582 THE POPULAR SCIENCE MONTHLY 



whole series of processes — i. e., the response of the living system to 

 stimulation. First we must note that since in any special case the 

 response as a whole is constant, all of its single component stages or 

 separate processes must also be constant, both in their character and 

 their interconnections. There must therefore be some one constant 

 initial process which is directly caused by the external event or stimulus, 

 and upon which the others automatically and inevitably follow. This 

 initial process thus constitutes the critical or activating event in the 

 physiological sequence. It alone is directly dependent on the stimulus ; 

 the others are dependent upon it. What is remarkable is that it should 

 be produced by such a variety of different agents. The problem first to 

 be considered may therefore be put somewhat as follows: What is the 

 nature of the initial change produced in the irritable living tissue by the 

 action of the external agent, and how does it happen that it can be caused 

 by such diverse agencies ? This problem has evident relations to a wide 

 group of physiological and psychological problems; thus the question of 

 the basis of the " specific energies " of the special sensory apparatus be- 

 longs here. In this case also the response — the conscious affective state 

 or sensation — is distinctive and its quality independent (within certain 

 limits) of the character of the stimulus. This is in fact characteristic 

 of all cases of stimulation. How this can be possible I shall now attempt 

 to indicate. 



Let us take the case of the simplest of the irritable tissues of higher 

 animals, one in which the excitation-process occurs in a highly character- 

 istic form, but unaccompanied by highly specialized physiological effects 

 like contraction or secretion. Such a tissue is nerve. What are the 

 essential features in the response of this tissue to stimulation? It is 

 first to be noted that the process set up by the external stimulus is self- 

 propagating. The disturbance, whatever its nature, which originates at 

 the point of stimulus is of such a kind that it imparts a stimulus to the 

 adjoining regions of the nerve beyond the original point of stimulus; 

 these on becoming active stimulate the next stretch of nerve, and in this 

 way the state of excitation passes along the entire nerve to its termina- 

 tion. Evidently there is an active change of some kind, forming an 

 essential component of the local nerve process, that acts as stimulus to 

 adjoining regions. Now there is no mechanical change in a nerve as the 

 impulse passes, little or no production of heat, 1 apparently a slight physi- 

 cal or chemical change involving a loss of carbon dioxide ; but none of 

 these is in itself sufficient to act as stimulus. There is, however, another 

 definite physical change which has this power: namely, the electrical 

 variation — the bioelectric process or action-current — which always ac- 

 companies the activity of a nerve or indeed of any other irritable tissue. 



i Such, e. g., as causes the transmission of the chemical change along a train 

 of gunpowder. 



