46 PROTOPLASMIC ACTION AND NERVOUS ACTION 



specific vital properties of the system. The given 

 physico-chemical change calls forth or occasions a 

 definite change of activity peculiar to the system. 

 The physiological problem of stimulation has reference to 

 the physical and chemical nature of the conditions 

 under which this specific vital reaction is called forth. 



Only a special acquaintance with a given living 

 system or organism can enable us to predict what its 

 behavior will be under a given stimulating condition. 

 Irritability as such, however, is a property which is 

 manifested under comparatively uniform conditions in 

 all organisms; i.e., the tendency to respond to certain 

 kinds of physical change is very widely distributed, if 

 not universal. Such responsiveness is a general character 

 of living matter and is largely independent of special 

 features of structure and organization. Thus apparently 

 all forms of protoplasm are influenced in their activity 

 by the electric current; in many cases — nerves, certain 

 receptors, muscles — very weak currents are sufficient 

 for stimulation; i.e., induce a sudden and profound 

 change in the activity of the system; in other cases the 

 sensitivity to the current is less and the response is 

 more gradual. The electrical sensitivity of living matter 

 is in fact one of its most characteristic peculiarities. 

 Evidently there is some feature of protoplasmic structure 

 or organization, common to all cells and organisms, 

 that renders all responsive to electricity, although in 

 varying degrees. The same is true (though perhaps less 

 universally) of mechanical influences or change of 

 temperature. Chemical sensitivity is also universal; 

 since all living matter depends for its existence upon the 

 incorporation and transformation of the assimilable 



