Comparative Electro-Physiology: A Physico-Physiological Study

Another observation which lends independent support to the view that the retina exhibits the true excitatory re- action, is found in the fact—noted by Van Genderen-Stort and Engelmann—that the cones exhibit a motile effect by retraction under light. It has. been supposed from this that the optic nerve contains not merely sensory, but also retino- motor fibres. But I shall show that it.is not that the endings only of the optic nerve, but also that nerve itself, which exhibit true excitatory contraction under stimulation (cf. figs. 324,404). All nerves, in fact, will be shown in a later

chapter to exhibit normal excitatory contraction, and it will be by the study of these motile responses in nerves, and their variations under different condition, that we shall be able unerringly to relate the abnormal responses sometimes seen in the retina to those changes of condition to which they are due. : With regard to these abnormal responses, I have already shown that various tissues exhibit them under the two different conditions of sub-tonicity and fatigue. With regard to nervous tissue in particular, however, I may refer here, by anticipation, to results which will be given in detail in Chapter XXXV. concerning the mechanical response of nerve and its variations. In normal conditions of ex- citability the nerve gives response by contraction, and this is its true excitatory response, concomitant to the electrical response of galvanometric negativity. This excitatory re- sponse, then, whether by contraction or by negativity, will here, as in preceding chapters, be designated ‘negative.’ We have seen that the maintenance of the normal condition of a highly excitable tissue depends on its supply of energy. Hence, when such a tissue is isolated from the organism of which it forms a part, it is liable to fall below its normal tonic level. This depressed condition, however, does not connote any permanent chemical depreciation, but only a temporary depression of its fund of energy. Under such an induced lowering of the tonic condition, the response is reversed to positive. But under continuous stimulation, excitability is again enhanced, and the abnormal positive is converted to normal negative.

Thus we obtain, in a somewhat sub-tonic tissue, the following results : (1) Under a short-lived or instantaneous stimulus, whose effective value falls below the true excitatory level, response is positive (expansion), (2) Under the continuous action of stimulus the effective value is at first below, but after a time rises above, excitatory efficiency. In consequence of this we obtain a first phase of response which is positive, followed by a second which is negative (expansion followed by contraction).

The second condition to induce a reversal of the normal response occurs in consequence of the fatigue due to previous over-stimulation. (3) A reversed positive response (expansion) in conse- quence of fatigue. | | The various anomalies which occur in the response of the eye will all be found resolvable into one or other of these cases. I shall first describe certain experiments which will demonstrate the reversal that is brought about by induced sub-tonicity. We have seen that the normal response of an eyeball, with two contacts at nerve and cornea, consists of a current from the nerve to the cornea. I have already given records of such normal responses, obtained by sub- jecting the preparation to equi-alternating electric shocks (fig. 249). In certain cases, however, in which the isolated eyeball of the frog had fallen into a sub-tonic condition, the response was found to be reversed, the nerve, under excita- tion, becoming relatively positive instead of normally nega- tive. It has already been said that such a reversal is due to great depression in the condition of the nerve. In dealing with these cases, therefore, it occurred to me that it ought to be possible to restore the normal response by the application of some exciting reagent—say, dilute Na,CO,—to the nerve in its depressed condition. As the result of this application I found the reversed response to be restored to the normal.

I obtained results precisely similar to these with isolated retina of the frog. The normal responses—a current from retina to nerve (fig. 238)—under equi-alternating shocks, were here found, in a depressed specimen, to be reversed. But the application of Na,CO, solution on the retinal surface brought the responses back to the normal. These abnormal responses, then, rectifiable to normal, are those due to sub- tonicity. And under fatigue finally, induced by long- continued, or over stimulation, I find the normal response of the eye to be reversed.

This is the place in which to refer to various anomalous effects observed. by previous investigators, and to offer satis- factory explanations of them from the results which I have already demonstrated. I shall therefore give a brief sum- mary of these from the admirable account of Biedermann. (a) ‘In light, frogs—z.e, such as have been exposed for hours to the full effect of daylight—the positive fore-swing light is entirely wanting, or appears as a trace only.’ ‘ Since this posztzve varzation of the existing current really means, as I have already shown, the ‘true excitatory negative, and the negative variation conversely, the occurrence of positive response, this observation means that a frog’s eye, previously exposed for a long time to light, gives positive response.

This, then, is a simple instance of the reversal of response from negative to positive under fatigue, which I have already dealt with above as case (3). (0) ‘The fact that the three phases of the retinal action current, due to transitory illumination, appear in sensitive preparations, even when, as with the electric spark, the impact of light is momentary, shows that the medium nega- tive (fosttzve) phase must not be regarded merely as the consequence of permanent illumination, since it is just this phase which alone appears in less excitable preparations with instantaneous light stimuli.’

‘With regard to this it may be said that the medium positive phase here referred to, as given by. the excitable retina under continuous stimulation, is not the same as that positive response which ‘alone appears in less excitable pre- parations with instantaneous light stimuli. The former is due to fatigue-reversal, while the latter is an instance of the abnormal positive response of a sub-tonic tissue to feeble stimulus. This will be understood from the following con- siderations. In the retina, under continuous stimulation, the first phase of response is the true excitatory negative. This gives place to a second, or positive, due to _fatigue-decline.

RESPONSE OF RETINA TO STIMULUS OF LIGHT 425 - i g And this is again succeeded bya transitory negative effect on the sudden cessation of: light. These constitute the three phases of retinal action-current just referred to, under con- tinuous stimulus. This sequence is wrongly represented in symbols as (+ — +), since the actual changes concerned are Now in highly excitable. tissues, under instantaneous stimulation, we observe a sequence of response apparently similar, the first being normal_ negative, the second positive, owing- partly to recovery and: partly to the positive after- effect ; and the last phase representing a return from this positive. These three phases, therefore, though apparently similar, are not really the same as those just referred to under continuous stimulation, where the ‘medium negative (postteve) phase’ was the result of fatigue-reversal. The so- called negative (fosztive) effect which alone appears on the instantaneous stimulation of relatively inexcitable tissues is, again, the positive response of a sub-tonic tissue to a stimulus deficient in true excitatory value, already described (p. 422) as case (1). Similar positive responses of vegetable tissues in a sub-tonic condition under the action of light were seen in fig. 240.

We come next to the question whether or not we may discover multiple responses in the retina analogous to those which have already been demonstrated in the case of vege- table tissues. The occurrence of such an effect has not hitherto been suspected. We have seen that, under the continued action of light, vegetable tissues exhibit multiple responses ; and since we have found a general close analogy to exist between the responses to light of the retina and of these, we should expect that similar multiple responses would be found in the eye also. The reason why these were not hitherto detected lay in the inevitable depression of excit- ability in the isolated retina or eyeball. In the retina of certain fishes, where the excitability does not appear to decline so rapidly, I have often obtained records of multiple responses For example, in the retina of Wallago attu fish the stimulus of °

light applied for three seconds gave as its after-effect multiple responses which lasted for ten minutes, the average period of each oscillation being twenty seconds. I have also obtained such multiple responses from the eye of vigorous bull-frogs, a photographic record of one of these results being given in fig. 252. We shall also see at the end of this chapter that it is quite easy to detect the occurrence of these multiple responses in the intact human eye under stimulus of light.

As we have seen that multiple response is the expression of energy previously absorbed, and held latent in the tissue for a time, and since, as has been stated, the retina itself exhibits multiple response, it is easy to see that this organ, on the cessation of light, will show after-effects. In my experiments on the effects of light on vegetable tissues I found, as has already been said, three different types of direct- and after- effect. The first of these related to those highly excitable tissues which under continuous stimulation gave

normal responses (—~+—+). In Fic. 252.—Photographic Re- gych cases, if the stimulus was cord of Multiple Response ; Continuous Action of Light 14.imum positivity the immediate after-effect was an increase of positivity. The formula was thus (—+ ++). In the third type, with sub-tonic tissues, the sequence, under continuous stimulation, was (+ —-+-—), and on the stoppage of stimulation at maximum negativity this negativity became suddenly augmented. The formula here was thus (+ —---). In the second or inter- mediate type, again, the formula of the direct and after- effects was either (— + -+-) or (— -:-).

I shall now discuss in some detail the various types of after-effects met with in the retina, corresponding, as I shall be able to show, with those met with in vegetables tissues. After-effects like those of Type I., had not hitherto been noticed in the retina for the reason that their demonstration can only be obtained in a tissue of normal high excitability, and not in one which has undergone depression in con- sequence of isolation. I was fortunate enough, however, to meet with a species of fish, Ophiocephalus marulius, whose vitality is so exceptional that it lives for days when taken out of water. When the fish is pithed, its heart continues beating vigorously for many hours. I made a preparation of the eye of this fish and carried out experiments on it under the action of light. |

In fig. 254 is given a record obtained with this specimen during the application of light and on its cessation. It will FIGs. 253, 254, 255. Parallel Records of Responses given by Plant and Retina, during and after Illumination, illustrative of Type I (— + +). In all these cases up-curve represents induced galvano- metric negativity ; down-curves, positivity. White background in this and following records represent light, and shaded, darkness. Fig. 253. Response of petiole of Aryophylium. Light was cut off on attainment of maximum positivity in the second of the multiple responses.

Fig. 254. Similar effect in response of retina of Ophzocephalus fish. Fig. 255. The same with another specimen. Light. was here cut off after the first oscillation. be seen that during the application of light the sequence was (—+—+). It will be noticed that, after completing two oscillations, and after the response-curve was even slightly reversed at its maximum positive phase the light was with- drawn. The immediate effect was a sudden increase of posi- tivity, followed by a series of after-effect oscillations. In the next figure (255), obtained with a different specimen of the same fish, light was withdrawn at the exact moment of maximum positivity, and the result is seen to be similar to the last—namely, an immediate enhancement of positivity,

followed by an after-oscillation. The essential similarities between these and corresponding records obtained on a fast- moving drum, of the response of the petiole of Sik wi det under light (fig. 253), are sufficiently obvious. When the excitability of the tissue is not so high, we may obtain after-effects of Type II., in which the formula is —-+-+--) or (—---). This was exemplified in vegetable tissues (cf. fig. 245 @).. In Ophziocephalus, 1 was able to obtain — this result also, when the specimen was slightly fatigued (fig. 256). With the eye of the frog, Kiihne and Steiner obtained .the record given in fig. 257, which is seen to be parallel to that given in fig. 256, its true significance being shown in the formula (— +---).

Fics. 256, 257. Parallel Records given by Plant and Eye; during and after Illumination, as illustrative of Intermediate Type II. (— + ---) Fig. 256. _ Response of retina of Ophzocephalus when slightly fatigued. Fig. 257. Response of frog’s eye (Kiithne and Steiner). A sub-case of Type II. is represented again, by (—-+--), where on the sudden cessation of light, there is a transient increase of excitatory response, This, as we saw, was due to the abrupt withdrawal of the antagonistic influence of a reversing force. I may state here that I have been able to demonstrate an exactly parallel effect with nerve of frog, where the excitatory negative effect during stimulus under- goes a brief and sudden augmentation on its cessation (p. 5 36). This sudden augmentation on the stoppage of stimulus has been taken as a proof of the existence of antagonistic pro- cesses of assimilation and dissimilation, rather than as due to molecular-derangement by external stimulus and its after-effect.

That, for its explanation, however, it is not necessary to postulate the two processes of assimilation and dissimilation is clearly seen from the fact that I have obtained: an exactly similar effect in inorganic. substances, suchas silver bromide, a record of whose response is seen in fig. 258. | We next turn to what has been- designated as Type III.,-in which the sequence of responses, owing to the depressed. condition of the -tissue, is. reversed, the formula here. being (+—+-—), while the direct and after- effects are represented by (+ —--:). asiecih —— This result was already obtained in the f sub-tonic tissue of the petiole of cauliflower, a record of this being given in fig. 259. Iwas able to detect similar effects in a, retina of Ophzocephalus, in

which response had become reversed under the sub-tonicity- due to long isolation (fig. 260). These results will explain the somewhat anomalous re- sponse which Kiihne and _ Steiner obtained with the isolated retinz of certain fishes (fig. 261). . rieet . Fic. 258. After-effect of multiple excitations during and after OF bight “on “Silver the exposure to light—in prepared speci- Bromide mens, where results must. be modified The thick line represents

response during light to.an unknown extent by the effects ~ (half a minute’s ex- of the responses of the intact. eye, ing darkness. Note = t t s ] as © the problem by means of visual sensation itself. Multiple excitation, as the after-effect of strong luminous stimulation, is here somewhat easy of demonstration. But its exhibition during the action of light presents unusual experimental difficulties. It is a well-known fact that if, after looking at a bright object for some time, we close the eyes, we see the image repeated many times, The occurrence of these after-images is somewhat different under a transitory flash of illumination, and with more persistent exposure to light. As the visual sensation is to be regarded as the excitatory effect, there will—owing to the fact that this excitation must reach its maximum some time after the cessation of an instantaneous exposure—be a persistence of the excitatory after-effect, with

FIGS. 259, 260, 261. Parallel Records of Responses given by Plant and Retin, during and after Illumination, illustrative of Type III. Fig. 259. Response of petiole of cauliflower. Light was here cut off on attainment of maximum negativity. Fig. 260. Response of retina of Ophiocephalus fish when de- pressed. Fig. 261. Response of isolated retina of fish as observed by Kiihne and Steiner. N.B.—The true excitatory negative response was by these observers described as the ‘ positive variation.’

its corresponding visual sensation. This is not so, however, when the eye is closed after looking at a bright object for a considerable time. In this case there is a positive rebound— opposite to the true excitatory negative effect—with con- comitant sensation of darkness. The next rebound is negative, giving rise to a repetition of the original sensation ; and these alternating phases may be repeated manytimes. With the eyes closed, the negative or luminous phases are the more prominent.

The same phenomenon may be observed in a somewhat different manner when, after: staring at a bright object, we look towards a well-lighted wall. The dark phases will now become the more noticeable. If, however, the wall be dimly lighted, both the dark and bright phases will be noticed alter- nately. It has been suggested that these phenomena might be due to some obscure form of fatigue, but the regular alter- nations observed clearly demonstrate them to be a case of oscillatory changes and multiple response. _

The demonstration of multiple responses in the human retina, as an after-effect, is thus seen to be easy. But their occurrence during continuous exposure of the eye to light is more difficult to prove, This arises from the fact that the waxing and waning of the effect are so gradual: as not to be noticeable, unless against some definite standard of com- parison. I shall now prove that, during the continuance of constant light, pulsatory visual effects are produced. These pulsations go unnoticed in visual sensation, not only for want of a standard of comparison, but also because of the remark- able fact which I have discovered, that while the impressions of each individual retina undergo variation, the sum total of the two remains always approximately constant. I have been able to provide the necessary comparison-standards by having two distinguishable images produced in the two eyes, the fluctuation of the visual excitation in one eye being thus capable of detection, by comparison with that in the other. It would have been impossible to detect this fluctuation, had the excitatory variation taken place in the two eyes simulta- neously, ze. if the maximum excitation in the one had occurred at the same moment as the maximum excitation in the other. But I have found that, as regards excitation there is a relative difference of phase, of half a_ period, between the two retinz, so that the maximum effect at a given moment in one eye corresponds to the minimum in the other. This constitutes the phenomenon which I have designated the Binocular Alternation of Vision. It is owing to this fact that the periodic excitations in each retina are unmistakably brought out by the following experiment, which consists in looking through a stereoscope that holds,

instead of photographs, incised plates with two inclined cuts, the right eye seeing the slit ‘inclined to the right, while the left sees that inclined to the left. When. the. observer looks through the stereoscope turned to the bright sky, his two eyes are acted on by strong stimulus of light through these divergently-inclined slits... The resultant sensation: forms the image of an inclined cross (fig. 262).. On now closing the eyes the multiple after-effect is observed as recurring images. © The relative difference of half a period, already referred to, is here perceived in a very interesting manner, for the after- image is not now the complete inclined cross. Instead of this, each of the two arms is seen in regular sequence alternately.

. . FIG. 263. Composite Indecipherable Inclined Slits ‘Word, of which Components are Seen Clearly on Shutting the Eyes Fic. 262. for Stereoscope and Com- posite Image formed in the Two Eyes That is to say, when the one is at its brightest the other has completely disappeared, and vice versa. The impression in each eye thus undergoes a periodic fluctuation. Some yery curious effects can thus be exhibited, when, instead of the two inclined bars, the incisions to be looked through com- pose a word. For instance, two letters forming half of the word may be seen by one eye, and the other two by the other. The result of this is that, as long as the eyes are open, we obtain an indecipherable image, due to super- position, like the following (fig. 263). »

But when the eyes are closed, then, owing to dznocular alternation of the multiple after-effect, the blur disappears, and the word is spelt out in repeated succession in the field of dark vision, as RO ME RO ME, and so on. In this curious instance one sees better with eyes closed than open! The next problem that presents itself is that of demon- strating the pulsatory nature of the visual sensation in each eye, under the constant action of light. The stereoscope, with its two inclined slits, is now taken and turned towards the bright sky, and when the design is looked at steadily it will be found that, owing to pulsatory excitation in each eye, and the binocular alternation of vision, when one bar of the cross begins to be dim the other becomes bright, and vce versa. The success of this experiment is determined by the fact that the image in each eye forms a comparison-standard for the other. And as the changes in the two eyes proceed in opposite directions, the visual fluctuations during the con- tinuous action of light, are brought out with the greatest distinctness. It may be stated here that the period of this visual oscillation has an average value of about four seconds. It is, generally speaking, shorter with young people, and longer with old. Even in the same individual, again, it is modified, according as the previous condition has been one of rest or activity. I give here a set of readings given by an observer :

In this connection it may be interesting to mention that the period of a single oscillation in a multiple response, measured in frog’s retina, under experimental conditions, was ten secands. In the case of the retina of Wadllago, the aver- age period was twenty seconds. Theory of Hydrostatic Pressure. and Theory of Statoliths—Question regarding active factor of curvature in geotropic response, whether contraction or ex- pansion— Crucial experiment by local application of cold—Reasons for delay in initiation of true geotropic response—Geo-electric response of shoot— Due to active contraction of upper side, with concomitant galvanometric negativity—Geo-electric response of an organ physically restrained.

IN the case of the action of external stimulus on plant organs it is possible, given the direction of incident uni- lateral stimulus, to predict the nature of the responsive movement. I have shown elsewhere, in my work on Plant Response, that all the actual movements of a plant organ can be deduced from the simple law that it is the more excited side that becomes concave. But though this law is sufficient guide in dealing with the action of a known ex- ternal stimulus, yet the problem becomes much more obscure when we have to account for any movement which occurs in response to a stimulus whose seat is internal. An example of this class is afforded in the responsive curvature associated