354 



THE AMERICAN NATURALIST [Vol. XLIX 



ditions of the higher organism and to indicate their role, 

 on the assumption that the internal mechanisms of the 

 organism are physico-chemical mechanisms. 



In the response of the respiratory mechanism to the 

 increased concentration of carbon dioxide or to lack of 

 oxygen in the blood, we have an instance of adaptation 

 which is not at once seen to be an obviously automatic 

 and inevitable result of the physico-chemical properties 

 of the environment. A striking characteristic of the re- 

 spiratory center is at once its sensitiveness to slight 

 changes in the concentration of carbon dioxide and its 

 tolerance to the accumulation of carbon dioxide in the 

 blood. The respiratory cells react to an extremely slight 

 increase of carbon dioxide which is insufficient to affect 

 the other cells, and remain sensitive to this increase after 

 the concentration has risen so high that the visible re- 

 sponses of certain other cells have ceased. The common 

 excitability of the respiratory and other motor nerve cells 

 to carbon dioxide may be supposed to result from the 

 disturbance or change produced in a complex system by 

 the accumulation of one end product of the reaction, and 

 to this extent to be an automatic result of the physico- 

 chemical constitution of the cell. 



The question raised by Mathison 4 ' 5 as to whether car- 

 bon dioxide is a stimulant for all nerve cells is of interest 

 in this connection. Carbon dioxide is certainly a stimu- 

 lant for the central nerve cells of the respiratory mech- 

 anism, but it is not necessarily a stimulant to the same 

 degree for all nerve cells. It is probable that all living 

 matter is more or less sensitive to the accumulation of 

 carbon dioxide since it is one of the waste products of 

 all destructive metabolism. The cell bodies of the respira- 

 tory neurones, by reason of the development of this 

 common property of excitability to carbon dioxide, have 

 become especially adapted to respond to slight variations 

 in carbon dioxide. The adaptation undoubtedly depends 

 upon a physico-chemical change in the respiratory neu- 



*« Journal of Physiology, 1910, XLI, p. 448. 



