THE ELEMENTS OF NERVE PHYSIOLOGY 37 



of the reaction behaves as a chemical stimulant. Electric 

 currents or, more correctly, changes in electric condi- 

 tion are among the most valuable forms of stimuli 

 for experimental purposes, though they do not play 

 any considerable part in the natural course of events. 



The power of a stimulus to elicit responses depends 

 upon the magnitude of the change from previous condi- 

 tions. This might be confidently predicted, but another 

 equally important fact might easily be overlooked. 

 Stimuli are effective not only in proportion to their am- 

 plitude, but also in proportion to the rate at which the 

 change takes place. In other words, changes sufficiently 

 gradual may fail of stimulating effect even though they 

 are most extensive. Demonstration of this is secured most 

 conveniently when the stimuli observed are electric; it is 

 found that rapid shifting of potential is far more exciting 

 than a slower change, while the alteration may be made 

 so gradual as to lose all efficacy. 



One more fact concerning stimuli deserves emphasis at 

 this point. The withdrawal of an influence which has 

 been operating upon the living mechanism may be as 

 truly a stimulus as would the application of a new form 

 of energy. Cold, which is to the physicist a negation, 

 must be counted a positive means of physiologic stimu- 

 lation. The same may be said of sudden darkness or 

 silence. When the baby has gone to sleep the parent 

 must not break off his lullaby too abruptly. He should 

 rather employ a judicious diminuendo, when, if successful, 

 he will have illustrated more than one biologic law. At 

 sea we wake from sleep when the regular throb of the en- 

 gine ceases. 



In the early speculations of philosphers respecting the 

 manner of action of nerves the picture frequently pre- 

 sented is that of tubular conduits giving direction to 

 spurts or pulses of fluid. One is reminded of the pneu- 

 matic appliances sometimes used in church organs to 

 transmit the effect of pressure upon a key to a distant 

 valve. All such comparisons are wholly faulty and we 



