THE CHARACTERISTICS OF STIMULI 41 



In the first place it does not follow that only positive fluctua- 

 tions of a factor, acting as a vital condition, result in excitation 

 in the existing vital processes. The withdrawal of water pro- 

 duces a diametrically opposite effect. A muscle, from which 

 water has been removed, if exposed to dry air or placed in a 

 hypertonic salt solution, shows violent excitation, which manifests 

 itself in great increase of irritability and development of fibrillary 

 contractions. The breaking of a constant current which has for 

 a long time flowed through a nerve or muscle also elicits a 

 momentary excitation. Further, the abrupt removal of light 

 may also bring about stimulation. To cite an example from 

 the physiology of the single cell, I should like to call to your 

 attention the interesting observations of Engelmann^ on the 

 Bacterium photometricum, of which he was the discoverer. 

 When the field containing these organisms is suddenly darkened, 

 all the individuals contained in the drop immediately dart forward 

 for some distance, at the same time, as is usually the case, quickly 

 rotating around their own axis, and then after a moment of 

 immobility, swim on quickly in another direction. An analogous 

 responsivity has also been shown by other single cell organisms, 

 as has been pointed out by several observers and especially by 

 Jennings.^ In all these cases the excitation was produced by a 

 lessening or total withdrawal of the factors which act as vital 

 conditions ; and even those who take the standpoint that only such 

 factors are to be considered as stimuli which produce an exciting 

 effect, are compelled to regard these alterations as stimuli, in 

 spite of the fact that they are negative variations of external 

 vital conditions. 



But further, the restriction of the term stimulation to those 

 alterations which increase the course of the changes in the living 

 substance involves the observer in still greater contradictions. 

 It can easily be shown that one and the same factor in one and 

 the same form of living substance has now an exciting, now a 

 depressing effect on the vital processes. This fact can be readily 



1 Th. W. Engelmann: "Bacterium photometricum ein Beitrag zur vergleichenden 

 Physiologie des Licht-und Farbensinns." In Pfliigers Arcliiv. Bd. 30, 1883. 



2 Jennings: "Behavior of the lower organisms." New York 1906. 



