CRYSTALLOID IN LIVING CELLS 329 



It would appear that in nerve and muscle at the period of 

 activity only, and in injured^ tissue (which is excited or active 

 tissue), there exists in reality a detachment of potassium ions from 

 the colloid which does not exist before or after the active period 

 (Macdonald). 



It has been already pointed out that for each constituent 

 passing into union with the bioplasm there exists an optimum 

 concentration or osmotic pressure of solution in the nutrient 

 medium of the cell which alone is compatible with normal physio- 

 logical activity; or rather it might better be put that there is a 

 range of suitable concentrations with a minimum and maximum 

 which must not be passed in either direction. 



This point is particularly well illustrated in the case of the 

 respiratory gases. For both oxygen and carbon dioxide there are 

 well- marked limits of pressure which are required to be satisfied 

 in order that the processes of respiration and oxidation in the 

 tissues may proceed in a normal fashion. 



Since the energy for all tissue activity is derived from the 

 oxidation of organic bodies it is obvious that there must be a 

 minimal pressure of oxygen below which life is impossible, but it 

 is not so obvious that there is an upper limit of oxygen concen- 

 tration at which life becomes equally impossible. Yet it is found 

 that when warm-blooded animals are exposed to a pressure of 

 about 3 atmospheres of pure oxygen, death occurs in a few 

 minutes after violent convulsions (Bert). Short of this excessive 

 pressure, exposure to over 1 atmosphere of pure oxygen for a 

 longer period leads, as shown by Lorrain Smith and L. Hill, to a 

 pneumonic condition of the lungs. 



If pure oxygen at atmospheric pressure be breathed for a shorter 

 time interval, the tissues become charged with oxygen at a higher 

 pressure than normal, and Hill and Flack have shown that for a 

 short period afterwards muscular work can be done at a more 

 rapid rate in such forms of exercise as sprinting, hill-climbing, and 

 working against resistance. After exhaustive muscular exertion 

 also the breathing of oxygen diminishes the dyspnoea and sense of 

 fatigue. 



On proceeding in the direction of testing the effects of per- 

 centages of oxygen less than the atmospheric, it is found that the 

 results obtained depend upon the type of mammal experimented 

 with and upon the state of quiescence or activity of the animal. 



