CHAP, ii.] RESPIRATION. 611 



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 

 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 diminution 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. 



It is a feature of the vascular system, and indeed of the other 

 mechanisms of the body, in which nervous factors intervene, that 

 they possess the power of adapting themselves to changed con- 

 ditions ; and as it is well known, the human organism somewhat 

 rapidly becomes accustomed to these moderate altitudes. Practice 

 and custom have far less effect, though they have some, on the 

 more fundamental processes depending on the actual supply of 

 oxygen ; and it is at the extreme altitudes, where in addition to 

 the other troubles a deficiency of oxygen definitely makes itself 

 felt, that the body seems to fail in adapting itself to the new 

 circumstances. 



The addition of these troubles not directly respiratory in 

 nature, when the supply of oxygen is diminished by a diminution 

 of the total pressure, perhaps explains why though an adequate 

 lowering of pressure will produce asphyxia, that asphyxia is 

 somewhat different from the ordinary asphyxia due to deprivation 

 of air or oxygen. Convulsions which are essential to ordinary 

 asphyxia are at times wholly absent; the nervous system under 

 the peculiar conditions does not respond to the stimulus of the 

 lack of oxygen; and other nervous symptoms, such as a rapid onset 

 of feebleness amounting almost to paralysis, are apt to make their 

 appearance. 



380. The Effects of Increase of Atmospheric Pressure. These 

 are in many ways remarkable. Up to a pressure of several atmo- 

 spheres of air, the only symptoms which present themselves are 

 those somewhat resembling narcotic poisoning. The animal 

 becomes sleepy and stupid, the result probably not so much of 

 respiratory changes, as of the effects of the increased pressure on 

 the whole organism to which we have just alluded. At a pressure 

 however of 15 atmospheres of air, or what amounts to the same 

 thing, of 3 atmospheres of oxygen, and upwards, a very remarkable 

 phenomenon presents itself. The animals die of asphyxia and 



392 



