44 IRRITABILITY 



and yet an immense number of instances of a like nature could 

 be cited to show the untenability of this view. 



It follows from this, that it is altogether impracticable to define 

 the stimulus itself in relation to the nature of the effects which 

 the stimulus has upon the substances in the living system. One 

 can only appreciate the nature of stimulation in relation to the 

 vital conditions and without considering the nature of the action 

 of the stimuli on the living substance. It is true that every 

 stimulus is followed by an alteration in living processes, but this 

 is to be expected when one clearly understands the nature of 

 vital conditions. A stimulus is in all cases an alteration in vital 

 conditions and, in that each of the vital conditions is necessary for 

 the continuance of life, it follows of necessity that every altera- 

 tion in the vital conditions, so intimately connected with the 

 living processes, will also be followed by an alteration in the 

 processes occurring in the living system. In short, response is 

 produced. Nevertheless, a definite alteration of an external vital 

 condition, depending upon the state of other vital conditions, 

 that is, according to the state of living substance at the moment, 

 can produce quite opposite eft"ects. Although it may appear 

 expedient to include in the conception of stimulation in given 

 instances, distinctions between stimuli according to the nature 

 of their effects upon the living substance, in all cases the con- 

 ception must under all circumstances be so formulated that it 

 comprises all alterations in the external vital conditions, either 

 positive or negative, that is to say, an increase or decrease, an 

 augmentation or diminution in those factors, acting as vital 

 conditions. 



Besides the quality there is another highly important factor 

 to be considered in the study of every alteration in the living 

 process, namely, its amount. The chemical concentration of the 

 medium, temperature, amount of light, the static and osmotic 

 pressure may undergo more or less variation. The electric 

 stimulus can rise from zero to great intensity and from great 

 intensity can fall to zero. The extent of the alteration deter- 

 mines the intensity of the stimulus. In relation to the intensity, 

 a differentiation of stimulation has been introduced, which is not 



