538 THE AIR AND LIFE. 



tlie diininntion be l>ronglit on by degrees. We see here some difference 

 betAveeu tlie plieuoineiia i)reseiited and those wbicli occur iu the case of 

 gradual reduction of pressure, a difference which is explained when we 

 examine the bodies of the vi(;tims by the fact that there are found 

 under the skin, in the tissues and in the vessels, free gases, a condi- 

 tion that nin'er occurs normally. The cause of death is revealed by 

 these gases. Blood and all the tissues constantly contain, as we know, 

 gases such ;is oxygen, nitrogen, etc., that are either free or combined 

 with the globules, and in proportions which vary with the external 

 pressure; that is, according to the degree of tension of the same gases 

 in the atmosphere. If the barometric pressure gradually decreases, 

 the tension of the gases in the organism decreases in the same manner. 

 The gases gradually pass from the blood into the atmosphere without 

 causing any disorder. But if the rarefaction is sudden, this gradual 

 operation can not be accomplished, and the result is tluit the gases in 

 the blood and tissues, being i a contact with an atmosphere in which 

 the pressure is much less than in the blood, are suddenly released in 

 the shape of bubbles and the circulation is paralyzed. To avoid acci- 

 dents of this kind men working in compressed air are always cautioned 

 against a too sudden return to tlie open air. They have little to fear 

 from the compressed air; all the danger lies in the removal of this com- 

 pression. ''We pay as we go out,'' as they put it in their picturesque 

 language. 



So nuicli for a decrease of pressure, whether rapid or slow. In the 

 case of slow depression the harm is done by a deficiency of oxygen, by 

 anoxyhemia, and it is for this reason that aeronauts take oxygen along, 

 so as to counteract the rarity of the atmosphere at great heights. In the 

 case of rapid de[)ression, whether the transition be from a high to an 

 intermediate or normal pressure, or from an intermediate to a low 

 pressure, the trouble arises from another cause, of a mechanical nature, 

 from the swift release of the gases contained in the tissues, and es])e- 

 cially in the blood, thereby producing a stoppage of the circula- 

 tion. The ])reseuce of free air or of any other gas in the A^essels is 

 known to quickly produce paralysis of the heart. Whenever the sud- 

 den change of pressure is from high to intermediate, anoxyhemia does 

 not intervene and the mechanical cause prevails. If the intermediate 

 pressure changes to a low pressure, anoxyhemia takes place if the 

 change is slow. Both anoxyhemia and disengagement of gases occur 

 if the change is sudden. 



Let us now consider the cases in which the pressure is increased 

 That which takes place in the depths of mines need not be discussed, 

 for the increase is insignificant, and its physiological effects may be left 

 out of account. The influence of au increased pressure is best studied 

 as affecting divers, for instance, and men engaged in sinking bridge 

 piers. These men work in ;i medium where the barometric pressure is 

 high, much higher than in the deepest mines, for tlie reason that the 



