580 TEE POPULAR SCIENCE MONTHLY 



by which the necessary supply of transformable energy and of building 

 material is secured. Both kinds of reactions are equally responses to 

 stimulation; and both are alike physiologically indispensable. In fact, 

 the characteristic self-conserving or regulatory power of organisms, with- 

 out which they could not continue to exist, depends essentially upon 

 their ability to respond in this way, i. e., upon their irritability. 



It is thus apparently not difficult to understand the general biological 

 significance of irritability. What is still largely obscure, however, is the 

 physico-chemical nature of the mechanism which renders possible the 

 response of an organism to stimulation. Physiological experimentation 

 has enabled us to simplify the problem to some degree. We find that 

 not only the intact living organism, but many of its isolated tissues and 

 even cells, react in characteristic ways to stimulation. This is especially 

 true of the tissues that subserve the motor activities of the animal, the 

 muscles, nerves and sense organs. Thus the problem of the nature of the 

 stimulation-process becomes one of the general problems of cell-physiol- 

 ogy, and may be stated as follows: What are the essential physico- 

 chemical peculiarities that render the irritable elements of these living 

 tissues so sensitive to stimulation? and what is the physico-chemical 

 nature of the process of stimulation itself? These are the questions 

 which I shall attempt briefly to discuss in this paper. Any answers 

 which can be given at present are incomplete and in part provisional. 

 But recently some definite progress — as it seems to me — has been made 

 toward their solution, and I shall try in what follows to give some 

 account of this recent work and of the more important general concep- 

 tions to which it has led. 



We have first to define more clearly what we mean by " stimulus " 

 and what by "response." We find on reflection that it is a difficult 

 matter to formulate definitions that are at once exact enough and com- 

 prehensive enough to characterize adequately all of the highly varied 

 phenomena included under these terms. We may perhaps best define a 

 stimulus as some change of condition that arouses a previously quiescent 

 tissue or organism to activity, or appreciably modifies the activity of one 

 already active; and the response as the resulting activity or change of 

 activity. In many cases the response may be negative in kind ; i. e. ; the 

 previous activity may be decreased or completely arrested ; inhibition is 

 in fact a very frequent mode of response and one perhaps fully equal in 

 importance to the more positive or active modes as a means of biological 

 adjustment. But what is perhaps the most characteristic peculiarity of 

 the relation between stimulus and response is the fact that there is, 

 broadly speaking, no definable relation of an energetic kind between the 

 two. One of the most striking and distinctive features of stimulation is 

 that an external event or change of condition which causes directly a very 

 slight alteration in the irritable system or organism may yet arouse in 



