268 PROTOPLASMIC ACTION AND NERVOUS ACTION 



This propagated effect, or "propagated disturbance" 

 (Keith Lucas' term), as it appears at a distance from 

 the point of stimulation, has its own definite pecuharities, 

 which are independent of the nature of the initiatory 

 local change or stimulus. For example, any single nerve- 

 impulse in a normal frog's nerve, by whatever means it 

 is initiated, travels at a constant velocity (assuming the 

 state of the tissue to be normal and the temperature and 

 surrounding conditions constant), and its most readily 

 observed physical accompaniment, the bioelectric varia- 

 tion, has a definite range of potential change and definite 

 time-relations. In other words, the propagated dis- 

 turbance differs from the local change in exhibiting 

 constant and specific features, qualitative and quantita- 

 tive, whose nature is determined by the special or 

 inherited constitution of the tissue. 



Almost any kind of sufficiently rapid local alteration 

 may initiate such a wave of physical and chemical 

 disturbance. The distinction between the local change 

 and the propagated effect is a fundamental one in any 

 theory of stimulation. The former is the ''releasing" 

 event and follows upon some simple physical change 

 produced in the tissue by the stimulating agent; the 

 latter is the distinctively physiological process, and as 

 such has specific characters of a complex kind, dependent 

 on the nature of the irritable system and as yet 

 imperfectly analyzed. 



Especially significant is the fact that all irritable 

 elements, apparently without exception, respond to 

 electrical stimuli, or are influenced in their already 

 existing activity by the electric current. The electrical 

 sensitivity of highly irritable tissues, such as vertebrate 



