RESPIRATORY INTERCHANGE UNDER DIFFERENT CONDITIONS 519 



dyspnea gradually becomes more evident until it eventually gives way to a respira- 

 tory and circulatory depression similar to that observed in deep narcosis (Fig. 259). 



Changes in Barometric Pressure. — It is also feasible to change the 

 pressures of the gases by altering the barometric pressure. This can 

 be done either by compressing the air surrounding us, or by changing 

 our altitude. Thus, a deficiency in oxygen may be produced either 

 by placing an animal into a chamber in which the oxyger> tension is 

 low or by bringing it to a higher altitude. As is indicated in the suc- 

 ceeding table, the pressure decreases the more, the higher the altitude. 



Fig. 259. — Record of the Carotid Blood-pressure During Dyspnea (Dog). 

 At L the tracheal tube was held shut until the blood-pressure began to drop. 



Sojourns in rarefied air give rise to a complex of symptoms which 

 are grouped under the term of mountain sickness. A person affected 

 in this way suffers from headache, nausea, vertigo, hemorrhages and 

 a general mental and bodily apathy. It is true, however, that the 

 altitude at which these symptoms appear is not the same for all 

 individuals, because a process of adaptation is frequently brought into 

 play which allows the continuance of normal function even at higher 



