74 IRRITABILITY 



excitation, and when there is decrease in the vital processes, we 

 speak of a depression. 



The conception of excitation and depression are purely empiri- 

 cal. They are terms for real things, referring, in fact, simply to 

 alterations in rapidity of life process, which can be as readily 

 observed as the process itself. I wish to lay particular stress on 

 this fact, for the reason that Cremer'- has recently made the 

 extraordinary statement that I have introduced hypothetical pro- 

 cesses into the definition of the conception of excitation. I have 

 always considered excitation as merely an increase or change of 

 intensity of the specific actions of a living system, and as such is 

 an established process without a trace of the hypothetical element.^ 

 If, however, the excitation process is to be regarded as something 

 absolute, as a mysterious state sui generis, which is entirely inde- 

 pendent and totally unlike the metabolism of rest, then, of course, 

 it would appear utterly incomprehensible and would be without 

 purpose. As an absolute process excitation is merely a meaning- 

 less word. Excitation and depression are relative conceptions 

 and can only acquire meaning when the process which is exci- 

 tated or depressed is more closely defined. This is the specific 

 vital process of a given organism, and the two conceptions only 

 have meaning in relation to it. The conception of the vital pro- 

 cess, however, is one directly gained from experience. However 

 complex or difficult to analyze the process may be, it still is as 

 little hypothetical as that of the combustion of carbon into carbon 

 dioxide, or the revolving of the earth around the sun. It can be 

 looked upon as something positive and real. Quite another 

 question is the manner in which we are to consider the mechanism 

 of the vital process. In analyzing this mechanism we cannot, at 

 least in the present state of our knowledge, entirely dispense with 

 hypothesis. But these hypotheses are in no way involved in the 

 definition of the process of excitation. If we look upon every 



1 Cremer: "Die allgemeine Physiologic der Nerven." In Nagels Handbuch der 

 Physiologic dcs Menschen. Bd. IV, Braunschweig 1909. 



2 In the first edition of my "General Physiology" in 1895 I have sharply and clearly 

 defined it as such, stating in formulating the general law of stimulation; that every 

 excitation is an increase either of individual parts or the whole of vital phenomena, 

 depression every decrease in the individual part or the whole of vital phenomena. 



