642 EFFECTS OF DIMINUTION OF PRESSURE. [BOOK n. 



inspired air may be changed, not only by altering the composition 

 of the air entering at the ordinary atmospheric pressure, but also 

 by altering the total pressure of the atmosphere without changing 

 its composition. The results of the latter are however complicated ; 

 we have then to deal not merely with the effects on the interchange 

 of gases in the lungs but with the effects on the whole organism. 

 All the complicated machinery of the body is adapted and arranged 

 to work under what we may call ordinary atmospheric pressure, 

 that is to say, within the limits of 760 mm. mercury at the sea 

 level and about 500 mm., corresponding to an altitude of 6000 feet, 

 this being the range of ordinary human dwellings. Any great 

 increase or decrease of pressure beyond these limits will affect not 

 only the exit of carbonic acid from and the entrance of oxygen 

 into the blood, but, in varying degree, all the physical and chemical 

 processes of the body. A gross instance of this is seen when an 

 animal is suddenly subjected to a great diminution of pressure, as 

 when it is placed in the receiver of an air-pump and the receiver 

 rapidly exhausted. The animal is soon thrown into fatal con- 

 vulsions, which are in part, but only in part, due to the liberation 

 of gas from the blood within the blood vessels ; the gas so set free 

 mechanically interferes with the circulation, as by obstructing the 

 play of the cardiac valves, or by plugging the smaller blood vessels, 

 and thus helps to bring the machine to a standstill. The free gas 

 found in the vessels upon examination after death is said to be 

 composed chiefly of nitrogen, the carbonic acid and the oxygen, 

 which probably were also set free, having been reabsorbed before 

 the examination was made. 



But, quite apart from gross effects of this kind, it is very 

 obvious that the organism must in many ways suffer from a 

 diminution of pressure. The complex and delicately balanced 

 vascular system is constructed to work at the ordinary atmo- 

 spheric pressure. The force of the heart-beat and the tonic 

 contraction of the small arteries are, so to speak, pitched to meet 

 the influence exerted on the outside of the blood vessels by the 

 ordinary pressure of the atmosphere ; and any great diminution 

 of that pressure must produce a greater or less disarrangement 

 of the vascular mechanism until it is counterbalanced by some 

 compensating changes. And a little reflection will supply many 

 other instances. 



We have already called attention ( 354) to the fact that, the 

 total pressure of the atmosphere remaining the same, the partial 

 pressure of the oxygen in the inspired air may be reduced as 

 low as about 76 mm. (10 p.c.) without seriously modifying the 

 respiration. In order to attain this diminution of the partial 

 pressure of the oxygen without changing the composition of the 

 atmosphere, the total pressure of the atmosphere must be reduced 

 to the limit of 300 mm., corresponding to an altitude of 17000 feet. 

 Now it is a matter of common experience that in ascending a 



