Fig. 104.—Curves giving the Relation between Intensity of Light and Magnitude of ResponseFig. 104.—Curves giving the Relation between Intensity of Light and Magnitude of ResponseIn (a) sensitive cell, (b) in frog’s retina.
Fig. 104.—Curves giving the Relation between Intensity of Light and Magnitude of Response
In (a) sensitive cell, (b) in frog’s retina.
In another set of records, with a different cell, I obtained the deflections of 6, 10, 13, 15, corresponding to light intensities of 3, 5, 7, and 9.
The two curves infig. 104, giving the relation between response and stimulus, show that in the case of inorganic substances, as in the retina (Waller), magnitude of response does not increase so rapidly as stimulus.
After-oscillation.—When the sensitive surface is subjected to the continued action of light, the E.M.effect attains a maximum at which it remains constant for some time. If the exposure be maintained after this for a longer period, there will be a decline, as we found to be the case in other instances of continued stimulation. The appearance of this decline, and its rapidity, depends on the particular condition of the substance.
When the sensitive element is considerably strained by the action of light, and if that light be now cut off, there is a rebound towards recovery and a subsequent after-oscillation. That is to say, the curve of recovery falls below the zero point, and then slowly oscillates back to the position of equilibrium. We have already seen an instance of this infig. 102. Above is given a series of records showing the appearance of decline, from too long-continued exposure and recovery, followed by after-oscillation on the cessation of light (fig. 105). Certain visual analogues to this phenomenon will be noticed later.
Fig. 105.—After-oscillationFig. 105.—After-oscillationExposure of one minute followed by obscurity of one minute. Note the decline during illumination, and after-oscillation in darkness.
Fig. 105.—After-oscillation
Exposure of one minute followed by obscurity of one minute. Note the decline during illumination, and after-oscillation in darkness.
Abnormal effects.—We have already treated of all the normal effects of the stimulus of light on the retina, and their counterparts in the sensitive cell. But the retina undergoes molecular changes when injured, stale, or in a dying condition, and under these circumstances various complicated modifications are observed in the response.
Fig. 106.—Transient Positive Augmentation given by the Frog’s Retina on the Cessation of Light L (Waller)Fig. 106.—Transient Positive Augmentation given by the Frog’s Retina on the Cessation of Light L (Waller)
Fig. 106.—Transient Positive Augmentation given by the Frog’s Retina on the Cessation of Light L (Waller)
Fig. 107.—Responses in Silver CellFig. 107.—Responses in Silver CellThe thick line represents response during light (half a minute’s exposure), and dotted line the recovery during darkness. Note the terminal positive twitch.
Fig. 107.—Responses in Silver Cell
The thick line represents response during light (half a minute’s exposure), and dotted line the recovery during darkness. Note the terminal positive twitch.
1. Preliminary negative twitch.—When the light is incident on the frog’s retina, there is sometimes a transitory negative variation, followed by the normal positive response. This is frequently observed in the sensitive cell (seefig. 96,b).
2. Reversal of response.—Again, in a stale retina, owing to molecular modification the response is apt to undergo reversal (Waller). That is to say, it now becomes negative. In working with the same sensitive cell on different days I have found it occasionally exhibiting this reversed response.
3. Transient rise of current on cessation of light.—Another very curious fact observed in the retina by Kuhne and Steiner is that immediately on the stoppage of light there is sometimes a sudden increase in the retinal current, before the usual recovery takes place. This is very well shown in the series of records taken by Waller (fig. 106). It will be noticed that on illumination the response-curve rises, that continued illumination produces a decline, and that on the cessation of light there is a transient rise of current. I give here a series of records which will show the remarkable similarity between the responses of the cell and retina, in respect even of abnormalities so marked as those described (fig. 107). I may mention here that some of these curious effects, that is to say, the preliminary negative twitch and sudden augmentation of the current on the cessation of light, have also been noticed by Minchin in photo-electric cells.
4. Decline and reversal.—We have seen that under the continuous action of light, response begins todecline. Sometimes this process is very rapid, and in any case, under continued light, the deflection falls.
(1) The decline may nearly reach zero. If now the light be cut off there is a rebound towards recoverydownwards, which carries it below zero, followed by an after-oscillation (fig. 108,a).
(2) If the light be continued for a longer time, the decline goes on even below zero; that is to say, the response now becomes apparently negative. If, now, the light be stopped, there is a rebound upwards to recovery, with, generally speaking, a slight preliminary twitch downwards (fig. 108,b,c). This rebound carries it back, not only to the zero position, but sometimes beyond that position. We have here a parallel to the following observation of Dewar and McKendrick:‘When diffuse light is allowed to impinge on the eye of the frog, after it has arrived at a tolerably stable condition, the natural E.M.F. is in the first place increased, then diminished; during the continuance of light it is still slowly diminished to a point where it remains tolerably constant, and on the removal of light there is a sudden increase of the E.M. power nearly up to its original position.’[18]
Fig. 108—Decline under the Continued Action of LightFig. 108—Decline under the Continued Action of Light(a) Decline short of zero; on stoppage of light, rebound downwards to zero; after-oscillation.(b) Decline below zero; on stoppage of light, rebound towards zero, with preliminary negative twitch.(c) The same, decline further down; negative twitch almost disappearing.
Fig. 108—Decline under the Continued Action of Light
(a) Decline short of zero; on stoppage of light, rebound downwards to zero; after-oscillation.
(b) Decline below zero; on stoppage of light, rebound towards zero, with preliminary negative twitch.
(c) The same, decline further down; negative twitch almost disappearing.
(3) I have sometimes obtained the following curious result. On the incidence of light there is a response, say, upward. On the continuation of light the response declines to zero and remains at the zero position, there being no further action during the continuation of stimulus. But on the cessation or ‘break’ of light stimulus, there is a response downwards, followed by the usual recovery. This reminds us of a somewhat similar responsive action produced by constant electric current on the muscle. At the moment of ‘make’ there is a responsive twitch, but afterwards the muscle remains quiescent during the passage of the current, but on breaking the current there is seen a second responsive twitch.
Résumé.—So we see that the response of the sensitive inorganic cell, to the stimulus of light, is in every way similar to that of the retina. In both we have, under normal conditions, a positive variation; in both the intensity of response up to a certain limit increases with the duration of illumination; it is affected, in both alike, by temperature; in both there is comparatively little fatigue; the increase of response with intensity ofstimulus is similar in both; and finally, even in abnormalities—such as reversal of response, preliminary negative twitch on commencement, and terminal positive twitch on cessation of illumination, and decline and reversal under continued action of light—parallel effects are noticed.
Fig. 109.—Certain After-effects of LightFig. 109.—Certain After-effects of Light
Fig. 109.—Certain After-effects of Light
We may notice here certain curious relations even in these abnormal responses (fig. 109). If the equilibrium position remain always constant, then it is easy to understand how, when the rising curve has attained its maximum, on the cessation of light, recovery should proceeddownwards, towards the equilibrium position (fig. 109,a). One can also understand how, after reversal by the continued action of light, there should be a recoveryupwardstowards the old equilibrium position (fig. 109,b). What is curious is that in certain cases we get, on the stoppage of light, a preliminary twitch away from the zero or equilibrium position, upwards as in (c) (compare alsofig. 107) and downwards as in (d) (compare alsofig. 108b).
In making a general retrospect, finally, of the effectsproduced by stimulus of light, we find that there is not a single phenomenon in the responses, normal or abnormal, exhibited by the retina which has not its counterpart in the sensitive cell constructed of inorganic material.
FOOTNOTES:[18]Proc. Roy. Soc. Edin., 1873 p. 153.
[18]Proc. Roy. Soc. Edin., 1873 p. 153.
[18]Proc. Roy. Soc. Edin., 1873 p. 153.
We have already referred to the electrical theory of the visual impulse. We have seen how a flash of light causes a transitory electric impulse not only in the retina, but also in its inorganic substitute. Light thus produces not only a visual but also an electrical impulse, and it is not improbable that the two may be identical. Again, varying intensities of light give rise to corresponding intensities of current, and the curves which represent the relation between the increasing stimulus and the increasing response have a general agreement with the corresponding curve of visual sensation. In the present chapter we shall see how this electrical theory not only explains in a simple manner ordinary visual phenomena, but is also deeply suggestive with regard to others which are very obscure.
We have seen in our silver cell that if the molecular conditions of the anterior and posterior surfaces were exactly similar, there would be no current. In practice, however, this is seldom the case. There is, generallyspeaking, a slight difference, and a feeble current in the circuit. It is thus seen that there may be an existing feeble current, to which the effect of light is added algebraically. The stimulus of light may thus increase the existing current of darkness (positive variation). On the cessation of light again, the current of response disappears and there remains only the feeble original current.
In the case of the retina, also, it is curious to note that on closing the eye the sensation is not one of absolute darkness, but there is a general feeble sensation of light, known as ‘the intrinsic light of the retina.’ The effect produced by external light is superposed on this intrinsic light, and certain curious results of this algebraical summation will be noticed later.
Fig. 110—Response-curves of the Sensitive Silver CellFig. 110—Response-curves of the Sensitive Silver CellShowing greater persistence of after-effect when the stimulus is strong.(a) Short exposure of 2″ to light of intensity 1; (b) short exposure of 2″ to light nine times as strong.
Fig. 110—Response-curves of the Sensitive Silver Cell
Showing greater persistence of after-effect when the stimulus is strong.
(a) Short exposure of 2″ to light of intensity 1; (b) short exposure of 2″ to light nine times as strong.
Effect of light of short duration.—If we subject the sensitive cell to a flash of radiation, the effect is not instantaneous but grows with time. It attains a maximum some little time after the incidence of light, and the effect then gradually passes away. Again, as we have seen previously with regard to mechanical strain, the after-effect persists for a slightly longer time when the stimulus is stronger. The same is true of the after-effect of the stimulus of light. Two curves which exhibit this are given below (fig. 110). With regard to the first point—that the maximum effect is attained some time after the cessation of a short exposure—the corresponding experiment on the eye may be made as follows: at the end of a tube is fixed a glass disc coated with lampblack, on which, by scratching with a pin, some words are written in transparent characters.The length of the tube is so adjusted that the disc is at the distance of most distinct vision from the end of the tube applied to the eye. The blackened disc is turned towards a source of strong light, and a short exposure is given by the release of a photographic shutter interposed between the disc and the eye. On closing the eye, immediately after a short exposure, it will at first be found that there is hardly any well-defined visual sensation; after a short time, however, the writing on the blackened disc begins to appear in luminous characters, attains a maximum intensity, and then fades away. In this case the stimulus is of short duration, the light being cut off before the maximum effect is attained. The after-effect here ispositive, there being no reversal or interval of darkness between the direct image and the after-image, the one being merely the continuation of the other. But we shall see, if light is cut off after a maximum effect is attained by longexposure, that the immediate after-image would be negative (see below). The relative persistence of after-effect of lights of different intensities may be shown in the following manner:
If a bold design be traced with magnesium powder on a blackened board and fired in a dark room, the observer not being acquainted with the design, the instantaneous flash of light, besides being too quick for detailed observation, is obscured by the accompanying smoke. But if the eyes be closed immediately after the flash, the feebler obscuring sensation of smoke will first disappear, and will leave clear the more persistent after-sensation of the design, which can then be read distinctly. In this manner I have often been able to see distinctly, on closing the eyes, extremely brief phenomena of light which could not otherwise have been observed, owing either to their excessive rapidity or to their dazzling character.[19]
After-oscillation.—In the case of the sensitive silver cell, we have seen (fig. 105), when it has been subjected for some time to strong light, that the current of response attains a maximum, and that on the stoppage of the stimulus there is an immediate rebound towards recovery. In this rebound there may be an over-shooting of the equilibrium position, and an after-oscillation is thus produced.
If there has been a feeble initial current, this oscillatory after-current, by algebraical summation, will cause the current in the circuit to be alternately weaker and stronger than the initial current.
Visual recurrence.—Translated into the visual circuit, this would mean an alternating series of after-images. On the cessation of light of strong intensity and long duration, the immediate effect would be a negative rebound, unlike the positive after-effect which followed on a short exposure.
The next rebound is positive, giving rise to a sensation of brightness. This will go on in a recurrent series.
If we look for some time at a very bright object, preferably with one eye, on closing the eye there is an immediate dark sensation followed by a sensation of light. These go on alternating and give rise to the phenomena of recurrent vision. With the eyes closed, the positive or luminous phases are the more prominent.
This phenomenon may be observed in a somewhat different manner. After staring at a bright light we may look towards a well-lighted wall. The dark phases will now become the more noticeable.
If, however, we look towards a dimly lighted wall, both the dark and bright phases will be noticed alternately.
The negative effect is usually explained as due to fatigue. That position of the retina affected by light is supposed to be ‘tired,’ and a negative image to be formed in consequence of exhaustion. By this exhaustion is meant either the presence of fatigue-stuffs, or the breaking-down of the sensitive element of the tissue, or both of these. In such a case we should expect that this fatigue, with its consequent negative image, would gradually and finally disappear on the restoration of the retina to its normal condition.
We find, however, that this is not the case, for the negative image recurs with alternate positive. The accepted theory of fatigue is incapable of explaining this phenomenon.
In the sensitive silver cell, we found that the molecular strain produced by light gave rise to a current of response, and that on the cessation of light an oscillatory after-effect was produced. The alternating after-effect in the retina points to an exactly similar process.
Binocular alternation of vision.—It was while experimenting on the phenomena of recurrent vision that I discovered the curious fact that in normal eyes the two do not see equally well at a given instant, but that the visual effect in each eye undergoes fluctuation from moment to moment, in such a way that the sensation in the one is complementary to that in the other, the sum of the two sensations remaining approximately constant. Thus they take up the work of seeing, and then, relatively speaking, resting, alternately. This division of labour, in binocular vision, is of obvious advantage.
Fig. 111.—Stereoscopic DesignFig. 111.—Stereoscopic Design
Fig. 111.—Stereoscopic Design
As regards maximum sensation in the two retinæ there is then a relative retardation of half a period. This may be seen by means of a stereoscope, carrying, instead of stereo-photographs, incised plates through which we look at light. The design consists of twoslanting cuts at a suitable distance from each other. One cut,R, slants to the right, and the other,L, to the left (seefig. 111). When the design is looked at through the stereoscope, the right eye will see, sayR, and the leftL, the two images will appear superimposed, and we see an inclined cross. When the stereoscope is turned towards the sky, and the cross looked at steadily for some time, it will be found, owing to the alternation already referred to, that while one arm of the cross begins to be dim, the other becomes bright, andvice versa. The alternate fluctuations become far more conspicuous when the eyes are closed; the pure oscillatory after-effects are then obtained in a most vivid manner. After looking through the stereoscope for ten seconds or more, the eyes are closed. The first effect observed is one of darkness, due to the rebound. Thenoneluminous arm of the cross first projects aslant the dark field, and then slowly disappears, after which the second (perceived by the other eye) shoots out suddenly in a direction athwart the first. This alternation proceeds for a long time, and produces the curious effect of two luminous blades crossing and recrossing each other.
Another method of bringing out the phenomenon of alternation in a still more striking manner is to look at two different sets of writing, with the two eyes. The resultant effect is ablur, due to superposition, and the inscription cannot be read with the eyes open. But onclosing them, the composite image is analysed alternately into its component parts, and thus we are enabled to read better with eyes shut than open.
This period of alternation is modified by age and by the condition of the eye. It is, generally speaking, shorter in youth. I have seen it vary in different individuals from 1″ to 10″ or more. About 4″ is the most usual. With the same individual, again, the period is somewhat modified by previous conditions of rest or activity. Very early in the morning, after sleep, it is at its shortest. I give below a set of readings given by an observer:
Again, if one eye be cooled and the other warmed, the retinal oscillation in one eye is quicker than in the other. The quicker oscillation overtakes the slower, and we obtain the curious phenomenon of ‘visual beats.’
After-images and their revival.—In the experiment with the stereoscope and the design of the cross, the after-images of the cross seen with the eyes closed are at first very distinct—so distinct that any unevenness at the edges of the slanting cuts in the design can be distinctly made out. There can thus be no doubt of the ‘objective’ nature of the strain impression on the retina, which on the cessation of direct stimulus of light gives rise to after-oscillation with the concomitant visual recurrence. This recurrence may therefore be taken as a proof of the physical strain produced on theretina. The recurrent after-image is very distinct at the beginning and becomes fainter at each repetition; a time comes when it is difficult to tell whether the image seen is the objective after-effect due to strain or merely an effect of ‘memory.’ In fact there is no line of demarcation between the two, one simply merges into the other. That this ‘memory’ image is due to objective strain is rendered evident by its recurrence.
In connection with this it is interesting to note that some of the undoubted phenomena of memory are also recurrent. ‘Certain sensations for which there is no corresponding process outside the body are generally grouped for convenience under this term [memory]. If the eyes be closed and a picture be called to memory, it will be found that the picture cannot be held, but will repeatedly disappear and appear.’[20]
The visual impressions and their recurrence often persist for a very long time. It usually happens that owing to weariness the recurrent images disappear; but in some instances, long after this disappearance, they will spontaneously reappear at most unexpected moments. In one instance the recurrence was observed in a dream, about three weeks after the original impression was made. In connection with this, the revival of images, on closing the eyes at night, that have been seen during the day, is extremely interesting.
Unconscious visual impression.—While repeating certain experiments on recurrent vision, the above phenomenon became prominent in an unexpectedmanner. I had been intently looking at a particular window, and obtaining the subsequent after-images by closing the eye; my attention was concentrated on the window, and I saw nothing but the window either as a direct or as an after effect. After this had been repeated a number of times, I found on one occasion, after closing the eye, that, owing to weariness of the particular portion of the retina, I could no longer see the after-image of the window; instead of this I however saw distinctly a circular opening closed with glass panes, and I noticed even the jagged edges of a broken pane. I was not aware of the existence of a circular opening higher up in the wall. The image of this had impressed itself on the retina without my knowledge, and had undoubtedly been producing the recurrent images which remained unnoticed because my principal field of after-vision was filled up and my attention directed towards the recurrent image of the window. When this failed to appear, my field of after-vision was relatively free from distraction, and I could not help seeing what was unnoticed before. It thus appears that, in addition to the images impressed in the retina of which we are conscious, there are many others which are imprinted without our knowledge. We fail to notice them because our attention is directed to something else. But at a subsequent period, when the mind is in a passive state, these impressions may suddenly revive owing to the phenomenon of recurrence. This observation may afford an explanation of some of the phenomena connected with ocular phantoms and hallucinations not traceable to any disease. In thesecases the psychical effects produced appear to have no objective cause. Bearing in mind the numerous visual impressions which are being unconsciously made on the retina, it is not at all unlikely that many of these visual phantoms may be due to objective causes.
FOOTNOTES:[19]As an instance of this I may mention the experiment which I saw on the quick fusion of metals exhibited at the Royal Institution by Sir William Roberts-Austen (1901), where, owing to the glare and the dense fumes, it was impossible to see what happened in the crucible. But I was able to see every detailon closing the eyes. The effects of the smoke, being of less luminescence, cleared away first, and left the after-image of the molten metal growing clearer on the retina.[20]E. W. Scripture,The New Psychology, p. 101.
[19]As an instance of this I may mention the experiment which I saw on the quick fusion of metals exhibited at the Royal Institution by Sir William Roberts-Austen (1901), where, owing to the glare and the dense fumes, it was impossible to see what happened in the crucible. But I was able to see every detailon closing the eyes. The effects of the smoke, being of less luminescence, cleared away first, and left the after-image of the molten metal growing clearer on the retina.
[19]As an instance of this I may mention the experiment which I saw on the quick fusion of metals exhibited at the Royal Institution by Sir William Roberts-Austen (1901), where, owing to the glare and the dense fumes, it was impossible to see what happened in the crucible. But I was able to see every detailon closing the eyes. The effects of the smoke, being of less luminescence, cleared away first, and left the after-image of the molten metal growing clearer on the retina.
[20]E. W. Scripture,The New Psychology, p. 101.
[20]E. W. Scripture,The New Psychology, p. 101.
We have seen that stimulus produces a certain excitatory change in living substances, and that the excitation produced sometimes expresses itself in a visible change of form, as seen in muscle; that in many other cases, however—as in nerve or retina—there is no visible alteration, but the disturbance produced by the stimulus exhibits itself in certain electrical changes, and that whereas the mechanical mode of response is limited in its application, this electrical form is universal.
This irritability of the tissue, as shown in its capacity for response, electrical or mechanical, was found to depend on its physiological activity. Under certain conditions it could be converted from the responsive to an irresponsive state, either temporarily as by anæsthetics, or permanently as by poisons. When thus made permanently irresponsive by any means, the tissue was said to have been killed. We have seen further that from this observed fact—that a tissue when killed passes out of the state of responsiveness into that of irresponsiveness; and from a confusion of ‘dead’ things with inanimate matter, it has been tacitly assumed that inorganic substances, like deadanimal tissues, must necessarily be irresponsive, or incapable of being excited by stimulus—an assumption which has been shown to be gratuitous.
This ‘unexplained conception of irritability became the starting-point,’ to quote the words of Verworn,[21]‘ofvitalism, which in its most complete form asserted a dualism of living and lifeless Nature.... The vitalists soon,’ as he goes on to say, ‘laid aside, more or less completely, mechanical and chemical explanations of vital phenomena, and introduced, as an explanatory principle, an all-controlling unknown and inscrutable “force hypermécanique.” While chemical and physical forces are responsible for all phenomena in lifeless bodies, in living organisms this special force induces and rules all vital actions.
‘Later vitalists, however, attempted no analysis of vital force; they employed it in a wholly mystical form as a convenient explanation of all sorts of vital phenomena.... In place of a real explanation a simple phrase such as “vital force” was satisfactory, and signified a mystical force belonging to organisms only. Thus it was easy to “explain” the most complex vital phenomena.’
From this position, with its assumption of the super-physical character of response, it is clear that on the discovery of similar effects amongst inorganic substances, the necessity of theoretically maintaining such dualism in Nature must immediately fall to the ground.
In the previous chapters I have shown that not the fact of response alone, but all those modifications inresponse which occur under various conditions, take place in plants and metals just as in animal tissues. It may now be well to make a general survey of these phenomena, as exhibited in the three classes of substances.
We have seen that the wave of molecular disturbance in a living animal tissue under stimulus is accompanied by a wave of electrical disturbance; that in certain types of tissue the stimulated is relatively positive to the less disturbed, while in others it is the reverse; that it is essential to the obtaining of electric response to have the contacts leading to the galvanometer unequally affected by excitation; and finally that this is accomplished either (1) by ‘injuring’ one contact, so that the excitation produced there would be relatively feeble, or (2) by introducing a perfect block between the two contacts, so that the excitation reaches one and not the other.
Further, 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).
Fig. 112.—Uniform Responses in (A) Nerve, (P) Plant, and (M) MetalFig. 112.—Uniform Responses in (A) Nerve, (P) Plant, and (M) MetalThe 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.
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.
Fig. 113.—Fatigue (A) in Muscle, (P) in Plant, (M) in MetalFig. 113.—Fatigue (A) in Muscle, (P) in Plant, (M) in Metal
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.
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 anexactly 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).
Fig. 114.—‘Staircase’ in Muscle, Plant, and MetalFig. 114.—‘Staircase’ in Muscle, Plant, and Metal
Fig. 114.—‘Staircase’ in Muscle, Plant, and Metal
Increased response after continuous stimulation.—An effect somewhat similar, that is to say, anincreasedresponse, due to increased molecular mobility,is also shown sometimes after continuous stimulation, not only in animal tissues, but also in metals (fig. 115).
Fig. 115.—Increased Response after Continuous Stimulation in Nerve and MetalFig. 115.—Increased Response after Continuous Stimulation in Nerve and MetalThe normal response in animal tissue is represented ‘down,’ in metal ‘up.’
Fig. 115.—Increased Response after Continuous Stimulation in Nerve and Metal
The normal response in animal tissue is represented ‘down,’ in metal ‘up.’
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.
Fig. 116.—Modified Abnormal Response in (A) Nerve and (M) Metal converted into Normal, after Continuous StimulationFig. 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.
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 pass 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 thesame 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 repetition, 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.