STIMULI AND THEIR ACTIONS 357 



1. Every change in the external vital conditions of an organism 



constitutes a stimulus. 



2. Every augmentation of a vital phenomenon, either of one or of 



all, constitutes excitation. 



3. Every diminution of a vital phenomenon, either of one or of all, 



constitutes depression. 



4. The action of stimuli can consist of excitation or depression. 



2. The Duration of Reactions 



Another question, that of the duration of reactions, which 

 naturally thus far has received much less systematic treatment, is of 

 no less interest, for it is in the closest relation with subjects, such 

 as those of adaptation, immunisation, etc., which are of far-reaching 

 practical importance. It is to be expected that these subjects, which 

 afford very promising problems for experimental cell-physiological 

 research, will soon attract more attention. For the present only 

 a few disconnected discoveries of a very general nature can be 

 specified. 



In general, it may be said that the duration of the reaction de- 

 pends primarily upon the duration and intensity of the stimulus, 

 and that after the cessation of the latter the reaction passes away 

 the more rapidly, the briefer and feebler the stimulus was. A 

 few special cases demand particular attention. 



To consider first the relations under prolonged stimulation, 

 usually during the continuance of the stimulation the reaction un- 

 dergoes a change in accordance with the intensity of the stimulus. 

 With feeble stimuli there is, after some time, an abatement and 

 finally a cessation of the reaction : the living substance becomes 

 accustomed, or adapted to the stimulus. Such phenomena may easily 

 be observed in very different objects and with very differ- 

 ent varieties of stimuli. Thus, as Engelmann (79, 1) and 

 others 1 have shown, it is possible to accustom many uni- 

 cellular organisms to relatively strong salt solutions which 

 at first call out distinct phenomena of stimulation. If an 

 Actinosphcerium that has extended its pseudopodia in the 

 customary, ray-like manner be placed in a weak solution of sodium 

 bicarbonate, it gradually draws in its pseudopodia from all sides 

 and becomes spherical. But soon minute projections reappear 

 upon the surface, extend and lengthen, until the organism 

 has assumed its original form and become completely normal. By 

 successively increasing the concentration, the same result can be 

 produced many times in succession. Such adaptations may be 

 brought about to weak solutions of poisons, high temperatures, 

 strong light, etc. If the stimuli are strong, no adaptation takes 

 1 Cf. Verworn ('89, 1). 



