Il8 THE HUMAN BODY. 



countries. We may consider therefore that the altitude has 

 no influence upon persons who have long lived at a given 

 level. But if we rise rapidly to a great height, the pulse 

 quickens very sensibly. Aerostatic ascensions and mountain 

 journeys furnish the proof of this. It is not to locomotion 

 or to muscular effort that this quickening of the pulse can 

 be attributed in the aeronaut or the traveller on horseback, 

 but it is chiefly due to the greater frequency of respiration in 

 an atmosphere of lesser density. The diminution of the 

 pressure of the atmosphere conduces also to the same effect 

 by relaxing the vessels; but the fall in temperature in pro- 

 portion to the rise in elevation seems to neutralize this last 

 effect by the contraction of the tissues which it induces. (See 

 Respiration, p. 99.) 



The establishment of the fact, that an increase in the 

 pressure of the atmosphere diminishes the frequency of the 

 pulse is due to the observations of Pravaz and Tabarie. Both 

 these authors state that the pulse falls to fifty and even to 

 forty-five pulsations per minute when the subjects are placed 

 in an apparatus for compressing air, and the pressure is 

 increased to that of two atmospheres and over. Results 

 entirely contrary were observed by M. Francois in the tubes 

 containing compressed air which he used in building the 

 bridge at Kehl in 1860. This physician observed that the 

 pulse invariably quickened in the labourers employed upon 

 this work under a pressure of about two atmospheres. 

 Other observations by M. Hermel establish the fact that the 

 pulse is sometimes retarded, and sometimes quickened in 

 compressed air to one hundred and fifty in a minute. The 

 phenomena observed in men who work in compressed air 

 seem to be due to complex causes, among which we must 

 note the vitiation of the air from deficient renewal. 



We shall not here discuss the numerous causes which may, 

 in pathological conditions, influence the circulation. 



