Response in the Living and Non-Living

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.

As regards maximum sensation in the two retine there is then a relative retardation of half a period. This may be seen by means of a stereoscope, carrying, mstead of stereo-photographs, incised plates through which we look at light. The design consists of two slanting cuts at a suitable distance from each other. One cut, R, slants to the right, and the other, L, to the left (see fig. 111). When the design is looked at through the stereoscope, the right eye will see, say R, and the left L, 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, and vice versa. The alternate fluctuations become far more conspicuous when R the eyes are closed; the pure oscillatory after-effects are then obtamed in a most vivid manner. After looking through the stereoscope for ten seconds or more, the eyes are Fie. TL SepEnoclosed. The first effect observed is scopic DESIGN one of darkness, due to the rebound. Then one luminous 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 a blurr, due to superposition, and the inscription cannot be read with the eyes open. - But on closing 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 belowa set of readings given by an observer :

PNM hada celeaicteass sas ou GrP iM ae aeons pA EETLOON .i5.00'ste onto sis ¥ive 4” | Di bay) | foaervseawat-oaeas 56" SS TERT ae Ry" ella Meas. esate eee 6:5!” 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—tIn 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 the

retina. 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 erouped 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. ’!

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 unexpected manner. 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 class 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 these

cases 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. 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 there is

other cases, however—as in nerve or retina 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 anesthetics, 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 dead

animal 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,! ‘ of vitalism, 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-controllmg unknown and inscrutable “ force hyper- mécanique.” 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 superphysical character of response, it is clear that on the discovery of similar effects amongst organic 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 in response 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). 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

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. 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. 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 an

exactly similar phenomenon in plants and metals, where successive responses to equal stimuli exhibited an increase, apparently by a gradual removal of molecular slugeishness (fig. 114). Fie. 115.—Increasep RESPONSE AFTER CONTINUOUS STIMULATION IN NERVE 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 in 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

3efore After Before After Fic. 116.—Mopririep ApnormaL Response 1n (A) Nerve and (M) Meran 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. usual negative response (fig. 28). On nearing the deathpoint, 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.

But these modified responses usually become normal when the specimen is subjected to stimulation either 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 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

Fic. 117.—ABouiTIon oF Response IN NervE, Puant, anp Mera BY THE ACTION OF THE SAME ‘ Porson’ The first half in each set shows the normal response, the second half the abolition of response after the application of the reagent. stimuli becoming effective after repetition, in the relation between stimulus and response, and in the effects produced by temperature ; also in the phenomenon of afteroscillation. These similarities went even further, the very abnormalities of retinal response finding their reflection in the inorganic.

Thus living response in all its diverse manifestations is found to be only a repetition 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 assumption 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 assumption of vital force. They are, on the contrary, physicochemical phenomena, susceptible of a physical inquiry as definite as any other in inorganic regions. Physiologists have taught us to read in the responsecurves 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 lifeprocess by poison.

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. a5 1: in plant, 19 After-images and their revival, 177 After-oscillation in photo-sensitive cell, 159, 163 Anesthetics, effect on response in nerve, 72 Chloroform, effect on nerve response, 72 a a plant response, 74

Electrical response. See Response, electrical Electric tapper, 24 Faticur, absence of, under certain conditions, in metal, 120 Insury, current of, in nerve, 7 Inorganic response. See Metal, electrical response in KUHNE on retinal current, 149 Kunkel on electrical changes by injury or flexion in plant, 14, 70 Liaut, after-effect of short exposure to, on photo-sensitive cell, 171 » 5 on retina, 171 decline and reversal of response under continuous, in photosensitive cell, 166

- a continuous, in retina effect of temperature on response of photo-sensitive cell produced by, 158 43 + retinal response produced by, 158 relation between intensity and response to, in photo-sensitive cell, 3 A Ha in retina, 162 response to, after-oscillation in photo-sensitive cell, 159, 163 - effect of increasing length of exposure in photo-sensitive cell, 159 ss Es _ in retina, 160 a in frog’s retina, 150, 151, 156, 164, 166 McKenprick on retinal response, 149 Mechanical recorder, 3 55 response, 1 5 stimulus by electric tapper, 24 m » by spring-tapper, 23 e “e by vibrator, 24 : ih conditions of maintaining uniformity of, 26 a 3 means of graduating intensity of, 22, 24, 96 Metal, electric response in, abnormal, 125 9 my abolition of, by ‘ poison,’ 148 ‘9 Po additive effect of superposition of stimulus on,

Minchin on photo-electric cell, 165 Molecular ‘ arrest’ in metals by ‘ poison,’ 145 Munck on electric response in sensitive plants, 14 Muscle, fatigue in, 38, 39, 40, 42. See also Fatigue prolongation of recovery by ‘ poison’ in, 144 relation between stimulus and response in, 52 staircase effect in, 122 NEGATIVE variation, response by method of, in metal, 87, 183 injured and uninjured contacts corresponding to Cu and Zn in voltaic couple, 8 enhancement of, after continuous stimulation, 127 modified, 128