ther, 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, the
variation depending on the intensity of the stimulus (figs. 4, 7, 54).

[Illustration: FIG. 112.--UNIFORM RESPONSES IN (A) NERVE, (P) PLANT, AND
(M) METAL
The normal response in nerve is represented 'down.' In this and
following figures, (A) is the record of responses in animal, (P) in
plant, and (M) in metal.]

#Relation between stimulus and response.#--In all three classes we have
found that the intensity of response increases with increasing stimulus.
At very high intensities of stimulus, however, there is a tendency of
the response to reach a limit (figs. 30, 32, 84). The law that is known
as Weber-Fechner's shows a similar characteristic in the relation
between stimulus and sensation. And if sensation be a measure of
physiological effect we can understand this correspondence of the
physiological and sensation curves. We now see further that the
physiological effects themselves are ultimately reducible to simple
physical phenomena.

#Effects of superposition.#--In all three types, ineffective stimuli
become effective by superposition.

Again, rapidly succeeding stimuli produce a maximum effect, kept
balanced by a force of restitution, and continuation of stimulus
produces no further effect, in the three cases alike (figs. 17, 18, 86).

#Uniform responses.#--In the responses of animal, vegetable, and metal
alike we meet with a type where the responses are uniform (fig. 112).

#Fatigue.#--There is, again, another type where fatigue is exhibited.

[Illustration: FIG. 113.--FATIGUE (A) IN MUSCLE, (P) IN PLANT, (M) IN
METAL]

The explanation hitherto given of fatigue in animal tissues--that it is
due to dissimilation or breakdown of tissue, complicated by the presence
of fa