Decreased Pressure 985 



In the first place, we know that, between two animals of the same 

 species, adult and in good health, the oxygen content of the arterial 

 blood is very variable; the variations, in our analyses, have been 

 (See Table XX) from 14.4 to 22.8 volumes of oxygen per 100 vol- 

 umes of blood; it seems evident then, a priori, that two animals 

 presenting this difference will not behave in the same way in re- 

 gard to decompression, and that the first will be more rapidly 

 affected than the second. 



In the second place, if we imagine two animals identical in the 

 oxygen content of their arterial blood, it may be that they are not 

 identical in the maximum capacity of their blood for oxygen; one 

 may be already nearly saturated, the other still far from his point 

 of saturation. The latter then, by speeding up his respiratory and 

 circulatory movements, can go further towards saturation and con- 

 sequently resist decompression better. But that is not all; since 

 he has the same quantity of oxygen, although not saturated, his 

 blood contains in an equal volume more hemoglobin than that of 

 the first, and this hemoglobin is less oxygenated. Now everything 

 shows that the oxy-hemoglobin is hard to dissociate, either in the 

 pump, or by the tissues, in proportion to its distance from its 

 saturation point. Our animal, for this reason too, will lose in 

 decompression less oxygen than the one which nevertheless had 

 the same quantity in the blood. 



We can go further yet: two animals of the same weight and 

 identical in the oxygen content and degree of saturation of their 

 arterial blood may differ considerably in the quantity of blood con- 

 tained in their vessels. And if this is so, it is clear that if, in a 

 given time, they consume the same quantity of oxygen within 

 their organisms, the blood of the one which has the least will give 

 up to the tissues the greater proportion of oxygen; in other words, 

 there will be a greater difference from the point of view of oxygen 

 content between his venous blood and his arterial blood than in 

 the animal with the greater quantity of blood. In our experiments 

 we have observed differences of this sort; we have seen, for ex- 

 ample, a certain dog have in his venous blood 9.2 volumes of oxygen 

 less than in the arterial blood (Exp. CXC) ; another (Exp. CXCII) 

 has only 3.3. If we suppose all other conditions identical, and if 

 we subject these two animals to a considerable diminution of 

 pressure, it is clear that the first will be painfully affected long 

 before the other, because his venous blood has a much smaller 

 reserve of oxygen. 



Now we reach the very foundation of the question. Let us con- 



