends quite to the base of the tail, and 

 as a result it is of great help in movement upon the land. Contraction 

 of this panniculus assists in flexing the lumbar region so that the hind 

 feet may be placed flat on the ground, and in galloping, contraction of 

 the panniculus after extension of the anterior limbs helps in pulling for- 

 ward the entire hinder end of the animal. In swimming the panniculus 

 can act from the other end and lend power to backward thrusts of the 

 fore flippers. This, combined with adductive motion, is also the func- 

 tion of the posterior part of the pectoralis. 



In the seal lateral movements of the hinder end are prerequisite to 

 swimming, and one would imagine that for it a panniculus of the sea- 

 lion type would be very useful, but as already said this has not been de- 

 veloped, possibly because any purely brachial stimulus for it has been 

 lacking. It is logical however, to expect lateral movements, accom- 

 plished mainly by the spinal musculature, to be markedly assisted by 

 ventral muscles, and this is brought about by an extraordinary develop- 

 ment of the pectoral muscles. Midventrally the latter extend from the 

 sternum (deep pectoral part) practically to the pelvis, although muscle 

 fibers do not occur quite so far caudad, while more laterally the abdom- 

 inal pectoral virtually reaches the knee. This abdominal division is ex- 

 tremely heavy and thick in its anterior portion. The whole postbrachial 

 part of the pectoral thus can operate to pull the hinder end sidewise while 

 the dorsal musculature counteracts a downward pull by the posterior pec- 

 toral. But such flexion tends to pull the arm to the rear and this must 

 be counteracted by antagonistic action of those lateral cervical muscles 

 that are attached to the arm. Thus the arm operates mechanically as a 

 sort of raphe, from which cervical muscles act in pulling the head to the 

 side while at the same time the posterior pectoral is doing the same for 

 the hinder end. 



[164] 



