STIMULI AND THEIR ACTIONS 351 



is zero : in other words, the complete lack of them corresponds to 

 the optimum. They can, therefore, have but one maximum, so 

 that for them only the right-hand portion of the diagram comes 

 into consideration. They are included in the general definition of 

 the stimulus, namely, every change of the external agencies that 

 act upon an organism ; this definition holds good as well for those 

 agencies which, like heat, function in a definite degree as vital 

 conditions, as for those which, like electricity, under usual 

 circumstances are absent from the environment of the organism, 

 and, therefore, do not exist as conditions of life. 



In considering the intensity of the stimulus, one more point 

 requires mention. Let us imagine an organism or part of an 

 organism, e.g., a muscle, under conditions in which no stimulus 

 affects it, and let us bring to bear upon it a stimulus, e.g., the gal- 

 vanic current, which varies in intensity from zero upward and can be 

 graded easily and delicately. Then we should expect the muscle 

 to exhibit phenomena of stimulation, i.e., to perform a contraction, 

 as soon as the intensity is increased above 0. But this does not 

 happen. The intensity can be increased considerably before the 

 muscle performs even the slightest twitch. Only when the 

 intensity has reached a certain degree does the muscle respond 

 with a contraction ; from here on the contraction is never wanting, 

 and, up to a certain degree, becomes more energetic the more the 

 intensity is increased. The stimulus, therefore, begins to operate 

 only at a certain intensity, and this point is termed the threshold of 

 stimulation. Below the threshold the stimulus is ineffective ; above 

 it the effect increases with increasing intensity of stimulus. For 

 the different forms of living substance the value of the threshold 

 is very different. Thus, nerve-fibres are put into activity by 

 extremely feeble galvanic stimuli, while Amoeba demands very 

 strong currents. The same is true of all other varieties of stimuli 

 in relation to the various forms of living substance. 



3. Trophic Stimuli 



For the sake of convenience our considerations thus far have 

 been based upon the idea that a certain contrast exists between 

 vital condition and stimulus, in so far as the former represents a 

 stable given state, and the latter every change of that state. 

 This sharp distinction cannot be maintained for the reason that 

 vital conditions are not wholly stable and continuous factors, but 

 in nature are constantly undergoing variations. Hence, under 

 certain circumstances certain vital conditions can be considered 

 also as stimuli, or what is the same thing, certain stimuli function 

 as necessary vital conditions. A few concrete cases will make this 

 at once clear. 



