Comparative Electro-Physiology: A Physico-Physiological Study

skin as alone active, the inner being, to his thinking, in- effective. This particular result may possibly be accounted for by supposing that he used a stimulus intensity which was not sufficiently strong. In my own experiments I obtained clear demonstration of the effectiveness of both surfaces in opposite ways electrically, though the effect obtained from the outer was undoubtedly the more intense of the two. By comparing these two experiments, then, on the grape skin and skin of frog, it will be seen that the inference that the vegetable protoplasm reacts in any way

o = c R A B A ae ee es B —e v f b a a Fic. 179. Electrical Response of Frog’s Skin to Rotary Mechanical (a) A, positive response of outer ; B, negative response of inner; (a’) A and B exhibit abolition of response in skin on boiling ; (4) Cc, current of rest from outer to inner ; R, excitatory response from inner to outer, being summated effect of positive response of outer and negative essentially different from that of the animal is quite unjus- tified. How widely applicable is the method of mechanical excitation by rotary stimulus will be seen in an attempt, successfully carried out, to determine the very difficult ques- tion of the characteristic response of the intact human skin. This will be seen in the following record of the results obtained with the skin of a forefinger. The responsive elec- trical changes represented by the down records, exhibit induced galvanometric positivity of the excited surface (fig. 180).

I shall next describe the results obtained by simultaneous excitation of the inner and outer surfaces of grape-skin. The responses now given, under stiniulation by thermal shocks, are seen in fig. 181, the resultant current being seen to be ‘ up ’—that is to say, from the inner to the outer. On observing the excitatory after-effect of equi-alternating shocks, the results were found to be the same, the responsive current being now once more from the inner to the outer.

I next took a series of records of the direct effect of equi-alternating shocks, the results of which were precisely the same as before. On applying stimulation, by exactly equal and alternating shocks, we, as already explained, of Electrical Responses of Fic. 181. - Photographic -Record of Upper Surface of Intact . Electrical Responses of Grape-skin Human Forefinger to Rotary to Thermal Shocks at Intervals of a Mechanical Stimulation. Minute Down responses here indicate Responsive current from inner to outer, induced galvanometric posi- tivity.

obtain a result which is due solely to the differential excita- tion of the two opposite surfaces. This is not complicated in any way by the factor of polarisation, although the latter could not have failed to be present if the exciting shocks had been one-directioned. Under the conditions of these equi-alternating shocks, then, a certain effect is often seen, in the phasic variation of the base-line, which is ex- tremely characteristic. We have already seen (p. 98) that when a tissue is subjected to repeated or continuous stimula- tion, its condition undergoes a phasic or periodic variation. Thus from a neutral or positive condition, it may pass into one of maximum contraction or galyanometric negativity, to

be subsequently reversed to positive once more. Such phasic changes, moreover, may be repeated. They find visible indications in appropriate shiftings of the base line of the record. Similar effects are also shown in the differential response as seen in the records given in fig. 182; this feature ; is very noticeable. We here ob- tain the resultant response of the two surfaces of grape-skin, from . the inner to the outer. As the result of the series of stimuli applied, the existing current of

FIG. 182. Photographic Re- cord of Electrical Responses of Grape-skin to Stimula- tion by Equi-alternating Electrical Shocks at In- tervals of a Minute Fic. 183. Photographic Record of Series causing shifting of base-line, Direction of responsive current from inner to down and up. outer. Note also variation of base-line. rest undergoes a periodic variation. If this had remained constant, the base-line would have been horizontal. In the present case, the original current of rest was from outside to inside. This, at first, underwent an increase; then a decrease ; to be followed, later, by another increase. Thus, in the course of about ten minutes, it exhibited an alterna- tion of almost one whole cycle.

In the next figure (fig. 183) I give a series of results obtained with frog’s skin in direct response to equi-alternating shocks. Here we find the usual ‘up’ responses, showing that, as before, the direction of the responsive current is from within to without ; and here also we see the existing current of rest undergoing a periodic change. It has now been fully demonstrated that the response of skin is determined by the differential excitabilities of its two surfaces, upper and lower, that of the lower being. the greater. That the resultant responsive current from lower to upper, is in such cases brought about by the greater excit- ability of the lower, has been fully shown, in a previous chapter, by experiments on the pulvinus of J/zmosa. I next made records of a long series of responses given by the last-named specimen, with the object of finding out whether or not these also exhibited a periodic variation of the resting-current similar to those just observed in the anisotropic skins of grape

Fic. 184. Photographic Record of Trans- and of frog. Electrical verse Response of Pulvinus of A/imosa : to Equi-alternating Electrical Shocks connections were made : ; . . The direction of the responsive current is with diametrically opposite from the more excitable lower to the ; less excitable upper. Note the cyclic points on the Upper and variation of the current of rest. organ, and they were subjected to equi-alternating shocks. Owing to the conducting power of the tissues, it was not now the upper and lower skin surfaces merely, but the upper and lower halves of the organ that became excited. And the responsive current was from the lower to the upper, as already demonstrated. In the particular record seen in fig. 184, the general resemblance to the responses of skin is sufficiently obvious. The interesting feature of this record is the periodic changes in the resting-

current, which exhibit a complete cycle in the course of thirteen minutes. Thus, as a consequence of the after-effect of stimulus, a cyclic variation of relative conditions is induced, as between any two anisotropic surfaces, such as those of skin or pulvinus. This cyclic variation of relative conditions is indicated by the concomitant variations induced in the resting-current, shown in the shiftings of the base line. I have been able, further, to demonstrate the interesting fact that such phasic variations are capable of exhibition | even through mechanical response. I have already ex- plained that autonomous pulsations, such as those of the lateral leaflets of Desmodium gyrans, may be regarded as the after-effect of stimuli previously absorbed and held latent by the tissue. In taking the record of a series of such pulsations, I have often found phasic variations to occur, similar to those obtained with long-continued response of skin or pulvinus. If, for example, the lower half of the pulvinus of the lateral leaflet of Desmodium undergoes an increase of turgidity above the average, that half will become more convex, and the base-line of the record will be correspondingly tilted. The converse will take place under the opposite change. Thus the phasic variations shown in the record (fig. 185) clearly indicate that the relative turgidities of the two surfaces of an anisotropic organ may undergo a periodic change. The corresponding electrical expression of this we have seen in the variation of the current of rest. This variation may sometimes be so great as actually to reverse the normal

current of rest. Thus, while under normal standard con- ditions the resting-current in the pulvinus of Mzmosa is from the upper half to the lower, across the organ, this normal direction may sometimes be found to be reversed. It may now be asked, What is it, in the case of the skin, which determines the respective directions of the resting- current and the current of response? We have seen that the current of rest in the frog’s skin, from outer to inner, is generally attributed to the possession of glands by the outer,

a supposition seemingly supported by Rosenthal’s discovery, already referred to, that an apparently similar ‘ingoing’ current was to be observed in the mucous coat of the frog’s stomach. Against this may be urged the conclusion, to which Hermann drew attention, that the skin glands are nor- mally nearly closed to the external surface, and cannot there- fore have any external galvanic relation. There are, moreover, other arguments. First, a similar current is observed in the case of grape-skin, where there is no special glandular layer. Second, the specific response of a glandular surface is

Fig. 185. Continuous Photographic Record of Autonomous Pulsaticn of Desmodium gyrans from 6 P.M. to 6 A.M. The lower record is in continuation of the upper. Note phasic variation. definite, and is by galvanometric negativity, whereas the response of the outer surface of frog or grape skin on excitation is by galvanometric positivity. And, thirdly, we shall see that the current observed in the mucous membrane of stomach is most probably not the natural current of rest, but the excitatory after-current.

It will be remembered, however, that we have always found the natural current to flow ‘in the tissue from the less to the more excitable, and the current of response in the opposite direction. In the skin, owing to physiological and histological modifications, the outer surface is reduced in excitability. The epidermal layers have little protoplasmic contents, and may be transformed in various ways, becoming corneated or cuticularised. The extent of such transforma- tion may be small or great, but the external layer will as a general rule become less excitable than the inner tissue. Hence, under normal conditions, we have a current of rest from without to within.

If the inner layer be only moderately excitable, or if its power of recovery from excitation be great, then the dis- turbance caused by the prepara- tion of the specimen will be slight, or will pass off quickly. It is to be remembered that as the inner surface is the more excitable, the responsive current due to the mechanical stimulus of preparation wil! be from inner to outer; and therefore its after-effect, proving in certain Fic. 186. Photographic Record cases persistent, may give rise of Electrical Responses in Skin to g current apparently the re- of Neck of Tortoise to Stimulus PP y of Equi-alternating Electrical verse of the true normal current. aera at Intervals of One Thus the direction of the current

The direction of the responsive of rest, which we should have current was from inner to inferred theoretically to be from outer. The so-called current ‘ of rest was also in this case, the less to the more excitable, owing to the excitatory after- may occasionally be found re- effect of preparation, from inner ; ; to outer. versed, owing to the excitatory after-effect of preparation. The current of rest, moreover, is liable to autonomous periodic variation, as we have seen.

The most satisfactory method of determining the relative excitabilities of two surfaces, then, lies in subjecting them to simultaneous excitation, and observing the direction of the responsive current. In the skin, unless the tissue was excitable inner to the less excitable outer, even in those cases where the normal direction of the resting-current had been reversed, as an excitatory after-effect of preparation. This fact is well illustrated in the following record, taken with the skin of the neck of tortoise. As an after-effect of preparation, the resting-current so called was here reversed, flowing from inner to outer. But the excitatory responsive current was nevertheless from the more excitable inner to the less excitable outer (fig. 186).

It was stated at the beginning of this chapter that the resultant response from inner to outer merely expresses the general fact that the vs excitability of the inner is greater than that of the outer. And this will a still remain true, even A : when the transformation ae of the external layer is — iN Se 3 not so great as actually e : 4 to reverse its individual g Fic. 187. Isolated Responses of Upper and galvanometric response Lower Surfaces of Skin of Tomato to Rotary

an - (a) A, negative response of feeble intensity in tivity. The experi- rs outer ahce : “ negative response of iach mental results obtained greater intensity in inner surface ; (4) Cc, cur- ‘ : ‘ rent of rest from outer to inner. Resultant with the skin of ripe excitatory response from inner to outer, due tomato form a case in to greater induced galvanometric negativity point. The natural ndeoat | current of rest is here, as usual, from the outer to the inner, and the excitatory responsive current in the opposite direction. But from the analysis of individual responses, on the outer and inner surfaces, obtained by means of the rotary apparatus for mechanical stimulation, it will be seen (fig. 187) that both the surfaces alike give the normal excitatory response of galvanometric negativity. This responsive negativity of the inner, B, is, however, very much greater than that of the outer. The resultant

response, then, representing as this does the difference in ; b dy degree between two negativities, is still from inner to outer, owing to the greater excitatory reaction of the inner. That the direction of the resultant response is actually from the inner to the outer is seen in the series of records given in fig. 188. The stimulus consisted of equi-alter- nating electrical shocks, applied at intervals of one minute. The record shows negative responses, followed appa- rently by the positive after-effect. In order to observe the peculiarities of this response in greater detail, the record was taken on a faster-moving drum (fig. 189). From this figure it will be seen that there was a short latent period of no responsive reaction. Response then rose to a maximum, and again sub- sided. After now reaching the zero position, the record proceeded in the positive direction, and again reverted back to zero. In_ similar records, the occurrence of

Fic. 188, Photographic Record of Series of Responses in Skin of Tomato under Equi-alternating Electrical Shocks applied at In- tervals of One Minute Direction of resultant current from inner to outer followed by feeble opposite after-effect. this latent period, and _ posi- tive after-variation, has been adduced by certain physio- logists as affording visible demonstration of the exist- ence of opposite processes of assimilation and dissimilation. It has been supposed that the various features of the response were the outcome of a sort of tug-of-war between the two opposed forces, the preliminary pause being the expression of a short-lived balance, while the subsequent negative and positive variations were to be regarded as indicating the predominance, now of the one process, and then of the other. That in the present case such an assumption is unwarranted will be evident when we observe the isolated responses of the upper and lower surfaces separately (fig. 187). In each of

these we see the normal response of galvanometric negativity, followed by recovery, without evidence of any antagonistic process, such as might give rise to subsequent positivity. The difference between these two responses lies simply in their time-relations. On simultaneous excitation of the two, the predominant negativity of the inner gives the first half of the negative response. The persistence of the excitatory reaction of the outer, on the other hand, after the subsidence of the effect on the inner, gives rise to the apparently

Fic. 189. A Single Response of Skin of Tomato to Equi-alternating Shock recorded on Faster Moving Drum positive after-effect. Thus, here the supposed tug-of-war between two opposite processes of assimilation and dis- similation is, in reality, between two normal responses having ‘different time-relations. It is from a failure to recognise the fact that the excitatory reaction is not con- fined to one, but takes place on both surfaces, that such erroneous assumptions as that referred to have often been occasioned,

The result which I have described—namely, a greater responsive negativity of the inner than of the outer, giving rise to a resultant responsive current from inner to outer—is that which occurs in the majority of cases with tomato. But, as establishing a continuity between this response and that of grape-skin, | may mention the interesting fact that in a few instances I obtained records in which, while the inner surface on excitation exhibited a strong negativity, the outer, under the same stimulus, exhibited a feeble positivity.

The normal response of skin is sometimes, however, found to be reversed, and no explanations have yet been Fic. 190. Photographic Record of Series of Normal Responses in Skin of Gecko offered to account for this. But I have shown two definite conditions of universal application, which are liable to bring about the reversal of normal response. These are, on the one hand, sub-tonicity, and, on the other, fatigue. Should the condition, in a given case, be the former of these, then the impact of stimulus will of itself, by raising the tonic condition, restore the normal response. ‘Thus in a case of abnormal positive response due to sub-tonicity, an inter- vening period of tetanisation will tend to convert the ab- normal response to normal. An abnormal positive will thus pass into diphasic, and thence into the normal negative.

For the following experiments, I took the skin of gecko, which can be detached from the body with very little injury. This animal offers remarkable facilities for many electro- physiological experiments. Its isolated tissues can be main- tained in a living condition for a very long time. Its sciatic nerve affords us a specimen about 15 cm.in length. Thus for electro-physiological investigation, it provides much greater advantages than the frog. Fic. 191. Photographic Record of Abnormal Diphasic Responses in Skin of Gecko, converted to Normal, after Tetanisation

Taking a specimen of gecko skin, which was in a favour- able tonic condition, I obtained the series of normal responses to equi-alternating shocks, which is given in fig. 190. The responsive current here flowed from the inner to the outer surface. I next took another specimen, which was in a less favourable tonic condition, and obtained records of its responses, here seen (fig. 191) to be diphasic. An intervening period of tetanisation is seen, however, to restore the normal response.

Epidermal, epithelial, and secreting membranes in plant tissues—Natural resting-current from epidermal to epithelial or secretory surfaces—Current of response from epithelial or secretory to epidermal surfaces—Response of Dillenia—Response of water-melon—Response of foot of snail—The so- called current of rest from glandular surface really due to injury— Misinterpretation arising from response by so-called ‘positive variation’ —Natural current in intact foot of snail, and its variation on section — Response of intact human armpit—Response of intact human lip—Lingual response in man—Reversal of normal response under sub-minimal or super- maximal stimulation—Differential excitations of two surfaces under different intensities of stimulus, with consequent changes in direction of responsive currents, diagrammatically represented in characteristic curves—Records ex- hibiting responsive reversals,

HAVING now seen how the responsive peculiarities of the epidermis may be elucidated by the responses of similar tissues in the plant, we shall next take up an inquiry as to the parallelism between the responses of epithelium and glands in animal and in vegetable tissues. And here, as in the last case, we shall find the obscurities of the one made clear by the study of the other. If we take the hollow peduncle of a Uviclis lily, and, cutting this into longitudinal halves, take a portion from the upper end of one, we shall observe noticeable differences between the investing membranes of the outer and inner surfaces. On the outer, as we have seen elsewhere, the cells are dry, thick-walled, and cuticularised. This surface then is naturally distinguished as epidermal. The internal mem- brane of the hollow tube, however, is very thin, and its cells very little differentiated (fig. 192). The internal membrane may thus be distinguished as epithelial.

from the top to the bottom, at the point where the peduncle rises from the bulb, we shall find that the epithelial layer of the upper end passes imperceptibly into a markedly secret- ng (glandular?) layer at the lower. By this, secretion is constantly taking place, filling up the hollow tube with fluid. In one instance which I measured, the amount of this secretion was as much as IO grammes in the course of the day. These secreting cells in this, which may be called the glanduloid layer, are very thin- walled and excessively turgid, and, from an evolutionary point of view, these gradual transitions from epidermal to epithelial, and from epithelial to secretory layers, observed under conditions of such great simplicity, are extremely interesting. |