GENERAL SURVEY AND CONCLUSION 183 



response which occur under various conditions, take 

 place in plants and metals just as in animal tissues. It 

 may now be well to make a general survey of these phe- 

 nomena, as exhibited in the three classes of substances. 



We have seen that the wave of molecular disturb- 

 ance in a living animal tissue under stimulus is accom- 

 panied by a wave of electrical disturbance ; that in 

 certain types of tissue the stimulated is relatively 

 positive to the less disturbed, while in others it is the 

 reverse ; that it is essential to the obtaining of electric 

 response to have the contacts leading to the galvano- 

 meter unequally affected by excitation : and finally that 

 this is accomplished either (1) by ' injuring ' one con- 

 tact, so that the excitation produced there would be re- 

 latively feeble, or (2) by introducing a perfect block 

 between the two contacts, so that the excitation reaches 

 one and not the other. 



Further, it has been shown that this characteristic 

 of exhibiting electrical, response under stimulus is not 

 confined to animal, but extends also to vegetable tissues. 

 In these the same electrical variations as in nerve and 

 muscle were obtained, by using the method of injury, 

 or that of the block. 



Passing to inorganic substances, and using similar 

 experimental arrangements, we have found the same 

 electrical responses evoked in metals under stimulus. 



Negative variation. In all cases, animal, vegetable, 

 and metal, we may obtain response by the method of 

 negative variation, so called, by reducing the excitability 

 of one contact by physical or chemical means. Stimulus 

 causes a transient diminution of the existing current, 



