CHAP, ii.] RESPIRATION. 643 



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 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 

 convulsions, exactly in the same way as when oxygen is deficient. 

 Corresponding with this it is found that the production of carbonic 



