350 GENERAL PHYSIOLOGY 



is possible only between these. If they are overstepped, death 

 develops. But all points between the two limits are not equally 

 favourable to life. The intensity of the life-process is less when 

 the vital condition is near its maximum or minimum, than when 

 it has an average value. That degree of any vital condition at 

 which life thrives best, at which the intensity of the life-process is 

 greatest, is termed the optimum. But the optimum is not always 



Optimum, 



Tod eb&n. Tod 



DIAGRAM OF VITAL CONDITIONS. 



intermediate between the maximum and the minimum, in many 

 cases it lies nearer the former, in others nearer the latter. 



In accordance with the above diagram of vital conditions the 

 conception of the stimulus may be at once appreciated. If an 

 organism exists at the optimum of any vital condition, e.g., of 

 temperature, then every deviation of the temperature, whether in 

 the direction of the maximum or the minimum, acts as a stimulus. 

 That degree of any vital condition to which the organism is 

 adapted, represents its optimum, it represents the indifferent point 

 of stimulation ; here the stimulus is equal to zero. If the 

 condition changes toward the maximum or the minimum, the 

 intensity of the stimulus simultaneously increases until it reaches 

 the maximum or the minimum. The stimulus, therefore, has a 

 minimum, which coincides with the optimum of the vital condition 

 in question, and two maxima, the one at the minimum, the other 

 at the maximum of the condition. With supra-maximal 

 stimulation death develops. If, therefore, a diagram of stimulation 

 be constructed, the same points must be designated as in the 

 diagram of vital conditions ; but other names must be given them, 

 for the optimum of the conditions becomes the zero-point of 



JVidlpunkt 



Tod Leben, Tod' 



DIAGRAM OF STIMULATION. 



stimulation, the minimum and the maximum both become 

 maxima. Every change of intensity between the zero-point and 

 either maximum acts as a stimulus. 



This diagram comprises all varieties of stimulus, even those 

 which, like certain chemical and electrical stimuli, under normal 

 conditions do not come into relation with the organism at all. 

 The intensity of these latter varieties considered as vital conditions 





