AQUATIC MAMMALS 



ses are continuously and uniformly present as far as the peduncle and in 

 these, as far as can be told from bony details, the post-thoracic apaxial 

 musculature is relatively homogeneous, so that the lumbar region seems 

 to constitute the base of the tail. In porpoises with an excessive num- 

 ber of lumbar vertebrae it seems that all spines except those near either 

 end lack zygapophyses, so that in these sorts there are involved two re- 

 gions, in some manner separately specialized, of the lumbo-caudal part 

 of the erector spinae, one in the lumbar and the other in the caudal area. 

 Conditions suggest that a need experienced for longer lumbar muscles 

 may have been instrumental in lengthening this region in certain por- 

 poises. This in turn suggests that there may be some difference in the 

 exact muscular action by which these two sorts of cetaceans accomplish 

 swimming. 



In Cetacea the spinous processes and parapophyses, constituting simple 

 transverse processes, of the lumbar series are greatly developed, while 

 other bony protuberances are either much reduced or entirely absent. This 

 corresponds to the simplification of the spinal musculature. The whale 

 swims by movements in the vertical plane of the caudal appendage or 

 flukes, and the spinal muscles are called upon for little else. Naturally, 

 with almost perpetual use these have acquired phenomenal thickness. In 

 the lumbar area the dorsal muscles are imperfectly divisible into two ser- 

 ies (iliocostalis and longissimus) while in the posterior thorax these may 

 be even more homogeneous, and continue onto the head with little 

 change either in mass or character. The muscular action concomitant to 

 swimming has already been discussed to a considerable extent so that here 

 little need be said, save to repeat that in whales, as contrasted to seals, 

 the back muscles can act more uniformly throughout the entire vertebral 

 length, permitting more latitude in possible shifting of the pivot of mo- 

 tion. 



The erector spinae operates to raise the tail. Consequently in whales 

 there must be an antagonist to depress the tail with potential force that 

 is approximately equal. This is provided by the extraordinary modifica- 

 tion of the infravertebral or hypaxial musculature of the tail base and 

 lumbar region. In the latter situation there are presumably psoas and 

 quadratus lumborum elements, but it is entirely out of the question to de- 

 cide whether all of these have become hypertrophied or some have in- 

 creased at the expense of others, for anteriorly they are so simplified that 

 only one superficial separation is possible. In what may be termed the 

 posterior lumbar area the hypaxial and apaxial musculature are of ap- 

 proximately equal mass, but whether they are each capable of exerting 



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