CHEMICAL SIGNS OF IRRITABILITY 15 



' In order to study the whole of the respiratory metab- 

 olism of a tissue, we should at least determine the 

 oxygen consumption as well as the carbon dioxide 

 production, and also generally the heat production. 

 Inasmuch as the present problem, however, is concerned 

 only with presenting direct evidence for the existence 

 of metabolic activity in nerve fibers, we shall attempt 

 to measure the carbon dioxide production alone; for 

 while the lack of consumption of atmospheric oxygen 

 may not necessarily indicate the absence of chemical 

 changes, the production of carbon dioxide will surely 

 prove the presence of metabolism, provided, of course, 

 that we can prove that such carbon dioxide is formed by 

 physiological processes. Furthermore, as carbon dioxide 

 is the only universal expression of the respiratory^ 

 activity in almost all anaerobic and aerobic plant and 

 animal tissues in normal condition, metabolic activity 

 is probably better represented by carbon dioxide pro- 

 duction than by oxygen consumption, although we 

 must restate here, of course, that the study of carbon 

 dioxide alone will never reveal completely the nature of 

 the metabolic activity. 



Method. The method which was finally devised to 

 detect and measure quantitatively the very minute 

 amounts of carbon dioxide which it might be expected* 

 would be formed consisted essentially in determining 

 the amount of carbon dioxide which was just sufficient 

 to produce a deposit of barium carbonate in a film of 

 half-saturated barium hydroxide solution. Barium car- 

 bonate is almost entirely insoluble in such a barium 

 hydroxide solution, and a very small amount of precipi- 

 tate can be detected with the aid of a small lens. The 



