474 GENERAL PHYSIOLOGY 



which death appears, since we have previously l become acquainted 

 with them. Over-stimulation, in its most general significance, is 

 nothing but that which has been termed elsewhere external causes 

 of death. The fact does not require special mention that over- 

 stimulation, when it consists either in an increase or a decrease of 

 the factors that act as vital conditions, always results finally in 

 death. It has already been seen that overstepping either the 

 minimum or the maximum of vital conditions leads to a fatal 

 outcome. 



In a previous chapter we came to regard life as a phenomenon 

 of nature that, like all other phenomena of nature, comes into exis- 

 tence when a certain complex of conditions is fulfilled. If the con- 

 ditions become changed, the phenomena also change ; if the former 

 wholly disappear, the latter also cease. In stimuli we have become 

 acquainted with sach changes of vital conditions. Under the in- 

 fluence of stimuli vital phenomena change, and they wholly cease, 

 when the stimuli overstep a certain limit. 



If we except the small number of cases, thus far largely un- 

 explained, such as the metamorphic processes of necrobiosis, where 

 vital phenomena are forced into a perverted path and are qualita- 

 tively changed under the influence of stimuli, we observe that within 

 certain limits stimuli cause only a single kind of effect, namely, a 

 gradual, quantitative change of the vital phenomena, either increas- 

 ing or decreasing the intensity of the latter. Hence in the vast 

 majority of cases stimuli do not call out new phenomena, but pro- 

 duce merely an excitation or depression of those general vital 

 phenomena already existing. 



It is here especially to be noticed that the different varieties of 

 stimuli produce in the same object wholly similar reactions. An 

 Amoeba may be made to retract its pseudopodia and assume a 

 spherical form by chemical, mechanical, thermal, and galvanic 

 stimuli ; the cells of a ciliated epithelium respond by an accelera- 

 tion of their ciliary motion to chemical, mechanical, thermal and 

 galvanic stimulation ; and by all of these agencies the production 

 of light can be induced in Noctiluca. 



This important fact shows that in every form of living substance 

 there must exist an extraordinary inclination toward a specific 

 sequence of processes. This sequence is continually present in 

 slight degree and finds its expression in the spontaneous vital 

 phenomena; but the slightest stimuli of all kinds augment the 

 discharge of the processes always in the same characteristic 

 sequence for each specific variety of living substance, just as the 

 nitroglycerine molecule can always be made explosively to dis- 

 integrate into the same constituents by mechanical, galvanic, or 

 thermal influences. 



1 Cf. p. 319. 



