1228 PHYSIOLOGY 



As a result of the oxygen starvation there is inadequate supply 

 of this gas to the heart, so that the circulation tends to fail, especially 

 on making the slightest muscular movements. At the same time 

 the oxygen starvation of the brain produces failure of judgment and 

 inability to carry out or to co-ordinate muscular movements properly. 

 The symptoms as a rule do not increase until death results, so 

 that, although there is an oxygen starvation of the body, there must 

 be some means by which the respiration is modified so as to obtain a 

 sufficiency of this gas for the lowered requirements of the body. That 

 the adaptation is effective is shown by the fact that most individuals, 

 if they remain at a height, gradually recover from the mountain 

 sickness and may finally be able to carry out muscular movements with 

 almost as great precision and force as they could previously on the 

 plains. The mechanism by which increased ventilation of the lungs 

 is attained is that already mentioned in dealing with the effects of 

 lack of oxygen, namely, the production of acid substances in the body. 

 The respiratory centre is thus stimulated by these acid substances, 

 especially lactic acid, as well as by the carbon dioxide tension of the 

 blood, and the joint action of these two substances (which probably 

 co-operate in raising the hydrion concentration of the blood) deter- 

 mines the marked increase in the lung ventilation. ' Since the carbon 

 dioxide is no longer the sole factor responsible for the ventilation, 

 the tension of this gas in the alveolar air is diminished. 



ACAPNIA. This diminution of carbon dioxide tension in the blood and alveolar 

 air has been regarded by Mosso as the essential factor in the causation of mountain 

 sickness and has been designated acapnia. It may be absent, however, in the most 

 marked cases of mountain sickness, where the respiratory centre has failed to 

 respond to the additional acid stimulation, and may be present to a marked 

 degree in individuals who are experiencing none of the ill effects of this 

 disorder. 



Another important means of rapid adaptation is by means of the 

 circulation. This is noticeable even in the case of persons sitting 

 quietly in a gas chamber who are subjected to gradually lower pressures. 

 It is evident that a deficient passage of oxygen from the alveoli to the 

 blood may, so far as the tissues and heart are concerned, be accom- 

 modated for by increasing the rapidity of the circulation, and this 

 is effected by a quickening of the pulse-rate. The following Table 

 shows the changes in the pulse- rate caused by exposure to varying 

 pressures in a gas chamber : 



PULSE IN GAS CHAMBER 



Pressure Pulse 



720 . . . 64 



650 72 



424 84 



