400 KELATION OF RESPIRATION TO OTHER PROCESSES [CH. XXVII. 



the momentary retardation leads to a certain amount of oxygen 

 want, and this is probably the cause of the lactic acid output, to 

 which we have drawn attention (see pp. 381 and 395). Lactic acid 

 production in muscle indicates incomplete oxidation. The acid 

 itself probably increases the blood -supply to the muscle automati- 

 cally by relaxing the arteriole walls. 



The symptoms produced by exercise and by decreased pressure 

 (as in mountain sickness) are not the same, probably because . the 

 former is accompanied by an increase in the production of carbonic 

 acid and of heat. Indeed most of the symptoms which are the 

 ordinary effects of exercise are due probably to these causes, the 

 actual oxygen supply to the brain being adequate during exercise. 



Mountain Sickness. 



The symptoms of mountain sickness differ in different persons 

 as do those of other forms of intoxication. In one person vomiting 

 will be the prominent feature, in another, absence of self-restraint, 

 in a third, inability to perform any task, such as arithmetic, involving 

 accurate cerebral processes. There is no doubt that the cause is 

 want of oxygen. This lack of oxygen causes the production of the 

 poisonous materials, which affect the brain and produce the symptoms. 

 These toxic substances are no doubt products of incomplete oxidation ; 

 it is not clear how far these are produced in the brain, and how far 

 they are carried to it in the blood. 



In considering what is the lowest pressure of oxygen consistent 

 with normal function, it must be borne in mind that with lungs of a 

 given size this limiting pressure varies directly with the number of 

 c.c. of oxygen which must be absorbed (Bohr). 



The effect of exercise is to increase the amount of oxygen 

 required by the tissues. The amount of oxygen required by the 

 same person under different circumstances varies very much. Thus 

 Zuntz resting and fasting on Monte Eosa required only 259 c.c. of 

 oxygen per minute, whilst in the act of climbing he wanted 1329 c.c. 

 of oxygen per minute. 



In the former condition he could have remained in comfort at an 

 altitude at which the pressure of oxygen in his alveoli was very low, 

 but as soon as he made any effort he realised that this pressure of 

 oxygen was insufficient to allow him to do muscular work. 



Now the alveolar pressure depends partly on the barometric 

 pressure, and partly on the nature of the respiration. The deeper 

 the respiration the more nearly does the pressure of oxygen in the 

 alveoli approximate to that in the atmosphere (i.e. the higher is the 

 alveolar pressure of oxygen). Thus it happens that men who take 

 deep respirations can tolerate altitudes which are impossible for 



