AQUATIC MAMMALS 



which time the oxygen in the lungs has been reduced to a point of from 

 9 to 1 1 per cent. Whether whales differ in their tolerance of free car- 

 bon dioxide in the lungs is as yet unknown, but it is assumed that they 

 must be much more tolerant, which would be accomplished at least par- 

 tially by alteration in the sensitivity in this respect of the respiration cen- 

 ter of the brain. 



But there is another aspect from which this gas must be considered. 

 In dogs the venous blood has a CO 2 content of about 45 cc. per 100 cc. 

 of blood. Not more than from 2 to 2.5 cc. of this can be held in 

 physical solution, while the remainder must be in chemical combination. 

 Carbon dioxide can enter the corpuscles and react with alkalis combined 

 with the hemoglobin to form a bicarbonate which in the lungs breaks 

 down again to liberate CO 2 . Not only should the efficiency of this 

 process be quantitatively facilitated in marine mammals, with their re- 

 ported increased hemoglobin content, but there may be qualitative aug- 

 mentation as well. Pressure may be of critical import in this connec- 

 tion, as it might well be if it should prove that the oil of whales can 

 actually take up some of the carbon dioxide from the blood. I regard 

 it as certain that any such physiological processes cannot operate with an 

 equal degree of efficiency at surface-water pressure and at the pressure 

 of a ton to the square inch. For this reason the possibility does not ap- 

 pear fantastic that the cachalot may not be able to hold its breath for 

 an inordinate length of time say fifteen minutes when at the surface, 

 while the increased pressure at the depth of one mile might so alter 

 its physiological processes that it would have no difficulty in remaining 

 below for an hour. There is no evidence whatever in support of such 

 a theory, but it must be taken into consideration in planning future 

 experiments. 



It is not always easy to determine the length of time during which a 

 mammal may hold its breath. Thus I have taken every opportunity 

 to observe seals and sea-lions but have never yet seen one submerge 

 for much more than a couple of minutes. And yet it seems certain that 

 at least the boreal seals, which spend much of their time beneath the ice, 

 must very greatly exceed this, say to the extent of at least ten and very 

 possibly twenty minutes. 



A man may hold his breath without undue discomfort for one 

 minute I have just done so. The reader may exceed this, and if he 

 first violently and completely empty his lungs several times he may be 

 able to last as long as two and a half minutes. But this is a very mod- 

 erate accomplishment. Beebe (1926) has stated that a sloth breathes 



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