Chap, ii.] RESPIRATION. 495 



jected 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 convulsions, 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 inter- 

 feres with the circulation, as by obstructing the play of the car- 

 diac 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 com- 

 posed 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 atmos- 

 pheric 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 (§ 285) 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 17,000 

 feet. Now it is a matter of common experience that in ascend- 

 ing 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 shews that 

 the minute blood vessels more directly exposed to the diminu- 

 tion of pressure are profoundly affected by it ; and what is true 

 of them is, probably, in various ways and to different degrees 

 true of the whole vascular system. The breathlessness which is 

 so marked a feature on these occasions seems due not so much 

 to the fact that the blood which reaches the respiratory nervous 

 centres is deficient in oxygen, as to the fact that the troubled 

 vascular system fails to deliver to those centres their blood in 

 an adequate fashion. 



