King's College, London.)

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1 THE EFFECT OF THE CIRCULATION ON THE ELECTRICAL RESISTANCE OF THE SKIN. BY F. AVELING AND R. J. S. McDOWALL. (From the Departments of Physiology and Psychology, King's College, London.) OF recent years, especially among psychologists, considerable attention has been drawn to the fact first noticed by Fere', that under conditions of emotion the electrical resistance of the skin of man may undergo considerable reduction, and this has given rise to a large number of observations in man on what is called the psycho-galvanic reflex. The efferent nervous impulses of the reflex are generally held to pass to the skin by way of the sympathetic nerve fibres, i.e. either by vascular or sweat fibres. The vascularity of the skin can be modified in a number of ways and it seemed desirable to determine how far the skin resistance varied with its vascularity. Only very few investigators appear to have succeeded, for reasons indicated below, in bringing about any change in resistance in anesthetised animals, and those have confined themselves to a study of the nature of the sensory stimuli which produce the fall or to the study of the relation of the fall of resistance to the possible action of the sweat glands. Attention to the latter has been rather emphasised by the fact that some observers have found that the fall of resistance was abolished by atropine (Schilf and Schubert, Fauville). It appears however to have been completely overlooked that the effect of atropine is not confined to its action.on glands and that it has a profound effect on blood vessels. Similarly it has been considered that the persistence of the fall of resistance after temporary occlusion of the blood supply to the heart necessarily indicated that it was not due to vascular changes. As we now know that the capillaries are largely independent of the arteries it will be clear that such evidence can no longer be taken as conclusive. Advantage was taken of the fact that the vasomotor system of animals under chloralose is appreciably more sensitive than if ether is used, especially if the experiment is prolonged. The experiments were made on cats. Ordinary zinc wash-leather electrodes were lightly bandaged on to the skin, the fur being clipped off if necessary with scissors and the whole soaked in saline. The electrodes were connected with a Wheatstone bridge and a galvanometer in the ordinary way used for observing resistances. In different animals the resistance varied from 30,000 to 60,000 ohms, but subsequently it was found could be changed

2 ELECTRIC RESISTANCE OF SKIN. as desired by the experimental procedures. Changes in resistance varied up to 2000 ohms. That these changes were really due to the skin could readily be demonstrated by removing it when it was found that the resistance of the circuit was less than 2000 ohms and the changes in this resistance were of such a small order as to be negligible. The results obtained were extremely definite provided fresh animals not suffering from shock were used. It was found from time to time that the reactions given before the preliminary ether anesthesia had worn off were absent or were appreciably less than those given later under the chloralose. This is important and no doubt accounts for the completely negative results which have been recorded by other workers. Direct vaso-constriction. This was brought about by the intravenous injection of adrenaline (1/50 mg.), which was found to bring about a marked fall in the resistance of the skin of the limbs and the pads of the feet. The fall of resistance corresponded with the usual rise of bloodpressure when the drug is injected. The return of the skin resistance to its previous height synchronised with the return of the blood-pressure. The change in the skin resistance like the rise of blood-pressure was best seen when the vagi were cut. Indirect vaso-constriction. It is well known that in haemorrhage there is constriction of the skin vessels as a result of increased action of the vasomotor centre. This is indicated by the pallor. When the animal was bled from the carotid artery there occurred a marked fall of skin resistance. Of interest in this respect was the effect of small doses of histamine (1/10 mg.), which have been shown by Dale and Richards to bring about capillary dilatation. The recovery from such small doses is extremely rapid and this has been shown by McDowall and Worsnop to be due to reflex arterial constriction through the operation of the vago-pressor reflex, and they have drawn attention to the similarity in the effect of the circulation of the arteries of such small doses to the effect of small haemorrhages. The similarity between large heemorrhages and large doses of histamine from which the blood-pressure does not recover was pointed out by Dale and Laidlaw. It was found that on the injection of a small dose of histamine there was a decided fall in the skin resistance which persisted after recovery of the blood-pressure. In this we have further evidence of the similarity referred to above between the action of haemorrhage and histamine. It is of interest to note in this respect that clinically in conditions of shock due to toxin absorption from wounds, there is a marked pallor of the skin and a sensation of coldness to the touch. PII. LX

3 318 F. AVELING AND R. J. S. McDOWALL. A similar fall in skin resistance was brought about by the action of cold which we know brings about a constriction of the skin vessels in order to prevent heat loss. Cold was applied to the limb above the electrodes and to the carotid artery; and cold fluid was injected into the cephalic end of the carotid all with the same result. Vaso-dilatation. This was brought about mechanically by compressing the inferior vena cava, when the electrodes were applied to the lower limbs. The procedure brought about a momentary fall of resistance but this was after half to one minute succeeded by a marked rise in skin resistance which persisted. This we may presume was brought about by the congestion of the part. Dilatation of blood vessels was also brought about by the administration of amyl nitrite, which also caused a marked rise in the skin resistance. Irritation of the part by plucking out the hairs previous to the application of the electrodes brought about a steady rise in skin resistance. When the electrodes were removed it was observed that the skin, previously pale, had become reddened as a result of the local dilatation of vessels. A rise of resistance was also obtained by the injection of hot saline into the blood stream but the results, although definite, were not very marked. It is then clearly evident that a vaso-constriction is associated with a fall in skin resistance and vaso-dilatation with a rise in skin resistance. A continuation of these experiments gave further results which could readily be interpreted from what has been said above. Asphyxia. This was almost identical with the effect of clamping the inferior vena cava. There was a preliminary fall of resistance apparently due to the immediately stimulating effect of applying the clamp to the trachea, but thereafter there was a rise of resistance which continued to rise rapidly till the death of the animal. The procedure can readily be shown to result in an enormous raising of the pressure in the veins which we may infer brings about local dilatation of blood vessels in the region of the electrodes. No doubt the effect of CO2 also plays a part. Acetyl-choline. If this drug is given in small doses insufficient to affect the heart its intravenous injection is associated with a rise in skin resistance. With larger doses, however, which brought about marked cardiac inhibitions there resulted a fall in skin resistance. Here we may understand that in the case of small doses there was the well-known vaso-dilator action of the drug, but with the larger doses when the

4 ELECTRIC RESISTANCE OF SKIN. 319 cardiac output was diminished as in haemorrhage the compensatory vaso-constriction counteracted the vaso-dilatation and brought about the fall in the skin resistance. This explanation is supported by the fact that a similar fall of resistance occurs if the output of the heart is reduced by impeding its action mechanically with the fingers. Pilocarpine. The effect of this drug was specially interesting in view of its action on the sweat glands and in view of the controversy which had taken place in regard to the possibility of the secretion of sweat being responsible for changes in the skin resistance of man. Two distinct stages were usually seen. In the first there was a short but distinct fall of skin resistance, in the second a distinct and prolonged rise. The fall accompanied the evanescent pallor stage and the rise the flushed stage during which secretion takes place. The changes in skin resistance were obtained on the skin of the limbs which does not sweat. They were then clearly independent of sweating. This evidence, together with that given above, especially that in relation to cold, indicates clearly that sweating per se is not responsible for the fall in resistance. These results support those of Waller, who found that atropine given subcutaneously in ;sufficient doses to produce its well-known inhibition of the sweat glands did not interfere with the changes of skin resistance which occur in certain emotional states in man. Atropine. As indicated in the first paragraph the result of the injection of this drug is of special significance. The effect of the injection (1 to 2 mgm.) was clearly to bring about a steady rise in resistance of 10 or 15 minutes without recovery. This confirms the work of Markbeiter. In the early stages of such doses or if smaller doses were administered, it was possible to interrupt the rise by procedures such as the injection of adrenaline or sensory stimulation which we have seen above bring about a fall of resistance, although even at this stage it might be evident from the dilatation of the pupil and the acceleration of the heart that the parasympathetic had been paralysed (confirmation of Waller). These results are readily explained by the vaso-dilator action of atropine. This action is often overlooked, though it is referred -to in most of the pharmacological text-books and is well known in man Ẇhen injected, the vaso-dilator effect of the drug tends in the first -instance to be masked by the accelerator effect on the heart which brings about a rise in blood-pressure. If, however, the vagi have been cut and the heart is already freed from vagus tone, or if the animal is suffering from shock, the same dose of atropine will be found to bring about a 21-2

5 3)20 F. AVELING AND R. J. S. MoDOWALL. profound fall of arterial pressure, which, indeed, may result in the death of the animal. Sensory stimulation. The common stimulus adopted was pinchin,g the skin of the fore limb with a pair of pliers. The stimulus resulted in a marked fall of skin resistance in the limbs or the pads of the feet. Repetition of the same stimulus did not always, however, bring about the repetition of the result which was normally of the same order as that obtained with a moderate dose of adrenaline (1/5 mg.). This change in reaction quite clearly did not depend on the higher centres as it was brought about in decerebrate cats. Remarks. From what has been said it seems reasonable to believe that the fall of skin resistance which occurs on sensory stimulation is due to constriction of the skin vessels. The sensory stimulation is, as is well known, accompanied by a rise of blood-pressure as would be expected from sympathetic stimulation. That the sympathetic is stimulated is seen by evidence elsewhere than from the circulation, e.g. the dilatation of the pupil, which occurs at the same time. We have seen the similarity of result to the effect of adrenaline which is known to constrict the skin vessels. Further, under conditions, e.g. hemorrhage, in which adrenaline no longer brings about a fall, there is no response to sensory stimulation although at the beginning of the experiment this could be readily obtained. The blood vessels may be presumed to be already constricted as the result of the effect of the haemorrhage and cannot be made to constrict further by sensory stimulation. This presumption is supported by the fact that often in such circumstances the vessels may be dilated and the skin resistance made to rise by the administration of a powerful vasodilator, such as acetyl-choline and especially by asphyxia or clamping the vein. This conception is also supported by Muller's results. He found in monkeys that section of the nerves supplying the heart only very temporarily reduced the fall of resistance obtained on sensory stimulation, although this was abolished by the local injection of 5 p.c. novocaine and adrenaline. It may be considered that the nerves concerned lie along the blood vessels, which, as we know, recover their tone fairly rapidly in spite of nerve section. Thus procedures which are known to bring about the constriction of vessels cause a fall in the electrical resistance of the skin and a dilatation of vessels is associated with a rise in the resistance. All other procedures which affect the skin resistance of the chloralosed animal can readily be explained as being due to changes in the vascularity of the part. The

6 ELECTRIC RESISTANCE OF SKIN. 321 utility of such a relation is evident in the prevention of loss of blood and the throwing of a maximum amount of blood into the circulation. This will, of course, be prior to the requirements of skin dilatation for the purpose of increasing the heat loss once the exercise is begun. We have seen that the higher cerebral centres are not required and that the reaction is apparently of a reflex nature. The reflex, however, does not appear to be limited strictly to the skin. A similar diminution is found to occur in muscles although in the latter instance the circuit has so little resistance that the changes which occur are very small compared with those of the skin. Quite apart from the results obtained, it will be evident that we have in the investigation of the resistance of the skin, a method by which the circulation through it may be studied with accuracy, and it will be obvious that this may be of considerable value in the investigation of the location of the action of drugs especially in relation to the distribution of vasoconstriction and vaso-dilator effects. It may also prove useful in relation to the study of sweat secretion. SUMMARY. It is shown that in the cat a fall of the electrical resistance of the skin is associated with the constriction of vessels and that a rise is associated with a dilatation. Reasons are put forward why the fall of skin resistance which occurs on sensory stimulation may be considered to be due to similar vasoconstriction. The resistance of the skin may apparently be used as an indication of its vascularity. In carrying out these experiments we are indebted for the assistance of Dr H. M. Wells and Dr R. C. McCarthy. The expenses of the research were in part defrayed by a grant from the Government Grant Committee of the Royal Society. REFERENCES. Dale and Laidlaw. This Journ. 52, p Fauville. Arch. Internat. de Physiol. 16, p McDowall and Worsnop. This Journ ; Proc. Physiol. Soc. p. xxxvi. Markbeiter. Proc. Roy. Soc. B, 91, p Muller. Monatschr. f. Psych. 33, p Schilf and Schubert. Pfluiger's Arch. 195, p Waller. Proc. Roy. Soc. B. 91, pp. 17 and In the above paper all reference to similar experiments in man have been omitted in view of the very voluminous and controversial nature of the literature and the fact that in man it is not yet clear that the changes in skin resistance may not be affected by other factors.