86 IRRITABILITY 



Finally, a third kind of secondary effect of stimulation claims 

 our interest. This is the secondary extension of the result of 

 stimulation from the part of a living organism directly and pri- 

 marily affected by the stimulus, to the surrounding structures. 

 All living substance has the capability of conducting an excitation, 

 which is produced locally through a catalytic stimulus, to a neigh- 

 boring part, not directly affected by the stimulus. It finds its 

 highest development in the nerve, but in no living structure is it 

 completely absent. This capability has been frequently termed 

 "conductivity of stimulation." It is more precise, however, to 

 speak of conductivity of excitation, for it is not the primary 

 influencing external stimulus which is conducted in the living 

 substance, but the excitation which it has produced. I have inten- 

 tionally considered only the excitating effects of stimulation, and 

 not those of the depressing reactions, as only excitations, not 

 depressions, are conducted by the living substance. These ques- 

 tions, however, demand a closer analysis. Here we were con- 

 cerned only with a survey of the general effects of stimulation. 

 If I, therefore, once more summarize the results which have been 

 gained, this is most clearly demonstrated by the following scheme : 



PRIMARY EFFECTS OF STIMULATION 



Excitation Depression 

 Functional Cytoplastic Functional 



SECONDARY EFFECTS OF STIMULATION 



Secondary excitation Secondary depression 



Conduction of excitation, Metamorphic processes, Self-regulation of 



metabolism 



This, however, is simply a scheme, like all other schemes, having 

 for its purpose a superficial survey of the subject. 



It brings to some extent order into the overwhelming mass of 

 manifold effects of stimulation but tells us nothing of the mech- 

 anism and genesis. Our further task must, therefore, be a more 

 thorough analysis of this field. 



