RESPIRATION I3I 



lower still. The answer is that the respiration of terrestrial mammals is 

 generally very shallow. Take our own lungs, for example. Their maximum 

 capacity is 5 litres of air, but they generally contain no more than 2 -5 

 litres, i.e. they are only half-filled. Nor do we inhale and exhale even 

 that smaller amount for, with normal exhalation, 2 litres of air are left 

 in the lungs, and only if we breathe out as hard as we can is the residue 

 reduced to one litre. Our thorax, which protects the lungs, is so constructed 

 that only if we drill a hole into it can we get rid of more air still. Even if the 

 lungs are completely collapsed about 300 c.c. of residual air are left in 

 them. We shall have to return to this subject, and meanwhile note the 

 fact that man inhales and exhales no more than half a litre of air with 

 every breath, while his lungs are capable of taking in 4 litres at a time. 



Cetaceans, on the other hand, particularly when they dive regularly, 

 fill their lungs to capacity and, moreover, change 80-90 per cent of their 

 supply with every breath, unlike terrestrial mammals for which the 

 corresponding figures are 10-15 per cent (Fig. 75). The difference is due 

 not only to the fact that Cetaceans expand and contract their thorax to 

 the maximum, but also because that maximum happens to be 10 per cent 

 greater than it is in terrestrial mammals - so much so that their relative 

 volume of residual air is only half that of the latter. This was discovered 

 when, by drilling a hole into the thoi^ax of a dead Bottlenose Dolphin, it 

 appeared that far less air escaped, i.e. that the lungs were less collapsible 

 than those of, say, dogs or horses, for the very good reason that most of the 

 air had been expelled during exhalation. Even so, some residual air there 

 certainly is, as is proved conclusively by the fact that the lungs float when 

 they are thrown into the water. 



Possibly, whales and dolphins swimming quickly near the surface do 

 not breathe as deeply as they do during diving, but divers certainly make 

 up for their small lung capacity or their small respiratoiy frequency by 

 inhaling and exhaling as much air as they can. As a result, they may be 

 said to have no means of taking in special quantities of air when special 

 emergencies arise. Man, if need be, can breathe more deeply than he 

 normally does, but these animals, whose lungs as we have seen are already 

 filled to capacity, can get more air only by increasing the frequency of 

 respiration. In other words, they must surface more frequently, and this 

 fact is used by modern whalers who no longer trail the whales, but chase 

 them with very fast corvettes. The speed of the hunt is such that the poor 

 beasts are forced up to the surface much more often than usual, thus 

 presenting the gunners with excellent targets. 



We must now ask ourselves whether deep breathing really has the same 

 eflfect as shallow breathing with greater lung capacity or a faster respiratory 

 rate. Obviously, Cetaceans take down adequate supplies of oxygen, for 



