Comparative Electro-Physiology: A Physico-Physiological Study

the sensitive substance has its surface divided into six parts, the opposite sextants being put in electric communication. The opposite sextants CC are coated with shellac to represent the non-conducting aspect ; the sextants AA are coated with graphite to repre- sent the semi-conducting aspect; and the highly conducting aspect is represented by the sextants BB coated with tinfoil. The three main aspects of the sensitive substance are thus represented in the model; it is to be understood that with sensitive substances, under the action of stimulus, the transi- tion from one aspect to the next is gradual, and not abrupt, as represented. ‘The sensitive substance is interposed between

two electrodes. The torsion of the wire by which the cylinder is suspended represents the force of restitution. The galvanometer coil, by its deflections, exhibits indirectly the molecular strain produced in the substance by the action of stimulus. Let us suppose that we start with the substance in its normal state A, with moderate conductivity, and let the corre- sponding galvanometer deflection be 50. Let the substance belong to the negative class which exhibits an increase of conductivity, or diminution of resistance, under the action of stimulus. The stimulus will therefore distort the substance to a state of increased conductivity, the increased conductive aspect BB being brought opposite the electrodes. The enhanced current thus produced causes a deflection of, say, 100 in the galvanometer. If the strain has not been excessive, the substance will return, on the cessation of stimulus, to its original position of equilibrium, and the galvanometer deflection will fall from 100 to the original value 50. If the substance belong to the positive class, the distortion will be in the opposite direction, and the effect of stimulus will be to induce a responsive increase of resistance.

The coil of the indicating galvanometer thus moves in perfect response to the varying molecular strain induced in the sensitive substance by the action of stimulus. The invisible molecular distortions are thus revealed by the visible deflections of the galvanometric indicator—the effect on one is merely the reflection of the effect on the other. A curve of the molecular effect, induced by the action of stimulus, may thus be obtained with the galvanometer de- flection as ordinate, and the time as abscissa. It is thus seen that these response-curves faithfully represent the in- visible molecular strain-effect due to the stimulus, and the subsequent recovery. =

I shall now describe how in practice, by this method of resistivity variation, we obtain responses of various sub- stances to the stimulus of visible or invisible radiation. The sensitive substance may be made the fourth arm of the Wheatstone’s bridge, and the responsive galvanometric de- flection and subsequent recovery of the spot of light—by the upsetting of the balance, under the action of stimulus of radiation—is recorded in the usual manner, on a moving photographic plate. Or the sensitive substance may be placed in series with a galvanometer, a small E.M.F. giving a steady permanent deflection. Taking first selenium as the. sensitive substance, the molecular change induced by the action of light, with its concomitant variation of resistance, causes a deflection of the galvanometer spot of light. On the cessation of the stimulus, molecular recovery takes place, and the deflected spot of light returns to its original position. A series of such responses will be found on referring to page 3, fig. 3. The parallel method employed in recording the responsive resistivity varia- tion of masses of metallic par- ticles of various kinds, under the stimulus of electric radia- : tion, will be understood from Fic. 367. Method of Resistivity "8+ 367. On obtaining records Variation of the responses given under Sensitive metallic particles placed in this method, I find, as I pointed tube in series with galvanometer 5 and E.M.F. This gives a steady out in the first chapter, that permanent deflection. Stimulus the responding substances are

‘of electric radiation induces a ; responsive variation of resistance of two different types. The with. concomitant variation of first, of which aluminium may galvanometric deflection. be taken as the example, re- spond by diminution, or negative variation of resistance. The second, illustrated by potassium or arsenic, respond by an in- crease, or positive variation of resistance. In living tissues also, tested by various modes of response, we have seen two opposite types to occur—highly excitable nerve giving one, say, negative, while skin, on the other hand, gave the positive. In the case of the inorganic substances referred to, we have extreme types, whose response is generally either positive or negative. There are, however, intermediate

cases, where it is liable to change of sign according as the stimulus is feeble or strong. Certain substances, again, cannot quickly recover from the after-effect of stimulus ; while, in others, recovery is fairly rapid. Recovery from intense stimulation is generally, other things being equal, more protracted than from feeble or moderate. Anything, however, which enhances molecular freedom or mobility will tend to hasten recovery. I shall now give several typical records in illustration of the peculiarities of this form of response by resistivity variation, under various conditions. The first example

Fic. 368. Photographic Record of Response of Aluminium Powder in Sluggish.Condition to Stimulus of Electric-Radiation. . The first two responses exhibit incomplete recovery, which becomes com- plete on application of warmth. Note that warmth, increasing force of recovery, hastens recovery and also diminishes amplitude of response, as seen in the two succeeding records. given, that of aluminium powder, will» be of the negative type, the response being by diminution of resistance. When the substance tested happens to be in a sluggish condition, recovery is very protracted. There is then a _ response- remainder of persistent negative variation, corresponding to the contraction-remainder in muscle, or persistent electro- motive negativity in other living tissues. But we know that a moderate rise of temperature is favourable to recovery ; and on applying gentle heat, at the end of the second response, with its incomplete recovery, the persistent effect is seen to be removed, and -there is an immediate completion of recovery (fig. 368).

anomaly, in the response of living tissues, where a slight rise of temperature increases conductivity, at the same time that it appears to diminish excitability, inasmuch as it brings about a lessened amplitude of response (Chapter XV.). This latter, however, may not really be due to diminution of excitability, since the same effect might equally well be brought about by an enhancement of the force of recovery. This view is supported by the further records given in fig. 368. We see here that incomplete recovery became complete, under the application of gentle heat. The next response given by this slightly warmed substance is seen to

Fic. 369. Photographic Record Showing Uniform Response of Alu- miniun Powder to Uniform Stimulus of Electric Radiation. show complete recovery within a relatively short time, this enhanced force of recovery bringing about at the same time a diminution in the height of response. The temperature of the substance was now again raised to a slightly higher degree, and the next response shows a still further diminished height and a considerably quickened recovery. When a substance is in a normal condition of excitability, its succes- sive responses to uniform stimuli are found to exhibit com- plete recovery, and to be of equal amplitude. Figure 369 shows such a series obtained with powdered aluminium.

We have seen that the increased molecular mobility con- ferred by warming hastens recovery. A similar hastening of recovery may be brought about by a mechanical tap, as has already been shown (fig. 364) in the case of magnetic response. In fig. 370 is seen an example of the same thing in tungsten, where recovery from the effect of electric radiation is hastened by a tap. It has been shown that the normal response by negativity in living tissues is liable to reversal under very feeble stimulation. This is better observed when the tissue is not highly excitable; because in this case it is easy to adjust the intensity of stimulation, so as to fall below the critical value for excitation. It is very interesting to ob- serve similar opposed effects, under feeble and moderately strong stimulations, in the re- sponse of inorganic substances. For the reason just mentioned, it is desirable to select a sub- stance for this purpose, which possesses a moderate degree of sensibility. Using a mass of tungsten particles, I found that under strong intensity of electric radiation—-brought about by placing the radiator within a short distance of the Fic. 370. Photographic Record

of Response of Tungsten substance—the response was ; : The incomplete recovery is hastened by the normal negative varia- by application of tap at points, tion, or diminution of resist- marked with downward arrow. Cf. fig. 364. ance. But when the intensity of stimulus was dimfnished by placing the radiator at a greater distance, then the response was converted to positive. A record of this abnormal effect under feeble stimulation will be given later.

Thus, having observed molecular response and its varia- tions by the Magnetic and Resistivity Methods of record, we now proceed to study the transmission of the state of excitation. We have seen that the essential condition of the transmission of excitation in living tissues lies in the propagation of molecular disturbance from point to point. The characteristics of such propagation must be—(1) that the transmitted molecular disturbance becomes enfeebled with distance, so that at a certain point the transmitted excitation would be reduced to zero; (2) that while a moderate stimulus is transmitted to a short distance, a stronger stimulus would be carried further ; and (3) that the intensity of excitation transmitted would depend on the conducting power of the intervening tract, this conductivity being capable of enhancement by certain agencies, and depressible by others.

I shall now proceed to show that in an iron wire a transmission of molecular disturbance takes place which is Fic. 371. Experimental Arrangement for obtaining Response in Iron by Induction Current similar to that at the basis of the transmission of excitatory changes, both, as I shall show, being modifiable by similar circumstances in a similar manner. For these investigations I have employed the Induction or Electro-motive Method of observation. In the experimental arrangement—a diagram- matic representation of which is shown in fig. 371—S is the stimulating or exciting coil appliéd at the point to be excited. The conducting region intervenes between S and R, which is the responding point, over which is wound the receiving coil, placed in series with either a telephone or a galvanometer. When the excitatory molecular disturbance reaches R, it gives rise to an induction current in the coil, which in turn causes a sound in the telephone, or a responsive deflection in the galvanometer. For the purpose of simplicity, we shall take north polar or K-excitation as

normal, and the resulting deflection of the galvanometer to the right as the normal response. ~The direct effect of the coil S on the coil R may be regarded as negligible, when they are separated from each other by a sufficient distance, and this would be even more true if the intervening iron wire were bent at an angle of 90 degrees. Employing this mode of obtaining records of response to K- or.A-excitation, we meet with several curious analogies to the responsive effects seen in living tissues, under the electrical mode of stimulation. Electrical excitation of nerve and muscle, for example, is most effective when it is longitudinal, and ineffective when transverse. The same is true of magnetic excitation of iron, where longitudinal excitation is effectively transmitted to a great distance, whereas transverse excitation is relatively ineffective.

Again, in the case of the electrical excitation of living tissues, it is at the instant of kathode-make, as we have seen, that excitation is induced. Continued action exhibits in at kathode-make, is induced again, but at anode-break. Similarly, in the case of an iron wire, the normal galvano- metric response is seen at the moment of K-magnetic excitation, but not during its continuance. The same excitation is also obtained, at break of A, or south polar magnetisation. .

We are led from such close analogies, not only to visualise, but also to obtain some insight into the sequence of molecular events which is the concomitant of excitation. I have already pointed out that excitation and its opposite, depression, being phenomena of molecular. distortion, it is to be expected that a particular-directioned distortional movement should be associated with one of these, and the opposite with the other. We also know the further suggestive fact that it is the sudden change of the environ- ment, inducing a sudden responsive molecular disturbance, that is most effective in bringing about excitation. The latent period, and a slowly-rising excitation, correspond to the

slow initiation of the molecular upset. After this the rate of molecular distortion will be rapid, and in this second period we find that the excitatory reaction also is at its maximum. In any case, it is rather during the period of increasing molecular distortion that we should expect to see the most intense excitation, than when a static condition of derangement has been attained. Thus it is at the moment of K-make that we obtain the excitatory indication, and not afterwards, when the molecules are being maintained in the distorted position. ,

Returning once more to the iron wire, we find that when the distorted molecules have been set free by the break of kK, there is a sudden movement of recovery in the opposite direction. If now the K-effect, with its particular-directioned molecular movement, be termed the excitatory, then the oppo- site movement must be regarded as one of depression, and it is interesting to note that in a living tissue there is an after- effect of depression at kathode-break. The anode-make, on the other hand, with its opposite molecular distortion, is, as ' one would expect, depressory. But at the break, the direction of the rebound of the released molecules being the same as that brought about by K-make, must be excitatory. The close parallelism which we have thus traced out, forces upon us the conclusion that the molecular actions which underlie the excitation of living tissues may be-polar in their character.

The fact that magnetic excitation undergoes diminution during transmission, can be shown by moving the receiving coil R further and further away from S, when the responsive sound in the telephone, or deflection in the galvanometer, will be found to undergo a graduated diminution, till, with a given stimulus, the effect, from being considerable, is reduced at a certain distance to zz/, Keeping this distance the same, however, a stronger stimulus will be found efficient to evoke response, and the responding coil will now have to be moved further, in order again to reduce the response to zero.

by an external agent, as. modifying the intensity of the transmitted effect. In order to study the phenomenon of conduction and its modification, as will be remembered, a delicate form of Conductivity Balance, fully described in Chapter XX XIII. was used. Excitation was here caused by S at a middle point, the transmitted excitatory effects at E’ and E being made to balance, This condition of balance was obtained when one arm, say the left E’, was kept at a fixed distance from Ss, and the other, or right, was moved towards S, or away from it, as required. When E was too far from S the excitatory effect would be smaller than at E’, and this under-balance would be indicated by a response, say downwards. When, again, E was too near to S, there would be an over-balance, the resultant response being upwards. Between these could be found a point of exact balance where the record was horizontal (cf figs. 289, 290).

A high degree of delicacy in the study of: similar phenomena in the case of iron wires may be obtained by S, magnetising coil, by which north-polar or K-impulses are sent out in two directions as shown by arrows. E BE’, receiving coils, adjusted at balance. M, permanent magnet, by ,which either A- or K-tonus is induced at the responding ends of the iron rod. T, tonic coil, by which A- or K-tonic molecular dispositions may be induced in one arm of the balance.

the employment of the Magnetic Conductivity Balance (fig. 372), which I shall now describe. The magnetic stimulator, S, consists of a pair of similar coils wound in opposite directions, When a magnetising current is suddenly sent through these two coils, in a proper direction, two equal north-polar impulses will be generated simultaneously, and travel, one to the right, towards E, and the other to the left, towards E’. [n order to obtain a balance of the excitatory effects at E and E’, we keep E’ at a fixed distance, and move E backwards and forwards till the balance is found. This process of balancing will be found graphically illustrated in the records given in fig. 373. E was placed at first too near to S, and the over-balance is seen as up-responses. The coil was then moved away very gradually, and the response of over-balance is seen at each step to undergo a diminution or approach towards balance. We next note the attainment of exact balance, where the record is seen to be horizontal. The coil is now moved still further to the right, and the con-

Fic, 373. Process of Balancing illustrated by Photographic Record | of Responses sequent increasing under-balance is exhibited by the gradually increasing reversed down-responses. Having thus obtained balance, we are able to record the variations induced in conductivity by a given agent. This is applied on the right arm of the balance, the subsequent upset of which, in one direction or the other, indicates the enhancement or depression of conductivity. Resulting up- responses will indicate enhancement, and down-responses depression. A well-known agent for the enhancement or depression of the conductivity of the nerve is the polar action of the kathode and anode. Moderate kat-electro- tonus enhances conductivity, whereas the anode depresses or inhibits it. The explanation which I have already offered, regarding anodic and kathodic effects on excitability, will also. be found applicable in the case of conductivity. An excitatory or kathodic effect will be facilitated in trans-

mission, if the molecules in its path are already incipiently orientated, so that the incident stimulus finds them pre- disposed to respond in that direction and to transmit the excitation. Hence the kathodic effect is more easily trans- mitted through a tract which is in a state of K-tonus, whereas it is retarded or inhibited under A-tonus. We shall now study the corresponding effect in magnetic conduction. Normal excitation in these experiments, it should be remem- bered, is taken as that which is brought about by the north- polar or K-effect. If there is a tonic coil, T, surrounding one arm of the balance, then, by sending a permanent current of moderate intensity round the coil, in one direction or the other, we may induce at will, in that arm, either K-tonus or A-tonus. The molecular disposition induced by the stimulus, and by K- and A-tonus respectively, will be under- stood from the diagrammatic representation given in fig. 372. Local variation of excitability at E may be induced by bringing near to’it either the north or south pole of a permanent magnet M.

I shall now exhibit the enhance- ment or depression of magnetic conductivity by K- or A-tonus. A balanced record is first obtained, and K-tonus then induced in the A-Tonus on Magnetic Con- right arm. Successive K-make ex- duction aes . . The first series exhibit by citations are now applied, starting resultant over-balance up- from the centre of the balance at wards the effect of K-tonus ‘ ; -% in enhancing ‘conductivity S, and proceeding onwards to left of right arm, The next and right simultaneously. The — series, with resultant down-responses, show de- resulting responses are recorded, pression by A-tonus.

avoided by timely interruptions of the galvanometer-circuit. It will be seen (fig. 374) from the upsetting of the balance in by reversing the current in the enclosing tonic coil, T, a de- pression of conduction is induced, as shown by the upsetting of the balance in a downward direction. - We next turn to the question of the variation of conduc- tivity induced by K-tonus, when moderate or excessively strong ; and it is here important to forecast from theoretical considerations what is to be expected under varying intensities of the polarising force. It is easy to understand that moderate K-tonus, inducing an incipient orientation of the molecules, will predispose them to easy upset in a particular direction, thus greatly facilitating the transmission of excitation from point to point. Thus a moderate K-tonus will enhance con- ductivity. But if the K-tonus in question be excessive, so that the molecules are already distorted to their maximum position, incident stimulus can then induce no further change, and under such circumstances there can be little transmission. Hence, under increasing intensity of K-tonus, we may expect to obtain increasing conductivity up to a certain point. But, beyond this, the conductivity will be decreased, or even actually inhibited.

These anticipations are seen fully verified in the accom- panying record (fig. 375), which shows the opposite effects on conductivity of moderate and strong K-tonus. The upsetting of the balance in an upward direction, K, shows the effect of moderate K-tonus. Strong K-tonus was next applied, with the effect of upsetting the balance in the opposite direction, K’. Thus we see that, while under moderate K-tonus the conductivity is enhanced, under a much greater intensity it becomes depressed. This will, I think, be found to explain a somewhat anomalous occurrence, which has been observed in regard to the conduction of excitation through a kathodic region in nerve-and-muscle preparation. A stimulus applied on the extra-polar region in nerve is

Fic. 375. Opposite Effects of kK-Tonus when moderate and strong found tobe transmitted through the kathodic area, inducing enhanced response of the indicating muscle, if the polarising current be weak. But when the intensity of the kathode is made stronger, even the strongest stimulus will fail to induce response. This is evidently due to the fact that a strong kathode induces a depression or abolition of conductivity. Moderate K-tonus, then; we have seen to induce enhanced conductivity, because of the favourable molecular disposition which it brings about. Even on the cessation of K-tonus this disposition remains, owing to molecular ‘ retentiveness, with its concomitant enhanced conductivity as an after- effect. This induction of a favourable molecular disposition or habit is an interesting phenomenon, which we shall meet with again. :

We shall next study the enhancement or depression of local excitability by K- or A-tonus. We saw, in Chapter XXXIIL, that by means of the Con- ductivity Balance we might determine the variations, not only of conductivity, but also of local excitability. In mag- netic experiments the responsive area at the right-hand end of the balance may be made either K-tonic or A-tonic, by bringing near it one or other pole of a permanent magnet. Under induced A-tonus, the molecular excitability is depressed, and

the balance upset in a down- A-Tonus on Magnetic Ex- . ‘ ae citability ward direction; while under 4 Ae From the resulting upset of the K-tonus excitability is enhanced, balance, A is seen to induce de- the resulting response being up- FF coun SNR EETER Es wards (fig. 376). A still more interesting case is that in which the stimulus itself fashions,:as it were, the path for its own conduction. The receiving coil is placed at such a distance from s