AQUATIC MAMMALS 



curvature is in the vertical plane, the result being that the tail is de- 

 pressed. 



The manati thus effects swimming by flexion of the hypaxial caudal 

 mass, and of the panniculus and rectus abdominis to depress the rear 

 half of the animal, and this may be assisted by some of the neck muscles 

 acting to depress the head. The hypaxial muscle is less extensive than in 

 the Cetacea because the lumbar region is shorter. It is made up of a 

 massive superficial and a robust deeper division, both caudad of the last 

 rib, and in addition, a smaller muscle which reaches within the thorax 

 and which probably represents a quadratus lumborum. The antagonist 

 of this ventral group is, of course, the erector spinae. The divisions of 

 this seem to be well fused, as one would expect. A spinalis dorsi con- 

 tinuous with a levator caudae internus was said to have been distinct, 

 while a broad sacro-lumbalis was confined to the thorax. 



As previously indicated one cannot be sure whether the theoretical 

 mechanical stimulus encountered by aquatic mammals is for an exactly 

 round thoracic cavity, or a round trunk as a whole. Personally I con- 

 sider it likely that this stimulus alone is rather feeble, or else that it is 

 overcome by much stronger ones. Just as a combination of factors has 

 resulted in the assumption of a trunk laterally flattened by speedy pelagic 

 fishes such as the mackerel, so might one expect to find that the swiftest 

 whales are so flattened but in the sagittal plane, to correspond with the 

 different plane of tail motion. As a matter of fact these two conditions 

 are complicated by at least two factors that should influence tail shape, 

 as discussed in the next chapter. 



The thoracic shape in whales is so variable that all our arguments fail 

 of application. It is impossible to ascertain the precise thoracic cross 

 section of a living, adult mysticete. Mounted skeletons indicate that 

 the cavity is either slightly broader than high, or else is approximately 

 circular. Allowing for the spinal musculature this would make the en- 

 tire trunk either approximately circular or else higher than broad. And 

 the latter seems to be the situation in at least the majority of porpoises, 

 to a really quite marked extent in some. Levitation by the lungs may 

 have been of influence in shaping those with broadened thorax, but this 

 argument takes us no farther. We might expect to find that the very 

 strong levitation of the lungs during deep diving had elevated the dor- 

 sal curve of the ribs well above the vertebral centra, but this is no more 

 marked than in many terrestrial mammals. 



As with the shape of its cross section there is really extreme variation 

 in the length of the cetacean thorax, it having become elongated in some 

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