982 Summary and Conclusions 



oxygen which the blood can absorb grows proportionately less as 

 the pressure lowers (See Part II, chapter II, subchapter V) ; in the 

 second place, if we suppose that the respiratory rhythm has not 

 changed, the quantity of oxygen which circulates in the lungs 

 during a given time diminishes in the same proportion as the pres- 

 sure. Now under normal pressure, the arterial blood, we have 

 seen, is never completely saturated with oxygen, the agitation of 

 the blood and the air not taking place with sufficient energy in the 

 lungs. 



The deviation must increase greatly when not only the coeffi- 

 cient of the oxygen absorption but also the intra-pulmonary cir- 

 culation diminishes. Indeed, at a half-atmosphere, for example, to 

 keep the conditions of intra-pulmonary mixing as they were at sea 

 level, everything must be doubled: the respiratory movements 

 must be double in amplitude and frequency; the heart beats must 

 be double in strength and number. That is evidently impossible. 



However, there is a tendency in this direction, as the accounts of 

 all the aeronauts give witness, as I have observed in the animals 

 and experienced myself in my apparatuses; at low pressures the 

 respiration quickens, the heart beats are stronger and more rapid, 

 and equilibrium can be nearly reestablished. We have seen, in 

 fact, that if the pulmonary ventilation increases, the arterial blood 

 may gain 3 or 4 volumes of oxygen per 100 volumes of blood. 



But this can be only momentary, and such gymnastics cannot 

 long continue without danger of emphysema and cardiac maladies; 

 and so this increase does not last, and when the balloon becomes 

 stationary, this dangerous acceleration does not continue in the 

 aeronauts: the oxygen then decreases fatally in their blood. 



Furthermore, when the pressure diminishes still more, the respir- 

 atory and circulatory acceleration not being able even for an in- 

 stant to compensate for the insufficiency of the intra-pulmonary 

 agitation of the air and the blood, the muscles of respiration, like 

 those of the heart, lose their energy and grow weary, since they 

 are receiving a blood that is insufficiently oxygenated, and yet are 

 compelled to carry on continuous labor. The respirations, always 

 numerous during activity, are shallow, so that the quantity of air 

 inspired in a given time is hardly the same in volume as at normal 

 pressure; in rest, they fall back to their ordinary number, while 

 remaining very shallow, and it even seems, according to the remark 

 of de Saussure, that one sometimes forgets to breathe. The heart 

 movements give similar results; their frequency increases, it is 

 true, but the cardiac tension drops considerably; in one of the 



