Decreased Pressure 981 



The question then appears to have been reduced to a remark- 

 able simplicity; but though the cause of the phenomena observed 

 can thus be expressed in a word, its consequences are so diverse 

 that they deserve to be studied in the different conditions in which 

 the diminution of pressure can act. 



1. Aeronauts. 



Let us begin with the simplest case, and let us consider first 

 the aeronaut, who, without making any effort, is lifted in the up- 

 ward course of his balloon. 



As he rises and the pressure diminishes, his blood loses its oxy- 

 gen, as my experiments have shown: a very slight weakening at 

 first, whose existence, nevertheless, my analyses have permitted 

 me to prove as soon as the pressure is not more than 56 centimeters. 

 Even then, the oxygen loss cannot have a very definite immediate 

 effect; the difference is like those one observes between individuals 

 who are in equally good health, like those which changes in respira- 

 tory rhythm or the different states of activity or of rest, of diges- 

 tion or of abstinence bring in the same individual. The aeronaut 

 cannot feel it. 



If he rises higher, the loss of oxygen increases: at 2000 meters it 

 was on the average 13%; at 3000, it becomes 21%; at 6500, 43%; at 

 8600 meters (26 centimeters pressure), the height at which Croce- 

 Spinelli and Sivel died, they must have lost half of the oxygen of 

 their arterial blood. My animals at 17 centimeters pressure had 

 lost 65%; their arterial blood then contained only 7 volumes in- 

 stead of 20 per 100 volumes of blood, less than ordinary venous 

 blood coming from a contracted muscle. This is the blood which, 

 in the arteries, was given the task of nourishing and animating 

 the muscles, the spinal cord, the sense organs, the brain! In con- 

 sidering these facts, we recall the celebrated experiment of Bichat, 

 on dark blood injected into the vessels of the nervous centers. 



We know that, in a general way, the effects of the rarefaction 

 of the air began to be felt quite plainly about the height of 4000 

 meters, corresponding to a pressure of 46 cm. It is also at about 

 this pressure that in our bells our animals ceased to move about 

 and showed signs of discomfort. Now the graph of Figure 31 shows 

 that at about this moment the proportion of oxygen in the blood 

 diminishes more rapidly; there is a remarkable agreement here. 



This decrease in the quantity of oxygen contained in the blood 

 is the prime factor. From it are derived all the symptoms of decom- 

 pression. Its cause, we have seen, is double: first, the proportion of 



