RESPIRATION 3^^ 



the same, but from 4.4 to i atmosphere, or in the proportion of 

 4.4 to I, only 20 per cent of the animals escaped symptoms, while 

 20 per cent died, 30 per cent had severe symptoms, and 30 per 

 cent had "bends," quite easily recognized in the animals by their 

 behavior and the manner in which they held the affected limb 

 (Figure 88). It seemed evident, therefore, that it is quite safe to 

 halve the absolute pressure rapidly. Before venturing on such 

 extensive rapid decompressions of divers under water we re- 

 peated the goat experiments on men in the steel chamber, Com- 

 mander Damant and Lieutenant Catto being the subjects. There 

 were no ill effects in a number of experiments, nor in subsequent 

 trials by them under water at sea; and rapid decompression to 

 half the absolute pressure is now the routine practice of divers, 

 and is not known to have ever resulted in harm. 



We were still, however, only at the beginning of the inquiry. 

 It was evident that the whole danger lay in the last stages of the 

 decompression. "On ne paie qu'en sortant," as was remarked by 

 Pol and Watelle, who were the first to give a medical account of 

 the symptoms of caisson disease.''^ The problem was to get divers 

 completely clear of the compressed air without paying. This 

 problem had resolved itself into that of avoiding the critical 

 supersaturation with nitrogen in any part of the body at or before 

 the last stage of decompression. 



Let us consider the process of saturation and desaturation more 

 closely. The blood passing through the lungs of a man breathing 

 compressed air will, in accordance with what has been explained 

 in Chapter IX as to the permeability of the lung epithelium to 

 gas, become instantly saturated to the full extent with nitrogen 

 at the existing partial pressure in the air. When this blood 

 reaches the systemic capillaries, most of the excess of nitrogen 

 will diffuse out and the blood will return for a fresh charge, this 

 process being repeated until at length the tissues are fully charged 

 with nitrogen at the same partial pressure as in the air. But the 

 blood supply to different parts of the body varies greatly, as we 

 have seen. The capacity of different parts of the body for dis- 

 solving nitrogen varies also. Thus the white matter of the central 

 nervous system has but a small blood supply and at the same time a 

 high capacity for storing nitrogen ; and the same remark applies to 

 fat. The gray matter, on the other hand, has an enormous blood 

 supply and no extra capacity for storing nitrogen. Other tissues, 



^ Pol et Watelle, Ann. d' hygiene publique, (2), p. 241, 1854. 



