BREATHING IN RAREFIED AND COMPRESSED AIR 425 



work -at the Elbe Tunnel, where the pressure was + 2 atmospheres. Very little advan- 

 tage could be demonstrated for the stage as compared with the uniform method at this 

 comparatively low pressure. The general conclusion which we may draw is that the 

 stage method should be employed, although it is not to be expected that it will ab- 

 solutely insure absence of decompression symptoms. Of course the great advantage 

 of the stage method is the saving of time, making it possible to persuade the workmen 

 to adopt it. , 



There are two other factors that are to be considered in hastening the desaturation 

 of the tissues; these are muscular exercise, and the breathing of an indifferent gas. 



It is clear, from what has already been said, that the gas dissolved in the tissues 

 will become removed in proportion to the mass movement of the blood, and it is prob- 

 ably true that muscular exercise, performed in the decompression chamber, is of as 

 great importance in preventing the subsequent development of symptoms as a much 

 prolonged decompression. In a man at rest, the circulation through the central nervous 

 system and the viscera is constantly influenced by the pumping action of the respiratory 

 movements, but in the capillaries of the muscles, joints, fat, etc., this influence is not 

 felt and the blood flows more slowly. It is consequently in these parts that bubble 

 formation is likely to occur, especially some time after decompression. The bubbles 

 cause the neuralgic pains the "bends" and "screws" so well known to caisson 

 workers. These could no doubt be entirely prevented by muscular exercise and massage 

 of the limbs during decompression. In illustration of these facts the following experi- 

 ment by Greenwood may be cited : During decompression from +75 pounds pressure in 95 

 minutes ' ' Greenwood flexed and extended all the limb joints at frequent intervals, 

 with the exception of the knees. Subsequently pain and stiffness were experienced in 

 t'he knees and nowhere else." In another experiment the knees also Avere flexed and 

 no pain was felt. 



But even in the parts with active circulation, the gas in the tissues may lag con- 

 siderably behind that in the blood, although the decompression has been properly 

 controlled. This has been shown by Leonard Hill in the case of the kidney. The 

 1 ' tissue ' ' gas in this case can be taken as the gas dissolved in the urine, by analyzing 

 which, therefore, at different stages of decompression, the excess of nitrogen over what 

 it should be at the external pressure, can be ascertained. On decompression from +30 

 pounds by two stages to zero, a considerable super-saturation was found to exist. The 

 excess of nitrogen can, however, be cleared out of the kidneys rapidly and completely 

 by breathing oxygen, which should therefore be administered during decompression in 

 cases where great care has to be exercised (Leonard Hill). 



When symptoms do appear, they can, in most cases, be relieved by recomprcssion, 

 and all modern caisson works are provided with a special chamber for this purpose. 

 We need scarcely say anything about this treatment here, as its value is so well known. 

 Suffice it to say that, although it is most likely to afford relief when applied as soon as 

 possible after the appearance of the symptoms, yet it is often efficacious when applied 

 several days after their onset. 



Quite apart from the dangers of decompression, it must of course be remembered 

 that the working conditions in a caisson are somewhat different from those at atmos- 

 pheric pressure, as the air, owing to its compression, is warmer and is loaded to satura- 

 tion point with moisture. This hot, wet air interferes with the heat-regulating 

 mechanism of the body, making hard muscular work very uncomfortable because of the 

 tendency of the body temperature to rise. The reaction of the body against this 

 tendency to hyperthermia consists in dilatation of the superficial capillaries and in- 

 creased heart action. 



When such working conditions are repeated day by day, the appetite is likely to 



