CHAPTER V 



STIMULI AND THEIR ACTIONS 



When investigating a phenomenon of nature the physicist is 

 not satisfied with determining the conditions under which it 

 exists ; he endeavours to learn also how it is affected when the 

 conditions are altered. 



Life is a phenomenon of nature. In the preceding pages we have 

 become acquainted with its manifestations and the conditions of 

 its appearance, and we have seen the results of an entire removal 

 of those conditions. It remains for us to learn how vital phenomena 

 are affected when the conditions are altered and new ones are allowed 

 to surround the living substance. Vital phenomena are called 

 spontaneous, when all the external conditions of life continue 

 unchanged, and phenomena of stimulation, when other influences 

 act upon them. This distinction is a valid one, but it must be 

 borne in mind that spontaneity is not absolute, that as a matter 

 of fact spontaneous vital phenomena depend upon the interaction 

 of living substance and the environment no less than do the 

 phenomena of stimulation. The former represent merely the 

 reaction of living substance to normal, constant external vital con- 

 ditions; the latter, the reaction of living substance to changed 

 external vitarl conditions. In many cases it is quite impossible to 

 decide whether a given phenomenon is spontaneous or a result of 

 stimulation, since even in nature the external conditions of an 

 organism do not remain constant, but frequently change in 

 a manner that eludes even the most exact methods of investigation. 

 In order, therefore, to study undoubted phenomena of stimulation 

 we have recourse to the experimental method, and produce the 

 phenomena artificially by causing stimuli to act upon living 

 substance. In so doing we secure the incalculable advantage of 

 keeping in hand and controlling exactly the conditions under 

 which the phenomena exist, and thus are able to experiment with 

 vital as with simple physical phenomena. 



