AQUATIC MAMMALS 



bony details the neck of the seal is little or no shorter than that of the sea- 

 lion, which as far as we can see has no stimulus whatever for a shortened 

 neck. In spite of the latter feature the seal is apparently enabled to 

 meet the condition imposed by its method of swimming that the mass 

 anterior to the thorax be not too great in volume (for proper efficiency) 

 by the fact that its head is relatively small. We cannot be sure, of course, 

 that the length of its neck is absolutely ideal for swimming. In fact 

 it seems likely that there is present some indeterminate amount of stimu- 

 lus for a shorter neck because in swimming or in resting posture on land 

 the neck is retracted to a marked extent. That this is so is indicated by 

 the fact that if a fish be held above the head of a seal in captivity it will 

 stretch the neck to a phenomenal degree, when it appears fully as long 

 as in the sea-lion. It thus seems probable that in antagonism to the 

 stimulus for a somewhat shorter neck during swimming, there may also 

 be some stimulus, connected with the acquisition of food, for the reten- 

 tion of a moderately long neck. Incidentally the Phocidae may be as 

 yet insufficiently specialized for either one of these to have gained de- 

 cided ascendency over the other. 



Examination of an embalmed specimen does not throw any light on 

 the manner in which this extensibility of the neck is made mechanically 

 possible, but it is doubtless due in part to unusual elasticity of the in- 

 tervertebral cartilages. The apparent retraction of the neck while swim- 

 ming is partly real (to as great an extent as the vertebrae will allow) and 

 partly illusory, both because of the unusual breadth through the base 

 of the neck and probably because tension of the lateral neck muscles 

 tends to draw the shoulders forward. 



It has been noted in the previous chapter that the muscles of the phocid 

 occiput are so distributed as to facilitate movements in the sagittal and 

 in the horizontal plane. Other muscles of the neck follow the same 

 plan, and are distributed so as theoretically to pull the head up or to 

 the side with less effort than in the sea-lion, and with a minimum of 

 diagonal twisting. If we analyze the swimming movements of the seal 

 it will be seen that practically the entire musculature of either side is 

 concerned. Neither the forward nor the hinder end can be thrown 

 to the side without curving the entire body ; hence the muscles concerned 

 in both head-swing and tail-, or pedal-swing really constitute one single 

 group, all the components of which have been specialized toward the 

 single end of efficient aquatic propulsion. Some operate from the an- 

 terior thorax, while others especially the deep and abdominal pectorals 

 on the one hand and the inferior atlantoscapular, cephalohumeral and hu- 



[144] 



