STIMULI AND THEIR ACTIONS 355 



viously in the latter as potential energy. Hence in this case the 

 irritability depends upon the fact that great quantities of potential 

 energy are accumulated in the living substance of the muscle, so 

 that the introduction of only a small quantity is needed to trans- 

 form it into actual energy. But such irritability and such a 

 reaction are not limited to living substance solely. Analogous 

 conditions may be established in lifeless bodies. A spring 

 stretched and held by a fine thread that maintains the tension in 

 equilibrium represents a body in which a great quantity of 

 potential energy is stored, although the body is in complete rest. 

 If the thread that holds the spring be touched lightly with the 

 edge of a sharp knife, the spring flies back with great force and 

 performs external work. By a small stimulus, represented by the 

 cutting of the thread, the potential energy of the spring has been 

 transformed into actual energy ; the cutting of the thread has, as 

 is said, "discharged" the energy of the spring. In explosive 

 bodies also there is such a discharge, and since there it is a dis- 

 charge of chemical tension, the similarity of it with the processes 

 of discharge in living substance is still greater, for in the latter 

 also potential energy is stored up in the form of chemical tension. 

 In a quantity of nitroglycerine the size of a pea there is contained 

 such a quantity of potential energy that it needs only a slight 

 impulse to produce a powerfully destructive effect. Like the 

 nitroglycerine molecule, living substance is explosive, although in 

 a manner that does not call forth so injurious effects. 



But the processes of discharge, as has been said, are only special 

 cases of reactions, and the relation between stimulus and reaction 

 may be wholly different in other cases ; for, on the one hand, there 

 are stimuli, such as fall of temperature, withdrawal of food, and exclu- 

 sion of oxygen, which consist not in the action but in the with- 

 drawal of energy ; and, on the other hand, there are reactions, such 

 as those of narcotics, which are expressed not by an increase, 

 but by a decrease and even a complete suppression of the produc- 

 tion of energy. Accordingly, it is characteristic of the process of 

 stimulation that no definite, generally valid, relation as regards the 

 quantity of energy exists between the stimulus and the reaction. 

 Hence, a conception of irritability that is to be generally valid must 

 be formulated as above. As regards reactions, it must be said : 

 The general action of all stimuli upon living substance consists in a 

 change of spontaneous vital phenomena. 



With the enormous multiplicity of vital phenomena in accordance 

 with the composition of living substance, and with the great variety 

 of stimuli, it is a priori conceivable that the phenomena of stimu- 

 lation must be very manifold. Moreover, to increase the variety of 

 the reactions still more, not only the different varieties of the 

 stimulus, but also the different intensities, as well as the time and 

 place of the stimulation, can call forth under circumstances very 



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