STIMULI AND THEIR ACTIONS 397 



metabolic process, such as we possess, e.g., in the quantity of 

 carbonic acid split off in the fermentation of yeast, it would be 

 possible to represent the individual factors of the vital process 

 and therewith the individual vital phenomena as a mathematical 

 function of temperature in the form of a curve of which the 

 abscissa would indicate the temperatures, the ordinates the 



KdUestarrG' Warmest&rre- 



Fig. 184. — Curve of excitation with increasing temperature. The abscissa represents the tem- 

 peratures, the ordinates the excitation 



intensity of the vital phenomenon in question (Fig. 184). Since 

 the individual factors of the vital process, both those belonging to 

 assimilation and those belonging to dissimilation, are dependent 

 upon temperature in very different degrees, in the construction of 

 these individual curves it would be possible to express in the 

 clearest and most graphic manner the very complex relations of 

 metabolism with every change of temperature. 



4. The Actions of Photic Stimuli 



When light is spoken of as a stimulus, the chemical, not the 

 thermal, activity of the light-rays is meant. In this sense, light, 

 when compared with other varieties of stimuli, is to a certain 

 extent peculiar, since it has been found that all varieties of living 

 substances do not react to it, while they do react to chemical, 

 mechanical, thermal, and galvanic stimuli. 



In the higher animals it is almost exclusively the sense-cells of 

 the visual organs that possess the capacity of reacting to light. 

 In most tissue-cells, so far as research thus far has gone, this 

 power is wanting. In some lower vertebrates, such as the re- 

 markable salamander, Proteus anguineus, which lives in the 

 streams of the Adelsberg grotto, the whole skin, as Rapheel 

 Dubois has shown, is sensitive to light-rays ; and in many inverte- 



