STIMULATION OF LIVING ORGANISMS 583 



The stimulated region undergoes a rapid change of electrical potential, 

 becoming externally negative relatively to its resting condition; the 

 neighboring still inactive regions being thus positive relatively to the 

 active region, the conditions for the flow of an electrical current between 

 stimulated and unstimulated regions arise. This current is undoubtedly 

 of sufficient strength to stimulate the tissue for some distance beyond the 

 immediate site of stimulation. The voltage of the action-current of 

 frog's nerve is at least thirty millivolts ; and a current between platinum 

 electrodes two or three centimeters apart differing in potential by this 

 degree is amply sufficient to stimulate an irritable nerve. The conditions 

 when the two regions of different potential are not externally applied 

 electrodes but portions of the nerve itself are not essentially different; in 

 either case a current flows along the nerve ; and if this current is intense 

 enough and arises suddenly enough it must stimulate the latter. There 

 is thus reason to believe that the electrical variation accompanying stimu- 

 lation is the main condition of propagation of the excited state. This 

 conclusion is supported by various experimental facts ; for instance, it is 

 found that the rate of development of the electric variation and the rate 

 of passage of the impulse are influenced to the same degree by changes of 

 temperature, and by certain chemical substances such as the anesthetics. 

 There are various other facts pointing in the same direction, and there 

 are also certain difficulties in the way of this conception ; but into these 

 we cannot enter here. The fact remains that the electrical variation is 

 the only known peculiarity of the local process that can account for its 

 self-propagating character; and recent determinations .of the minimal 

 current needed for excitation indicate that the bioelectric currents are of 

 sufficient intensity to serve as the basis for this propagation. 



It is clear that propagation of the state of excitation from the im- 

 mediate site of stimulus over the entire cell or nerve fibre is indispensable 

 to stimulation of any irritable element as a whole by any local stimulus ; 

 so that if the above view is correct we must regard the electrical variation 

 as perhaps the most essential feature of the stimulation process. If so, 

 we can understand why the electrical current has such universal stimu- 

 lating action. In passing a current through a tissue we are artifically 

 setting up differences of electrical potential between different portions of 

 the irritable elements, and according to the above conception this should 

 always cause excitation if the current is strong enough and rises to its 

 maximum with sufficient rapidity. That this is in fact the case needs no 

 emphasis. The electrical current is recognized as the most universal 

 form of stimulus ; and all irritable cells and elements, virtually without 

 exception, respond to its action. 



We conclude then that the critical or initiatory event in stimulation 

 is an electrical change, consisting essentially in a sudden decrease in the 

 electrical potential of the external surface of the irritable element at the 



