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, 
