406 THE RESPIRATION 



same number of minutes to desaturate that it took to saturate, and the 

 parts of the body that will lag behind the others, in being desaturated, 

 are those with a sluggish circulation. 



When the mass movement of the blood is increased by muscular exer- 

 cise, the rate of saturation and desaturation with nitrogen is increased 

 in proportion. During active work the increase in movement of the 

 blood may be four or five times over the normal, so that the tissues of 

 the caisson worker become much more quickly desaturated during decom- 

 pression than the above figures would lead one to expect. 



Application of Foregoing Laws in Practice 



With regard to the application of these principles in the decompression 

 of caisson workers, it is impracticable to occupy as much time as it takes 

 to saturate the body even at comparatively low pressures. If the great 

 dangers attending work in compressed air are to be avoided, we must 

 either insist on very gradual decompression or we must show how the 

 dissolved gases may be got rid of by some modification in the decom- 

 pression procedure. With this object in view, we must determine what 

 difference of pressure may be allowed between the external air and the 

 body without the formation of bubbles. Actual experience shows that 

 there is no risk of bubble-formation, however quick the decompression, 

 after exposure to + 15 pounds pressure ( i. e., 2 atmospheres absolute) . 

 "Now, the volume of gas capable of being liberated on decompression 

 to any given pressure is the same, if the relative diminution of pressure 

 is the same" (Haldane 35 ). On reduction from 4 to 2 atmospheres, 

 the same volume of gas will tend to be liberated as on reduction from 2 

 to 1 atmospheres that is to say, no bubbles will form. The practical 

 conclusion is "that the absolute air pressure can always be reduced to 

 half the absolute pressure at which the tissues are saturated without 

 risk." Thus, after saturation at 90 pounds absolute pressure (H- 5 atmos- 

 pheres), a man can be immediately decompressed to 45 pounds (+ 2 

 atmospheres) in a few minutes without risk, but from this point on the 

 decompression must be conducted slowly, so as to insure that the nitrogen 

 pressure in the tissues is never more than twice the air pressure. The 

 great advantage of this method is that it makes the greatest possible use 

 of difference of pressure between tissues and blood in order to get rid of 

 the gas that these contain. 



When the decompression from the start is gradual, the desaturation 

 of the tissues will progressively lag behind that of the blood, and the 

 tendency to the liberation of free gas will become greater. In such a 

 case the decompression is far too slow at first and far too rapid later. 



