[Ill.u.s.tration: FIG. 70.--FATIGUE IN PLATINUM]

#Fatigue.#--In some metals, as in muscle and in plant, we find instances of that progressive diminution of response which is known as fatigue (fig. 69). The accompanying record shows this in platinum (fig. 70). It has been said that tin is practically indefatigable. We must, however, remember that this is a question of degree only. Nothing is absolutely indefatigable. The exhibition of fatigue depends on various conditions.

Even in tin, then, I obtained the characteristic fatigue-curve with a specimen which had been in continuous use for many days (fig. 71).

While discussing the subject of fatigue in plants, I have adduced considerations which showed that the residual effect of strain was one of the main causes for the production of fatigue. This conclusion receives independent support from the records obtained with metals.

[Ill.u.s.tration: FIG. 71.--FATIGUE SHOWN BY TIN WIRE WHICH HAD BEEN CONTINUOUSLY STIMULATED FOR SEVERAL DAYS]

In this connection the important fact is that the various typical fatigue effects exhibited in living substances are exactly reproduced in metals, where there can be question neither of fatigue-product producing fatigue effects, nor of those constructive processes by which they might be removed. We have seen, both in muscles and in plants, that if sufficient time for complete recovery be allowed between each pair of stimuli, the heights of successive responses are the same, and there is no apparent fatigue (see page 39). But the height of response diminishes as the excitation interval is shortened. We find the same thing in metals. Below is given a record taken with tin (fig. 72). Throughout the experiment the amplitude of vibration was maintained constant, but in (_a_) the interval between consecutive stimuli was 1', while in (_b_) this was reduced to 30". A diminution of height immediately occurs. On restoring the original rhythm as in (_c_), the responses revert to their first large value. Thus we see that when the wire has not completely recovered, its responses, owing to residual strain, undergo diminution.

Height of response is thus decreased by incomplete recovery. If then sufficient time be not allowed for perfect recovery, we can understand how, under certain circ.u.mstances, the residual strain would progressively increase with repet.i.tion of stimulus, and thus there would be a progressive diminution of height of response or fatigue. Again, we saw in the last chapter that increase of strain necessitates a longer period of recovery. Thus the longer a wire is stimulated, the more and more overstrained it becomes, and it therefore requires a gradual prolongation of the interval between the successive stimuli, if recovery is to be complete. This interval, however, being maintained constant, the recovery periods virtually undergo a gradual reduction, and successive recoveries become more and more incomplete. These considerations may be found to afford an insight into the progressive diminution of response in fatigued substances.

[Ill.u.s.tration: FIG. 72.--DIMINUTION OF RESPONSE DUE TO SHORTENING THE PERIOD OF RECOVERY The stimulus is maintained constant. In (_a_) the interval between two successive stimuli is one minute, in (_b_) it is half a minute, and in (_c_) it is again one minute. The response in (_b_) is feebler than in either (_a_) or (_c_).]

#Fatigue under continuous stimulation.#--Fatigue is perhaps best shown under continuous stimulation. For example, in muscles, when fresh and not fatigued, the top of the tetanic curve is horizontal, or may even be ascending, but with long-continued stimulation the curve declines. The rapidity of this decline depends on the nature of the muscle and its previous condition.

In metals I have found exactly parallel instances. In tin, so little liable to fatigue, the top of the curve is horizontal or ascending; or it may exhibit a slight decline. But the record with platinum shows the rapid decline due to fatigue (fig. 73).

[Ill.u.s.tration: FIG. 73 (_a_) The top of response-curve under continuous stimulation in tin is horizontal or ascending as in figure.

(_b_) In platinum there is rapid decline owing to fatigue.]

Taking any of these instances, say that in which fatigue is most prominent, it is found that short period of rest restores the original intensity of response. This affords additional proof of the fact that fatigue is due to overstrain, and that this strain, with its sign of attendant fatigue, disappears with time.

#Staircase effect.#--We shall now discuss an effect which appears to be the direct opposite of fatigue. This is the curious phenomenon known to physiologists as 'the staircase' effect, in which successive uniform stimuli produce a series of increasing responses. This is seen under particular conditions in the response of certain muscles (fig. 74, _a_).

It is also observed sometimes even in nerve, which otherwise, generally speaking, gives uniform responses. Of this effect, no satisfactory theory has as yet been offered. It is in direct contradiction to that theory which supposes that each stimulus is followed by dissimilation or break-down of the tissue, reducing its function below par. For in these cases the supposed dissimilation is followed not by a decrease but by an increase of functional activity. This 'staircase effect' I have shown to be occasionally exhibited by plants. I have also found it in metals. In the last chapter we have seen that a wire often falls, especially after resting for a long time, into a state of comparative sluggishness, and that this molecular inertness then gradually gives place to increased mobility under stimulation. As a consequence, an increased response is thus obtained. I give in fig. 74, _b_, a series of responses to uniform stimuli, exhibited by platinum which had been at rest for some time.

This effect is very clearly shown here. So we see that in a substance which has previously been in a sluggish condition, stimulation confers increased mobility. Response thus reaches a maximum, but continued stimulation may afterwards produce overstrain, and the subsequent responses may then show a decline. This consideration will explain certain types of responses exhibited by muscles, where the first part of the series exhibits a staircase increase followed by declining responses of fatigue.

[Ill.u.s.tration: FIG. 74.--'STAIRCASE' EFFECT (_a_) in muscle (after Engelmann).

(_b_) in metal.]

#Reversed response due to molecular modification and its transformation into normal after continuous stimulation (1) in nerve.#--Reference has already been made to the fact that a nerve which, when fresh, exhibited the normal negative response, will often, if kept for some time in preservative saline, undergo a molecular modification, after which it gives a positive variation. Thus while the response given by fresh nerve is _normal_ or negative, a stale nerve gives _modified_, i.e. reversed or positive, response. This peculiar modification does not always occur, yet is too frequent to be considered abnormal. Again, when such a nerve is subjected to tetanisation or continuous stimulation, this modified response tends once more to become normal.

It is found that not only tetanisation, but also CO_2 has the power of converting the modified response into normal. Hence it has been suggested that the conversion under tetanisation of modified response to normal, in stale nerve, is due to a hypothetical evolution of CO_2 in the nerve during stimulation.[16]

#(2) In metals.#--I have, however, met with exactly parallel phenomena in metals, where, owing to some molecular modification, the responses became reversed, and where, under continuous stimulation, though here there could be no possibility of the evolution of CO_2, they tended again to become normal.

If after mounting a wire in a cell filled with water, it be set aside for too long a time, I have sometimes noticed that it undergoes a certain modification, owing to which its response ceases to be normal and becomes reversed in sign. I have obtained this effect with various metals, for instance lead and tin, and even with the chemically inactive substance--platinum.

[Ill.u.s.tration: FIG. 75.--ABNORMAL POSITIVE (UP) RESPONSE IN NERVE CONVERTED INTO NORMAL (DOWN) RESPONSE AFTER CONTINUOUS STIMULATION T (WALLER) The galvanometer is not dead-beat, and shows after-oscillation.]

The subject will be made clearer if we first follow in detail the phenomenon exhibited by modified nerve, giving this abnormal response.

The normal responses in nerve are usually represented by 'down' and the reversed abnormal responses by 'up' curves. In the modified nerve, then, the abnormal responses are 'up' instead of the normal 'down.' The record of such abnormal response in the modified nerve is shown in fig. 75. It will be noticed that in this, the successive responses are undergoing a diminution, or tending towards the normal. After continuous stimulation or tetanisation (T), it will be seen that the abnormal or 'up' responses are converted into normal or 'down.'

I shall now give a record which will exhibit an exactly similar transformation from the abnormal to normal response after continuous stimulation. Here the normal responses are represented by 'up' and the abnormal by 'down' curves. This record was given by a tin wire, which had been molecularly modified (fig. 76). We have at first the abnormal responses; successive responses are undergoing a diminution or tending towards the normal; after continuous stimulation (T), the subsequent responses are seen to have become normal. Another record, obtained with platinum, shows the same phenomenon (fig. 77).

[Ill.u.s.tration: FIG. 76 AND FIG. 77 Abnormal 'down' response in tin (fig. 76) and in platinum (fig. 77) transformed into normal 'up' response, after continuous stimulation, T.]

On placing the three sets of records--nerve, tin, and platinum--side by side, it will be seen how essentially similar they are in every respect.[17]

This reversion to normal is seen to have appeared in a p.r.o.nounced manner after rapidly continuous stimulation, in process of which the modified molecular condition must in some way have reverted to the normal.

[Ill.u.s.tration: FIG. 78.--THE GRADUAL TRANSITION FROM ABNORMAL TO NORMAL RESPONSE IN PLATINUM The transition will be seen to have commenced at the third and ended at the seventh, counting from the left.]

Being desirous to trace this change gradually taking place, I took a platinum wire cell giving modified responses, and obtained a series of records of effects of individual stimuli continued for a long time. In this series, the points of transition from modified response to normal will be clearly seen (fig. 78).

[Ill.u.s.tration: FIG. 79.--THE NORMAL RESPONSE _a_ IN NERVE ENHANCED TO _b_ AFTER CONTINUOUS STIMULATION T (WALLER) The normal response in nerve is recorded 'down.']

[Ill.u.s.tration: FIG. 80.--ENHANCED RESPONSE IN PLATINUM AFTER CONTINUOUS STIMULATION T]

#Increased response after continuous stimulation.#--We have seen that responses to uniform stimuli sometimes show a staircase increase, apparently owing to the gradual removal of molecular sluggishness.

Possibly a.n.a.logous to this is the increase of response in nerve after continuous stimulation or tetanisation, observed by Waller (fig. 79).

Like the staircase effect, this contravenes the commonly accepted theory of the dissimilation of tissue by stimulus, and the consequent depression of response. It is suggested by Waller that this increase of response after tetanisation may be due to the hypothetical evolution of CO_2 to which allusion has previously been made.

[Ill.u.s.tration: FIG. 81.--ENHANCED RESPONSE IN TIN AFTER CONTINUOUS STIMULATION T]

But there is an exact correspondence between this phenomenon and that exhibited by metals under similar conditions. I give here two sets of records (figs. 80, 81), one obtained with platinum and the other with tin, which demonstrate how the response is enhanced after continuous stimulation in a manner exactly similar to that noticed in the case of nerve.

The explanation which has been suggested with regard to the staircase effect--increased molecular mobility due to removal of sluggishness by repeated stimulation--would appear to be applicable in this case also.

It would appear, then, that in all the phenomena which we have studied under the heads of 'staircase' effect, increase of response after continuous stimulation, and fatigue, there is a similarity between the observations made upon the response of muscle and nerve on the one hand, and that of metals on the other. Even in their abnormalities we have seen an agreement.

But amongst these phenomena themselves, though at first sight so diverse, there is some kind of continuity. Calling _all_ normal response _positive_, for the sake of convenience, we observe its gradual modification, corresponding to changes in the molecular condition of the substance.

Beginning with that case in which molecular modification is extreme, we find a maximum variation of response from the normal, that is to say, to _negative_.

Continued stimulation, however, brings back the molecular condition to normal, as evidenced by the progressive lessening of the negative response, culminating in reversion to the normal _positive_. This is equally true of nerve and metal.

In the next cla.s.s of phenomena, the modification of molecular condition is not so great. It now exhibits itself merely as a relative inertness, and the responses, though positive, are feeble. Under continued stimulation, they increase in the same direction as in the last case, that is to say, from less positive to _more positive_, being the reverse of fatigue. This is evidenced alike by the staircase effect and by the increase of response after tetanisation, seen not only in nerve but also in platinum and tin.

The substance may next be in what we call the normal condition.

Successive uniform stimuli now evoke uniform and equal positive responses, that is to say, there is no fatigue. But after intense or long-continued stimulation, the substance is overstrained. The responses now undergo a change from positive to _less positive_; fatigue, that is to say, appears.

Again, under very much prolonged stimulation the response may decline to zero, or even undergo a reversal to _negative_, a phenomenon which we shall find instanced in the reversed response of retina under the long-continued stimulus of light.

We must then recognise that a substance may exist in various molecular conditions, whether due to internal changes or to the action of stimulus. The responses give us indications of these conditions. A complete cycle of molecular modifications can be traced, from the abnormal negative to the normal positive, and then again to negative seen in reversal under continuous stimulation.

FOOTNOTES:

[16] 'Considering that we have no previous evidence of any chemical or physical change in tetanised nerve, it seems to me not worth while pausing to deal with the criticism that it is not CO_2, but "something else" that has given the result.'--Waller, _Animal Electricity_, p. 59.

That this phenomenon is nevertheless capable of physical explanation will be shown presently.

[17] In order to explain the phenomena of electric response, some physiologists a.s.sume that the negative response is due to a process of dissimilation, or breakdown, and the positive to a process of a.s.similation, or building up, of the tissue. The modified or positive response in nerve is thus held to be due to a.s.similation; after continuous stimulation, this process is supposed to be transformed into one of dissimilation, with the attendant negative response.

How arbitrary and unnecessary such a.s.sumptions are will become evident, when the abnormal and normal responses, and their transformation from one to the other, are found repeated in all details in metals, where there can be no question of the processes of a.s.similation or dissimilation.

CHAPTER XV

INORGANIC RESPONSE--RELATION BETWEEN STIMULUS AND RESPONSE--SUPERPOSITION OF STIMULI