THE ACTIVE FORCES OF LIVING ORGANISMS 59 



the degree of oxidation of the tissues. A slight 

 increase of carbonic acid stimulates contraction 

 whenever the tissues are sufficiently oxidized ; and 

 similarly a slight increase in the amount of oxygen 

 in the blood, if absorbed by the tissues, stimulates 

 contraction whenever they previously have been 

 insufficiently oxidized. A great proportional excess 

 of either gas in the blood tends, on the contrary, 

 to diminish metabolism, and in some cases brings 

 about spasmodic action of the nerve centres. The 

 increase in the amount of carbonic acid which takes 

 place during violent exercise makes its influence felt 

 by rendering muscular contraction after a while more 

 feeble. Here, however, we have to consider not so 

 much the temporary fluctuations as the long- continued 

 changes due to differences of altitude. There is 

 reason to suppose that, although the mode of func- 

 tioning of the nerve-cells is at once modified by such 

 changes, they still retain for some time something of 

 the influence of the preceding conditions to which 

 they were subjected. It is probably this compromise, 

 so to speak, between the new and the old which is in 

 a great measure the cause of improvement. This 

 double action, indeed, suggests an explanation, and 

 probably the only one, of the bracing effect of high 

 altitudes ; for when an individual goes from a low 

 place to a high one, not only does his nervous system 

 come under the influence of a relatively greater 

 amount of oxygen in the blood, but it still remains for 

 some time under the influence of the pre-existing 



