RESPIRATION IN AIR 85 



during work, and the maximum debt allowed does not seem 

 to be essentially different when the size of the animals is taken 

 into consideration. The small animals having a high rate of 

 resting metabolism per unit weight can as a rule contract a 

 debt corresponding only to a few minutes' metabolism, but 

 it must be admitted that the domestic duck will stand 27 

 minutes' asphyxiation (Richet, 1899). The maximum debt 

 of a seal weighing 30 kg corresponds to 20-25 minutes. In 

 man the maximum debt incurred during work corresponds 

 to 40 minutes. The large whales of 50 to 100 tons can remain 

 submerged for about an hour, and in the case of the sperm 

 whale perhaps more. After such a dive they will stay near 

 the surface and "blow" at short intervals for some time. 

 The bottlenose, which can stay down for up to two hours, 

 weighs only 8-15 tons. Although there are large discrep- 

 ancies there is some relation to size, and the conception of a 

 relation to rate of metabolism is strengthened by the fact that 

 cold-blooded divers will stand submersion for very long periods. 



There are certain indications in experiments on man ex- 

 posed to varied tensions of oxygen during work, and in experi- 

 ments and observations on diving birds and mammals, that a 

 continuous or intermittent supply of a little oxygen increases 

 the amount of debt which it is possible to contract. No 

 explanation can be given at present. 



The main difference between a diver and a non-diver ap- 

 pears to be in the regulation of the circulation which allows 

 the diver to reduce greatly the blood-flow to the muscles and 

 perhaps also to other organs and to reserve the supply mainly 

 for the central nervous system. 



Animals diving to considerable depths have one more 

 serious difficulty to encounter. It was observed on human 

 beings, diving by means of a helmet or in a "caisson," that 

 unless special precautions were taken they were liable to 

 "caisson disease." They are supplied with air at a pressure 

 corresponding to the depth, that is, approximately one extra 

 atmosphere for every 10 meters. Breathing compressed air is 

 not in itself harmful unless the oxygen pressure is raised above 



