32 THE ACTIVE FORCES OF LIVING ORGANISMS 



effect of the stimulus decreases, the molecules of the 

 nerve-cells conforming to the change in the conditions 

 of their environment take upon themselves an altered 

 form of vibration — one, that is to say, which corre- 

 sponds to a state of diminishing oxidation. If the 

 stimulus to oxidation were abnormal or excessive, 

 the amount of carbonic acid produced would naturally 

 be so also, and the presence of a larger quantity of 

 this gas in the blood than usual- — larger, perhaps, 

 than the circulatory and respiratory systems could 

 readily cope with — would tend to modify the vibra- 

 tions of the molecules of the nerve-cells. An excess 

 of carbonic acid in the b ood, even when the supply 

 of oxygen is made ample by artificial means, produces, 

 it is said, a narcotic effect, acting not only on the 

 respiratory centre, but on the central nervous system 

 and brain.* Carbonic acid also stimulates the 

 respiratory muscles and causes deeper breathing, and 

 if it act in the same manner on the tissue of the 

 lungs and pulmonary vessels, either directly or indi- 

 rectly, it would also produce contraction, it would 

 seem, in these parts, and so diminish the aeration of 

 the blood. If we look upon all contraction as the 

 passage from a higher to a lower state of oxidation, 

 it is easy to see how carbonic acid, which represents 

 the latter, should cause increased muscular action so 

 long as the muscular tissue is fairly well supplied with 

 oxygen. 



There are many drugs, including probably the 

 * Poster, loc. cit., p. 601. 



