THE TRUNK 



In the case of mammals which are exclusively aquatic gravitation does 

 not enter the question. It is true that at least most whales are slightly 

 heavier than the water which they displace and that for equalized nota- 

 tion they need a lung-full of air. Hence there is some slight force of 

 gravity experienced; but when distributed over the entire animal it is 

 absolutely negligible. In consequence the conformational stimuli ex- 

 perienced by the thorax are those supplied by muscle pull, levitation of 

 the lungs within the cavity, such amount of tendency for a circular trunk 

 as progression through the water may determine, the influence which the 

 location of heavier bones may introduce, and at times a possible fifth 

 factor. In regard to the latter, certain odontocetes indulge in a rocking 

 motion during progression. The killer whale especially is in the habit 

 of swimming slowly and taking a fresh breath every few seconds. Ac- 

 cordingly the head is first elevated above and then depressed below the 

 surface, and the back rocks to and fro in the sagittal plane. Undoubtedly 

 this has had some effect, however slight, upon the conformation of the 

 entire animal. But it is impossible to know the relative importance of 

 any of these influences. 



It is popularly believed that aquatic life augments the lung capacity 

 and in consequence that there is an increase in the diameter of the thorax. 

 This is entirely logical and may be conceded. Certainly the thorax of the 

 sea otter seems to be considerably more capacious than in its river cousin. 

 But usually there is no sure yardstick whereby this can be measured and 

 it must be inferred. For one thing there are no thoroughly aquatic 

 mammals sufficiently close to terrestrial forms for adequate comparison; 

 and an expansion of the thorax may merely indicate that there has been 

 a shortening of the abdomen, or an increase in the size and capacity of 

 the alimentary tract may have crowded the thoracic cavity. 



Unfortunately I have neglected to examine the cross section of the 

 chest of any fresh pinniped or cetacean, and a preserved specimen soon 

 becomes so distorted by its own weight it is valueless for this pur 

 pose. Nor can great reliance be placed in a mounted specimen. 



Among the smaller aquatic mammals the only one (so far as I know) 

 having details of the thorax that merit attention in the present connection 

 is the insectivore otter (Potomogale). Dobson (1882) showed that in 

 this animal the pectoral muscles are surprizingly unlike the general pat- 

 tern that is so characteristic of insectivores. Rather is it suggestive of 

 conditions in the sea-lion. I believe, however, that this resemblance is 

 fortuitous and that there is shown merely an intermediate stage in even- 

 tual convergence toward a similarity in appearance to pectoral conditions 



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