Comparative Electro-Physiology: A Physico-Physiological Study

Stimulation, in which friction of the terminal area of a pumice- stone electrode constituted the stimulus, whose intensity was determined by the number of rotations (p. 291). The third non-electrical mode of stimulation employed was that of thermal shocks. The area to be stimulated was, in this case, enclosed within a thermal loop of platinum or german-silver wire, the requisite thermal variation being produced by the passage of a heating electrical current round the loop. The intensity of the stimulus could in this case be varied by increasing the intensity or duration of the heating current (p. 38). And finally I have shown that the drawbacks inci- dental to the electrical mode of stimulation might be over- come by the use of equi-alternating shocks, the indefinite polarisation factor being thus neutralised (p. 251).

As the intensity of stimulus is gradually increased, it is found that the amplitude of response reaches a limit. Beyond this, increase of stimulus evokes no increase of response. On the application of a very strong stimulus, then, there is an amount of energy which is unable to find expression in the single response given by the tissue. Under such circumstances, the excess of energy is held latent, and often finds responsive expression later in a series of multiple responses. This phenomenon of multiple response to a single strong stimulus I find to be of very extensive occurrence. As examples of the different kinds of tissues in which this may be observed, may be mentioned the stems and petioles of various plants (fig. 138), the digesting leaves of Drosera (fig. 209), the pitcher of Mepenthe

I have also shown that thers is no strict line of demarcation between the phenomena of such multiple response and autonomous response so-called. Bzophytum, for example, which, usually speaking, exhibits a single response to a single moderate stimulus, and multiple response to a strong stimulus, will, under exceptionally favourable tonic conditions—that is to say, when it has absorbed from its surroundings an excess of energy—exhibit responses which are apparently autonomous. A typically autonomous plant like Desmodium gyranus, again, when deprived by unfavour- able circumstances of that excess of energy which it requires, will be reduced to the condition of a multiply responding plant. merely. It then responds by a single response to moderate, and by multiple responses to strong, stimulus. When the energy imparted by strong stimulus is exhausted, these multiple responses come to a stop, to be once more renewed, on a fresh accession of strong stimulus. Or a lateral leaflet of Desmodium, originally quiescent, may be put into, and maintained in, a state of pulsation by the action of sunlight.

It is from the stored-up energy derived from _ its surroundings that the tonic condition of the plant is so raised as to maintain its so-called autonomous activity. From this it will be seen that, strictly speaking, there is no such thing as automatism. Movement can only be produced by the immediate action of stimulus, or by energy previously absorbed. In recording the autonomous pulsation of the lateral leaflets of Desmodium gyrans, it is found that while the down-movement brought about by the contractile action of the lower half of the pulvinule is very rapid, the up-move- ment due to recovery, and to contraction of the upper half of the organ, is relatively slow. The two alternating excitatory impulses, in the lower and upper halves respec- tively, are in the ratio approximately of 1°5 to 1, This

explains the peculiar electrical responses of Desmodium gyrans, which are concomitant with the autonomous mechanical pulsations of the leaflet. I find that, corre- sponding with one complete mechanical pulsation, there are two electrical pulses. Of these the principal electrical wave coincides with the down-movement of the leaflet, and the smaller with the up. The electro-motive intensity of the principal wave is nearly 1°5 times that of the subsidiary. |

In a particular experiment, for example, while the value of the former was ‘0024 volt, that of the latter was, ‘oo16 volt. These electro-motive variations are expressions of funda- mental excitatory effects, and not dependent on the mechanical movement of the leaflets. For when the responding leaflet is physically restrained, the electro-motive responses exhibit even greater intensity than before. This will be seen in the simultaneous records of mechanical and electrical pulsations given in fig. 145.

An important subject of inquiry lay in the accurate determination of the death-point. This investigation afforded striking demonstration of the fact that it is a single excitatory reaction which is expressed in different ways under different modes of record. It has been shown that when the experimental tissue is subjected to a gradual rise of temperature, there is a definite point at which an excitatory spasm occurs, marking the initiation of death. If a continuous record be taken of the concomitant variation of length, increasing expansion is found to be converted at this point into a sudden contraction. In an anisotropic organ like the pulvinus of Mzmosa the erectile movement of the leaf is abruptly transformed into one of fall. A curled tendril exhibits at this point a sudden uncurling. Taking, again, the electro-motive method of record for the detection of the death-point, the increasing positivity of .the specimen is spasmodically reversed to negativity. Finally, on employing for the record the method of resistivity variation the increasing is seen to become suddenly changed into a diminishing resistance. It is found,. how-

ever, employing numerous specimens, that these mechanical and electrical spasms take place, under normal conditions, at the same point. In the case of phanerogamous plants, this is found to be at or very near 60° C. (figs, 328, 329, 330). That these mechanical and electrical spasms, further, constitute a true case of excitatory response, is proved by the fact that induced physiological depression also induces depression of the death-point. Fatigue may thus lower the death-point by as much as 19° C. :

The response of contraction, initiated at the death-point, is later converted into fost-mortem relaxation, and galvano- metrically the negativity initiated at the same moment becomes subsequently a post-mortem positivity. With regard to the so-called Current of Injury it. was shown that this arises as the after-effect of strong stimulus. It should be remembered that a cut, or the application of a heated wire, constituting mechanical and thermal sections respectively, will act as a strong stimulus, and, further, that the after-effect of excitatory galvanometric. negativity is persistent when the stimulus is strong.. The excitatory effect, moreover, is transmitted from the point of application to greater or less distances, according to the strength of stimulus and the conductivity of the tissue. As this trans- mitted effect undergoes diminution with distance, it is obvious that the most intense negativity will be induced at the point of section, undergoing a gradual diminution as we move further away from it. If, taking a given length of isotropic tissue, we make two opposite terminal sections, we shall clearly have a symmetrical. distribution of electrical potential as regards the middle or equatorial zone, the two ends being most negative, while the equator is relatively most positive (fig. 110). . Two points symmetrically situated as regards this equator would thus be equi-potential, while a-symmetrical points would show appropriate differences of potential, a zone near the equator being relatively positive to one which is further away from it, or nearer to the terminal section. These considerations, supported as they

are by experimental results, account satisfactorily for the particular electrical distribution in a muscle-cylinder. It is often supposed that dead tissue is negative to living. But I have shown that this is not the case, the dead being actually positive to the living. It has already been mentioned, in connection with experiments described, on the mechanical and electrical spasms of death, that at traction and negativity, while the post-mortem effect is one of relaxation and positivity. This explains the peculiar electrical distribution which I have observed, in the explora- tion of tissues, of which some parts were dead, others dying, and still others, again, fully alive. It was there shown (figs. 113, 115) that the greatest negativity occurred on the death-frontier. Proceeding in either direction from this point, whether towards the living or towards the dead, it is found that these points are increasingly positive, or decreasingly negative. But the maximum positivity of the dead portion is greater than that of the living. From this it is clear that the dead is positive to the living.

From these facts, that the dying is negativé, and the dead relatively positive to the living, it is clear that the so-called current of injury is liable to reversal. In the case of the former, the current of injury will be from the dying to the living; in the latter, from the living to the dead. This demonstration of the occurrence of a hitherto unsuspected reversal, demonstrates to us the possibility of many complications, and wrong theoretical inferences, For response by the negative variation of the current of injury is usually taken as the concomitant of the chemical process of dissimilation, while the positive variation is held to be associated with assimilation. Now, by the reversal of the so-called current of injury, one identical excitatory reaction may be made to appear, now as a negative, and again as a positive variation. This is. sufficient to indicate the unreliability of the so-called Method of Negative Variation,

an unreliability of which we shall further meet with many glaring instances. Pe ; An assumption more or less current is, that in order to obtain response, there must be an antecedent current, by whose negative variation it can be detected. Hence the supposed necessity of a current of injury prior to response. The real reason, however, for thus injuring one of the contacts is so to depress its excitability that, on diffuse stimulation, the excitatory response of the uninjured may remain unbalanced, and therefore unannulled. That it is this depression of excitability, and not the current of injury as such, which is the essential condition for obtaining resultant response, is seen from the fact that excitatory response may still be obtained, even when the so-called current of injury is zero, or reversed positive (fig. 116).

Anisotropic organs—Laws of response in anisotropic organs—Natural current of rest and current of response— Reversal of natural current of rest-—Unreliability of positive and negative variations of current of rest-—Determination of the differential excitability of a tissue—Resultant response of skin due to induced stronger negativity of inner surface and feebler negativity (tomato skin) or positivity (skins of grape and frog) of outer—Response of intact human skin — Response of intact human lip—High excitability of secretory and glandular surfaces—Response of glandular foot of snail—Response of intact human tongue—Response of digestive organs—Phasic alternations of secretion and absorption—Multiple response of digestive organs—Phasic changes induced by previous activity—Response of digestive organs of Mepenthe and Drosera —Electro-motive peculiarities of skin and mucous coat of stomach not similar —Normal response by galvanometric negativity in mucous coat of stomach of frog, gecko, and tortoise— The root as a digestive organ—Excitatory secretion and galvanometric negativity of young roots—Phasic alternations of secretion and absorption — Cognate subject of ascent of sap—.Sap-wood not really dead— Proofs of physiological character of suctional response—Water-movement a mode of excitatory response—Response of electrical organs—Two types, Torpedo and Malepterurus—Vegetal analogues to electric plates of two types, Torpedo and Malepterurus, in Pterospermum and pitcher of MWepenthe—- Multiple character of response of electric organs—Response of electrical organs constitutes an extreme case of differential excitability of anisotropic structures--Similar effects with inorganic structures—Excitatory effect of light on plant tissues—Phasic alternations—Initiation of multiple and autono- mous response by light—Three types of direct and after-effects—Response of retina like, and

not different from, that of other tissues—Error introduced by method of negative variation—Multiple responses in retina and their visual correspondences —Binocular Alternation of Vision—Three types of direct and after-effects in retina under light—Geo-electric response. I SHALL next pass in review another class of phenomena, the want of a clear understanding of which is at the root of many supposed anomalies in the response of animal tissues. I allude to the natural anisotropy, with consequent differential excitability, of various organs,

_ As an example of a differentially excitable organ we may take the pulvinus of J/tmosa, in which the lower half is more excitable than the upper. In this case, strictly localised stimulation of either the upper or the lower evokes con- traction and galvanometric negativity of that particular half, the effect in the lower half being the greater. But if the stimulus be diffused, whether internally or externally, the response will be differential, by the greater contraction or galvanometric negativity of the more excitable. From this we arrive at the general law of the electrical response of anisotropic organs.

1. On simultaneous excitation of two points A and B, the responsive current flows in the tissue from the more to the less 2. Conversely, if under simultaneous excitation, the responsive current be from B to A, B is the more excitable of these two points. The second of these two laws enables us to determine the relative excitabilities of any two points. As a simple example of the anisotropy induced in a tissue by the unequal action of the natural stimuli of the enviroment, we may take a tubular organ, such as the hollow peduncle of Uriclis lily. Here the exposed outer surface, constantly subjected to external stimuli such as light, becomes as it were fatigued, and reduced in excitability. Other histological modifications follow on this, the external cells becoming thus cuticularised and protoplasmically defective. Owing to the depression of excitability on this epidermal surface, the intensity of its normal excitatory change by galvanometric negativity is decreased, a change which, in the case of certain skins, culminates in responsive positivity. The inner surface of the hollow peduncle, which may be regarded as epithelial, being, on the other hand, protected, remains normally excitable and is thus more so than the outer surface. The outer sur- face, however, probably by reason of the action of the external stimuli to which it is constantly exposed, is naturally negative, relatively to the protected and more excitable inner

surface. And it will generally be found true that while this natural cnrrent of rest is from the less excitable A to the more excitable B, the current of response, on the other hand, which occurs on excitation, is from the potentially more excitable, and therefore now more excited B, to the less excitable and therefore less excited A. Such is the course of events in the normal or primary condition. But under the excitation due to preparation, or - accidental disturbance, the more excitable surface becomes the more excited, and relatively to the other, galvanometri- cally negative. In consequence of this, the natural current is reversed, and we have a resting-current due to the after- effect of injury or accidental excitation, flowing from the more to the less excitable. Thus, while the natural current, in the primary condition, was from the less excitable A to the more excitable A, that is to say, A > B, this reversed current of rest, due to accidental excitation or injury, is from B->A. Even now, however, & may be more excitable than- A, hence fresh stimulation will induce a responsive current from B to A. In the primary condition, such a responsive current would have appeared as a negative varia- tion of the natural current AZ. But when the primary condition has been so modified that the natural current is reversed, and has become B-A, the normal responsive current B-+A will appear as a positive variation. Still another variation is possible, when the normal response itself undergoes reversal owing to fatigue, under which condition this abnormal response, relatively to the reversed current of rest, appears as if it were the normal negative variation (fig. 119). It has, however, been shown that if we discard this unreliable test of response, by the variation induced in an antecedent current of rest—the so-called negative varia- tion—it will be found that the responsive current always flows from the more to the less excited.

In order to determine which of two points in an anisotropic tissue is the more excitable, it is necessary, as now under- stood, to determine the direction of resultant response, under stimulation which is equal and simultaneous. In order to do this, we may employ such a non-electrical form of stimu- lation as the mechanical or the thermal. For this it is possible to employ (1) the Vibratory Stimulator; (2) the Rotary Mechanical Stimulator ; or (3) stimulation by thermal shocks. When results are obtained according to these methods, there can be no uncertainty as to those compli- cations of effects which might conceivably arise when the electrical form of stimulus is employed. The last-named may, however, be used without misgiving, when stimu- lation is effected by equi-alternating shocks, The ordinary Ruhmkorff’s make- and break-shocks are not suitable for this purpose, inasmuch as the effective intensity is unequa] for make and break, besides which the polarisation-effect may not be exactly neutralised. The equi-alternating shocks, from which these defects have been eliminated, are obtained by means of (1) a rotary reverser in the primary coil (fig. 170), or (2) a motor-dynamo (fig. 172). The responses again, under these electrical forms of stimulation, may be photo- graphically recorded as either the direct or the after-effect of stimulus. It was shown, by the employment of all these various methods of stimulus, mechanical, thermal, and elec- trical, that the responsive current to be obtained with an anisotropic organ was definite in direction, being always, under normal conditions, from the more excitable B to the less excitable A. I shall now proceed to recapitulate briefly the results obtained by these methods in various cases of anisotropic tissues, such as skin, epithelium, glands, animal, and vegetal digestive organs, and electric organs generally. Taking first the skin of tomato it has been shown that the separate responses of the outer and inner surfaces are unequal.

The outer, owing to cellular modification under the stimuli of the environment, gives only a feeble negative response, whereas the internal surface gives a much stronger normal response by galvanometric negativity. On simul- taneous excitation of both inner and outer surfaces, the responsive current is found to flow from the inner to the outer. Here the resultant’ current’ is ‘brought about~ by the difference between the stronger responsive negativity of the inner, and the feebler responsive negativity of the outer

specimens of tomato skin, however, the modification of the outer surface is so great that its individual response is reversed to positive, that of the inner being the normal strong negative. The resultant response, then, is still from inner to outer, but equals the summated effect of the two ne From this we pass to the response of grape-skin, which resembles the latter of these two cases. The response of the skin of frog is also of this type, and it may be said of skins in general that their response is from the. more excitable inner surface to the less excitable outer. This conclusion has been verified by experiments on various skins, both vegetable and animal. Among the latter of these may be mentioned the skin of the neck-of tortoise, and that from various parts of the body of gecko.-

When the skin is isolated with very great care, so as to reduce to a minimum the excitatory effect of preparation, it is found-that the natural current of rest is from the less excitable outer to the more excitable inner surface; the excitatory current being in,the opposite direction. Owing to the excitatory effect of preparation; the current of rest of the skin of tortoise-was found reversed. The responsive current, however, was found to flow from inner to outer, thus proving that the inner surface was the more excitable. In illustration of the great practicability of the methods employed, I may refer to the photographic records obtained of the response of the skin of the intact human forefinger (fig. 180). .

In describing the differential excitability of the hollow peduncle of Uviclts lily, it was shown that protected surfaces are, as a rule, more excitable than those which are exposed, and have thus undergone a greater degree of modification. On taking the plagiotropic stem of Cucurbita, the lower surface of which is protected from light, it is. found that, while the current of rest flows from the exposed upper to the protected lower surface, the direction of the responsive current is opposite, namely from the lower to the upper, proving that the protected lower is the more excitable of the two. Similarly, in the case of the intact human lip, I found that the resting current was from the epidermal to the epithelial, the responsive current being in the opposite direction (fig. 196). . Again, on testing the differential response of armpit and shoulder, I found that the respon- sive current was from armpit to shoulder, the former being thus the more excitable of the two (fig. 194).

We have seen that the lining membrane of the inner surface of the peduncle of Uvric/zs lily is very thin, and that, in distinction to the outer or epidermal membrane, it may be regarded as epithelial. As we approach the bulb-end of the peduncle, this inner layer of cells is found to be highly turgid, and secretion is found to take place into the hollow tube. The inner surface of the carpellary leaf of Dzllenta indica, again, secretes a mucilaginous substance. In these two cases there are no definite glands, But definite glands are found to occur on the inside of the pitcher of Wepenthe. In all these cases the secreting layer, whether provided with glands or not, is found to be very highly excitable, and to respond by strong galvanometric negativity. Taking a carpel of Ditllenia indica, it is found that the natural current is from the outer epidermal to the inner secreting surface, the respon- sive current being in the opposite direction. On making very careful connections, with the skin of the protruded body of the snail, and the glandular under-surface of its foot, it is found that the natural current is from the non-glandular to the glandular, but the responsive current from glandular to non-glandular. As an example of the way in which the true natural current of rest may be reversed by the excitatory effect of preparation, I showed that, while in the intact snail the natural current was from non-glandular to glandular— the gland being in this case relatively positive, to the extent of -0013 volt—after the sectioning of the foot, the original

natural current was reversed, owing to the greater relative excitation induced at the glandular surface, which now became relatively negative, to the extent of —-‘0020 volt. With the intact human tongue, further, I found that a very strong responsive current was induced on excitation, from the lower to the upper surface, thus showing that the lower was the more excitable of the two. The response of digestive organs may now be passed in review. In these, as in glandular organs, excitatory response is supposed to take place by secretion. In connection with this, it must be borne in mind that in the tissue of the pulvinus of J/zmosa, on the removal of the impervious skin, excitation induces secretion of the contained fluid, which, again, is re-absorbed on the cessation of excitation. We know the pulvinus to be contractile, and may therefore regard this secretion as an effect of contraction, causing expulsion of water. Apart from the differential action of the upper and lower halves of the organ, and the magnifying petiolar index, the fundamental contractile action would, in the case of J/zmosa, as in others, have passed unnoticed. This goes to show that it is not impossible that the phe- - nomenon of secretion through a permeable membrane may be associated with excitatory contraction. In favour of such continuity, it may be urged that tissues, hitherto regarded as non-motile, have been shown to exhibit excitatory contrac- tion. In digestion, as a whole, we have to recognise two different processes, those, namely, of secretion, and of sub- sequent absorption. Parallel to these, we find that the electrical response of digestive organs exhibits phasic alter- nations of negativity and positivity.

It was shown that the pitcher of Wepenthe—which may be regarded as an open stomach—affords us unique facilities for the observation of the normal responses of digestive organs. In experimenting with the animal stomach, the specimen has to be cut open, in order to make the necessary connections ; and, owing to the highly excitable character of the organ, this gives rise to intense excitatory action, the after-effect of which is necessarily to reverse the normal current of rest, With a pitcher of Mepenthe in a fresh condition, the natural current of rest is from the outer to the inner, the responsive current being in the opposite direction, and the glandular surface, on simultaneous exci- tation of the two, becoming galvanometrically negative (fig. 203). Digestive organs, moreover, tend to -exhibit multiple responses, the response to a single strong stimulus, say thermal, or of mechanical section, consisting, whether in animal or vegetable organs, of a series that may persist for nearly an hour (figs. 206, 209, and 213), When the pitcher of Nepenthe contains a large number of captured flies, that is to say, when it has been subjected to long-continued stimu- lation, it exhibits a phasic change, the responses now becoming reversed to positive (fig. 205). This, as pointed out above, is probably significant of absorption. In Drosera, the normal response of the glandular surface is by induced negativity, but on long-continued stimulation, this is reversed to positivity (fig. 208).

In the animal stomach, the sine current of rest is generally from the glandular to the non-glandular surface. From the fact that the skin of the toad, which is also possessed of imbedded glands, gives a current of rest from the outer surface to the inner, it has been supposed that the mucous coat of the stomach of the frog had the same electro- motive reaction as its outer skin, That this, however, is not the case is seen from the fact that on excitation the skin becomes galvanometrically positive, while the mucous mem- brane of the stomach becomes galvanometrically negative. The observed current of rest in the stomach would appear, from WVepenthe, to be, not the natural current of rest, but the reversed current, due to the excitatory effect of preparation. The normal effect of excitation in the stomach, I uniformly find, in such different instances as frog, gecko, and tortoise, to be by galvanometric negativity of the mucous surface (figs. 210, 211, and 212). On applying a strong thermal stimulus

responses, of which the first were negative, the second part diphasic, and the last portion reversed positive (fig. 213). Looking at the phenomenon of digestion, we see that it consists first of a secretory process, by which certain solid substances are dissolved, and secondly of the absorption of these dissolved substances. Similar functions are subserved in vegetable life by the root, by which solid inorganic food- absorbed. The proof of the former is seen in the well- known corrosion-figures produced by growing rootlets on a marble surface. I have also been able to demonstrate the phenomenon of excitatory secretion in young roots by allow- ing them to absorb dilute salt solution, and then under exci- tation to secrete it into highly dilute silver nitrate solution: This last was attended by the visible formation of a white precipitate. The electrical response of young roots of Colocasia, moreover, I found to be by induced galvanometric negativity (fig. 214), which, under long-continued stimulation, was apt to show reversal to positivity. The older roots, on the other hand, under the same intensity of stimulation, gave response by galvanometric positivity (fig. 215). The former of these responses, there is every reason to believe, is as- sociated with secretion, and the latter with absorption.

This question of the absorption of inorganic food materials by the plant is naturally connected with the subject of the Ascent of Sap, which is regarded as one of the most difficult problems in plant physiology. The non-physiological theories advanced are admittedly inadequate to the explanation of this phenomenon. That the ascent, nevertheless, could not be due to physiological action was held to have been proved by the facts (1) that water-conduction takes place pre- ferentially through sap-wood, assumed to be dead ; and (2) that poisonous solutions, such as would kill a living tissue, have been found to be transported through the roots, or the cut ends of their trunks, to the tops of trees.

I have, however, been able to show that these objections are not valid. For in the first place, the supposed dead wood, concerned in the transport of sap, through the trunks of trees, can be proved, by electrical tests, to be fully alive. This living wood responds to stimulation by in- duced galvanometric negativity, such response disappearing on the death of the tissue, as, say by drying, after which it cannot be restored. The response of living wood is also depressed by anesthetics, and abolished by poisons (figs. 216, 217). As regards the argument based on the transport of poison, it has been shown that as the active elements concerned in the transport of sap are diffused throughout the length of the trunk, the death of one indi- vidual zone, to which the poison has ascended, does not abolish the suctional activity of the zone above. It is only when the plant has been killed throughout, by the arrival of the solution at its top, that the complete arrest of suction could be expected to take place. And this is found to be the case. Various agents, on the other hand, which are known to induce changes, whether of exaltation or depres- sion, in the physiological activity, are found to induce corre- sponding modifications in the rate of suction. A very delicate means of investigating this question has been shown to be that afforded by the records obtained with the Shoshungraph (fig. 218). Here, under the ordinary method of record, the slope of the curve indicates the normal rate of suction, and the effect of various agencies is immediately shown by the resulting flexure of the curve. This method of record, again, becomes extremely sensitive, when it is carried out under balanced conditions. By means of these records, it has been shown that depressing agents, such as cold or anesthetics, depress or arrest suction, whereas warmth exalts it.

It has been shown, further, that just as the multiple activity of the Desmodium leaflet is arrested, when the latent energy of the plant falls below par, so also, under similar circumstances, the suctional activity falls into abeyance, and that, as in the one case, so also in the other, the activity is renewed, by the application of an external stimulus. It has also been shown that the latent period which elapses, before the initiation of this responsive variation to external stimulus, is longer when the plant is in a sub-tonic condition than in the same plant when its tonic condition has been slightly raised by previous stimulation. Crucial experiments, finally, have been described, showing that water-movement is a mode of excitatory response.