EFFECTS OF CHANGES IN ATMOSPHERIC PRESSURE. 389 



so acts as a narcotic poison. The peculiar effects of nitrous oxide (laughing 

 gas) are similarly due to the direct action of the gas in the blood on the 

 central nervous system. Some gases are irrespirable and may interfere with 

 respiration, even causing suffocation, on account of their causing spasm of 

 the glottis, and this is said to be, to a certain extent, the case with an atmos- 

 phere which is wholly or largely composed of carbonic acid. 



322. The effects of changes in atmospheric pressure. Diminution of 

 pressure. The partial pressure of the oxygen in the 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 ar- 

 ranged 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 be- 

 yond 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 ex- 

 hausted. The animal is soon thrown into fatal convulsions, which are in 

 part, but only in part, due to the liberation of gas from the blood within 

 the bloodvessels ; the gas so set free mechanically interferes with the circula- 

 tion, as by obstructing the play of the cardiac valves, or by plugging the 

 smaller bloodvessels, 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 pro- 

 bably 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 atmospheric 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 bloodvessels 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 coun- 

 terbalanced by some compensating changes. And a little reflection will 

 supply many other instances. 



We have already called attention ( 297) to the fact that, the total pres- 

 sure 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 per cent.) 

 without seriously modifying the respiration. In order to attain this dimi- 

 nution of the partial pressure of the oxygen without changing the composi- 

 tion of the atmosphere, the total pressure of the atmosphere must be reduced 

 to the limit of 300 mm., corresponding to an altitude of 17,000 feet. Now 

 it is a matter of common experience that in ascending a mountain "distress" 

 is felt long before such an altitude is reached. The distress felt on such 

 occasions is probably due not so much, if indeed at all directly, to the 

 diminution of oxygen as to a general disarrangement of the organism and 

 perhaps more particularly of the vascular system. The nose-bleeding which 

 is so frequent an occurrence under the circumstances shows that the minute 



