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.

[Ill.u.s.tration: FIG. 116.--MODIFIED ABNORMAL RESPONSE IN (A) NERVE AND (M) METAL CONVERTED INTO NORMAL, AFTER CONTINUOUS STIMULATION (A) is the record for nerve (recording galvanometer not being dead-beat shows after-oscillation); the abnormal 'up' is converted into normal 'down' after continuous stimulation. (M) is the record for metal, the abnormal 'down' being converted into normal 'up' after like stimulation.]

But these modified responses usually become normal when the specimen is subjected to stimulation either strong or long continued (fig. 116).

#Diphasic variation.#--A diphasic variation is observed in nerve, if the wave of molecular disturbance does not reach the two contacts at the same moment, or if the rate of excitation is not the same at the two points. A similar diphasic variation is also observed in the responses of plants and metals (figs. 26, 68).

#Effect of temperature.#--In animal tissues response becomes feeble at low temperatures. At an optimum temperature it reaches its greatest amplitude, and, again, beyond a maximum temperature it is very much reduced.

We have observed the same phenomena in plants. In metals too, at high temperatures, the response is very much diminished (figs. 38, 65).

#Effect of chemical reagents.#--Finally, just as the response of animal tissue is exalted by stimulants, lowered by depressants, and abolished by poisons, so also we have found the response in plants and metals undergoing similar exaltation, depression, or abolition.

We have seen that the criterion by which vital response is differentiated is its abolition by the action of certain reagents--the so-called poisons. We find, however, that 'poisons' also abolish the responses in plants and metals (fig. 117). Just as animal tissues pa.s.s from a state of responsiveness while living to a state of irresponsiveness when killed by poisons, so also we find metals transformed from a responsive to an irresponsive condition by the action of similar 'poisonous' reagents.

The parallel is the more striking since it has long been known with regard to animal tissues that the same drug, administered in large or small doses, might have opposite effects, and in preceding chapters we have seen that the same statement holds good of plants and metals also.

#Stimulus of light.#--Even the responses of such a highly specialised organ as the retina are strictly paralleled by inorganic responses. We have seen how the stimulus of light evokes in the artificial retina responses which coincide in all their detail with those produced in the real retina. This was seen in ineffective stimuli becoming effective after repet.i.tion, in the relation between stimulus and response, and in the effects produced by temperature; also in the phenomenon of after-oscillation. These similarities went even further, the very abnormalities of retinal response finding their reflection in the inorganic.

[Ill.u.s.tration: FIG. 117.--ABOLITION OF RESPONSE IN NERVE, PLANT, AND METAL BY THE ACTION OF THE SAME 'POISON'

The first half in each set shows the normal response, the second half the abolition of response after the application of the reagent.]

Thus living response in all its diverse manifestations is found to be only a repet.i.tion of responses seen in the inorganic. There is in it no element of mystery or caprice, such as we must admit to be applied in the a.s.sumption of a hypermechanical vital force, acting in contradiction or defiance of those physical laws that govern the world of matter.

Nowhere in the entire range of these response-phenomena--inclusive as that is of metals, plants, and animals--do we detect any breach of continuity. In the study of processes apparently so complex as those of irritability, we must, of course, expect to be confronted with many difficulties. But if these are to be overcome, they, like others, must be faced, and their investigation patiently pursued, without the postulation of special forces whose convenient property it is to meet all emergencies in virtue of their vagueness. If, at least, we are ever to understand the intricate mechanism of the animal machine, it will be granted that we must cease to evade the problems it presents by the use of mere phrases which really explain nothing.

We have seen that amongst the phenomena of response, there is no necessity for the a.s.sumption of vital force. They are, on the contrary, physico-chemical phenomena, susceptible of a physical inquiry as definite as any other in inorganic regions.

Physiologists have taught us to read in the response-curves a history of the influence of various external agencies and conditions on the phenomenon of life. By these means we are able to trace the gradual diminution of responsiveness by fatigue, by extremes of heat and cold, its exaltation by stimulants, the arrest of the life-process by poison.

The investigations which have just been described may possibly carry us one step further, proving to us that these things are determined, not by the play of an unknowable and arbitrary vital force, but by the working of laws that know no change, acting equally and uniformly throughout the organic and the inorganic worlds.

FOOTNOTES:

[21] Verworn, _General Physiology_, p. 18.