BREATHING IN COMPRESSED AND IN RAREFIED AIR 407 



Theoretically, therefore, tlie decompression should be rapid at first and 

 very slow later. 



Before recommending the adoption of this principle of stage de- 

 compression in caisson work, Haldane and his coworkers made numerous 

 observations on the incidence of decompression symptoms in laboratory 

 animals. They assert that the stage method is decidedly safer than the 

 uniform method, the advantage being particularly after short exposures. 

 On the other hand, Leonard Hill could make out no definite advantage 

 for the stage method. The two methods have also been compared in 

 actual caisson work at the Elbe Tunnel, where the pressure was + 2 

 atmospheres. Very little advantage 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 absolutely 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 breath- 

 ing 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 probably 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 decom- 

 pression. 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 oc- 

 cur, 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 experiment by Greenwood may be cited: 

 During decompression from + 75 pounds pressure in 95 minutes "Green- 

 wood flexed and extended all the limb joints at frequent intervals, with 

 the exception of the knees. Subsequently pain and stiffness were ex- 

 perienced in the knees and nowhere else." In another experiment the 

 knees also were flexed and no pain was felt. 



But even in the parts with active circulation, the gas in the tissues 

 may lag considerably behind that in the blood, although the decompres- 



