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.
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 fatigue-products, while recovery is due to assimilation, for which material is brought by the blood-supply—has long been seen to be inadequate, since the restorative effect succeeds a short period of rest even in excised bloodless muscle. But that the phenomena of fatigue and recovery were not primarily dependent on dissimilation or assimilation becomes self-evident when we find exactly similar effects produced not only in plants, but also in metals ([fig. 113]). It has been shown, on the other hand, that these effects are primarily due to cumulative residual strains, and that a brief period of rest, by removing the overstrain, removes also the sign of fatigue.