Part 15 (1/2)

Pa.s.sing 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).

[Ill.u.s.tration: 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 cla.s.ses 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 rest.i.tution, 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.

[Ill.u.s.tration: 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 a.s.similation, 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 a.s.similation 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 c.u.mulative residual strains, and that a brief period of rest, by removing the overstrain, removes also the sign of fatigue.

#Staircase effect.#--The theory of dissimilation due to stimulus reducing the functional activity below par, and thus causing fatigue, is directly negatived by what is known as the 'staircase' effect, where successive equal stimuli produce increasing response. We saw an exactly similar phenomenon in plants and metals, where successive responses to equal stimuli exhibited an increase, apparently by a gradual removal of molecular sluggishness (fig. 114).

[Ill.u.s.tration: FIG. 114.--'STAIRCASE' IN MUSCLE, PLANT, AND METAL]

[Ill.u.s.tration: FIG. 115.--INCREASED RESPONSE AFTER CONTINUOUS STIMULATION IN NERVE AND METAL The normal response in animal tissue is represented 'down,' in metal 'up.']

#Increased response after continuous stimulation.#--An effect somewhat similar, that is to say, an increased response, due to increased molecular mobility, is also shown sometimes after continuous stimulation, not only in animal tissues, but also in metals (fig. 115).

#Modified response.#--In the case of nerve we saw that the normal response, which is negative, sometimes becomes reversed in sign, i.e.

positive, when the specimen is stale. In retina again the normal positive response is converted into negative under the same conditions.

Similarly, we found that a plant when withering often shows a positive instead of the usual negative response (fig. 28). On nearing the death-point, also by subjection to extremes of temperature, the same reversal of response is occasionally observed in plants. This reversal of response due to peculiar molecular modification was also seen in metals.