Theories and Experiments 475 



The changes displayed by the curves in both cases have the same 

 causes and result from a purely mechanical influence exerted by the 

 increase of the pressure upon the elastic vessels filled with liquid. 



The obliquity of the ascending line, which has become more pro- 

 nounced in compressed air, indicates a greater resistance of the walls 

 of the arteries to the impulse of the heart. 



The lessened amplitude of the oscillation has two causes: 



The diminution of the excursion, of the arterial vessels, the 

 diastole of which is less because of the increased resistance of the 

 walls, and the systole is less because, on account of the pressure, the 

 arteries contain more blood in proportion to their caliber, and cannot 

 contract as much; 



The decrease in volume of the arteries which, in compressed air, 

 become smaller and contain less blood absolutely; which is shown 

 clearly by the escape of the liquid from our apparatus after a long 

 stay in compressed air ... . 



Now there are compressed gases in the blood, which are liberated 

 by the decrease in the air pressure, and which consequently are kept 

 there by a high atmospheric pressure: and this is a reason why the 

 mass of the blood, remaining constant, is reduced under high air 

 pressure to the smallest volume possible .... 



From these facts it follows that the action of the compressed air 

 is exerted most strongly upon the peripheral vessels. (Ibid., p. 375.) 



In the following chapter we shall see the value of this odd ex- 

 periment, the only one which Vivenot performed in the course of 

 his researches, and what importance the conclusions which he 

 draws from it may have. 



The ideas of G. Lange 27 agree perfectly, as one might have 

 expected, with those of his collaborator Vivenot. He adds, how- 

 ever, a few original explanations; this one, for example, which has 

 reference to the decrease in the number of respiratory movements: 



The need of breathing does not result from the influence exerted 

 upon the outstretched spinal cord by a blood with a reduced oxygen 

 content; it is the carbonic acid of the blood which produces this need, 

 the intensity of which grows proportionately to the quantity of this 

 acid which the blood contains. That is why every increase in the 

 quantity of carbonic acid exhaled will decrease the need of breathing, 

 and consequently the number of inspirations, unless a more active 

 combustion of the blood restores an excess of carbonic acid equal or 

 superior to the quantity exhaled. 



The decrease in frequency of the respiratory movements would 

 therefore be explained by a greater exhalation of carbonic acid in 

 compressed air, at least during the stay under the bell and the time 

 immediately following this stay. (P. 23.) 



But G. Lange knows very well that this hypothesis, then agree- 

 ing with the theory of Brown-Sequard, cannot account for the per- 



