46 IRRITABILITY 



be observed or not. The conception of the threshold of stim- 

 ulation, therefore, has meaning only when used in relation to 

 a certain indicator. The threshold of the same living system 

 may be different for different indicators. When we use the 

 term threshold we must necessarily know the indicator employed 

 in its determination. The threshold stimulus produces only 

 barely perceptible effects. The amount of response in most 

 living substances increases with the intensity to a certain limit. 

 If this limit is reached, that is, if the response is maximal, the 

 stimulus of the weakest strength necessary to produce this result 

 is termed the maximal stimulus, whereas all intensities lying 

 between the threshold and the maximal stimulus are termed 

 submaximal stimuli. If the intensity of the stimulus is increased 

 above that of the maximal, the response, as in the case of the 

 muscle, does not increase, and therefore one could say that all 

 intensities above the maximal could also be called maximal 

 stimuli. 



In realty, however, the response to stimuli of different inten- 

 sities is never equal, even though it may appear so, when meas- 

 ured by an indicator, as for instance, the height of the maximal 

 muscle contractions. This is clearly shown, for example, when 

 the electrical stimulus is increased far beyond that intensity which 

 is necessary to produce maximal effect. Injury is thereby pro- 

 duced, which is manifested, for instance, in the muscle contrac- 

 tion by the nature of its course and also by its height. One is, 

 therefore, justified in a certain sense in calling the intensities of 

 the stimulus, which are above the value which barely produces 

 maximal contraction, " supermaximal stimuli," notwithstanding 

 this is logically far from being a happy expression. The term 

 "maximal stimulus," then, is limited to the intensity of the stimu- 

 lus which just produces a maximal effect. I wish to point out this 

 distinction between maximal and supermaximal stimulus, as there 

 is often a lack of clearness in the use of these terms. 



In that the nomenclature of intensity of stimulation is based 

 upon the intensity of response, the question arises as to the rela^ 

 tion between the intensity of stimulus and the amount of response. 

 It is well known that this question has met in one special field 



