resting currents, and they are usually due to the injured surface undergoing molecular changes causing it to become of a different potential from that of any part that has not been injured. If living matIf living matter has a fresh surface exposed by an incision, the surface begins to die, that is to sav, it rapidly undergoes molecular changes, and the dying matter becomes of lower electrical potential than the matter that is not dying, so that if the dying surface is connected with one electrode while the non-dying surface is connected with. the other, a current passes through the galvanometer from the non-dying to the dying. Another way of expressing the same fact is to say that any local injury to living matter always disturbs electrical equilibrium, because the injured part becomes very quickly of lower potential. Now, it is evident that chemical action, as occurs in dying of tissue, will be greatest on the injured surface. We may suppose that this surface, acting like the zinc, the positive, plate in a Daniell's cell, generates currents which pass through the muscle to its surface, issue from the surface (thus the positive pole) to the galvanometer, and back from the galvanometer to the injured surface, which thus represents the negative pole. Such currents, therefore, are evidently not of much physiological importance, except that they differentiate between different planes of vitality.

But the case is different when the tissue or organ discharges its normal function. This will be readily understood if we examine what occurs in a contracting muscle. The normal function of a muscle is to contract, that is to say, there is a movement of its protoplasm by which it shortens in length while it increases in thickness. Now, suppose a muscle laid on the electrodes so that a resting" current is manifested by the deflection of the needle of the galvanometer; let the nerve supplying the muscle be irritated so as to cause contraction of the muscle; instantly the needle of the galvanometer moves in the opposite direction, and it may pass even beyond the zero point. This is due to the generation of a new current in the muscle, in a direction opposite to that of the resting current. The proof is this: let us compensate the resting current before causing contraction of the muscle, by sending a portion of a current from a Daniell's element in the opposite direction, so

that the galvanometer is brought to zero; then cause the muscle to contract, and the new current, the action current, as it may be called, sends the galvanometer needle to the opposite side of zero. This phenomenon of a new current in the opposite direction is known technically as the negative variation, and it is of importance physiologically because it is the indication of changes occurring in the muscle that are associated with its contraction. It is a vital phenomenon because it can only occur when the muscle is alive. The action current may be accounted for by suppos ing that at the nerve terminations in the muscle there is some kind of local change, just as occurs on the cut surface of a muscle. This local action, probably chemical, generates a current which passes through the muscle in the reverse direction to that of the resting current; that is to say, it flows to the cut surface, passes out by it to the galvanometer, and returns from the galvanometer to the point of entry of the nerve. The cut surface, therefore, during the action current, becomes the positive pole, while the uninjured surface is the negative pole of the little muscle battery; exactly the reverse state of matters to what obtained while the muscle was at rest.

Early observations seemed to show that this negative variation was a kind of wave of negativity that swept through the muscle, and was over and gone before the muscle contracted; but recent experiments of Burdon Sanderson, in which he simultaneously photographed the movement of the muscle and the movement of the mercury in the capillary electrometer, demonstrate that the negativity extends into the time of the contraction, or, in other words, that the two phenomena go closely together. The electromotive force between the longitudinal and transverse sections of the resting gastrocnemius muscle of a frog is from .03 to .08 volt, and the negative variation may amount to as much as .04 volt.

Similar action currents occur in nerve. A nerve has a resting current; but when the molecular disturbances which, for want of a better term, we call a nerve current, passes along it, there is a negative variation.

If

Electrical phenomena may also be discovered in the central nervous organs. we bring the electrodes of the galvanome

ter into contact with the surface of the brain, electrical changes occur when light falls on the eye. Recently, Gotch and Horsley have explored the spinal cord with electrodes connected with the capillary electrometer, and they have found electrical variations in the motor strands of the cord when motor centres in the cerebral cortex were irritated. Thus, in a sense, they tapped the wires of the living telegraphic system and got information as to the paths in the cord along which motor, and even sensory, impulses travel. It seems to be only a matter of experiment to discover electrical changes in all the cerebral nervous organs. Could we pic ture to ourselves the changes in the brain when its higher centres are in a state of molecular disturbance, as when one is thinking rapidly in a lecture, now adapting his words to his ideas, now thinking ahead as to what he will say next, now noticing the effects of his words on the audience, now becoming conscious that he is obscure, and again that he is succeeding in making things plain, now watching the clock and noting the inevitable flight of time-could we, in such circumstances of mental turmoil, examine the phenomena of the brain, we would, in all probability, obtain evidence of rapid changes of potential, and of currents flashing in a thousand directions, pursuing paths the intricacies of which are many times greater than if all the telegraphic and telephonic wires of London were concentrated in one vast exchange.

Take another illustration. Place a frog's eye on the electrodes; we at once obtain a resting current, as above indicated. Keep the little eye in the dark, and the resting current becomes less and less as the tissues die; but allow light to fall upon it-even a flash of light lasting the thousandth of a second, or the light of a vesta at a distance of several yards and there is usually, first, a positive variation, then a falling off, if the light is allowed to act; and, lastly, if the light is suddenly cut off, there is almost invariably a second (positive) increase, followed by a (negative) diminution of the current Holmgren, Dewar, and the writer). It is highly probable that similar electrical phenomena are related to the action of stimuli on all the terminal organs of sense. The skin of all animals shows a current passing from the surface inward. This

has been supposed to be due to abrasion of the surface, and the skin current must be distinguished from that due to secreting glands in that organ. Thus the skins of fishes show a true skin current, although they contain no glands. The glands of the skin, however, produce currents, and it can be shown that when the secretory nerves of the glands are irritated, there is a positive variation or increase coincident with the secretion of sweat. Such electromotive phenomena connected with secretion have been demonstrated in the pad of the cat's foot, which contains numerous glands, and also in the submaxillary salivary gland. Thus the phenomena of secretion are undoubtedly connected with electrical changes.

One of the most interesting demonstrations of electrical phenomena in living structures is that of the variations connected with the beat of the heart. If the heart of a frog be laid on the electrodes, so that one electrode touches the base while the other touches a cut or injured surface at the apex, the needle of the galvanometer immediately begins to swing backward and forward, and it is easy to show that the swings are coincident with the beats. This remarkable phenomena has received much attention. It is almost impossible to trace the direction of the currents while the heart is beating; but if the rhythmic beat is arrested by applying a ligature around the heart at the junction of the sinus venosus with the auricle, as was first shown by Stannius in 1852, it is then possible to bring about a single beat by stimulating the heart either at the base or at the apex. Suppose, now, that the heart was connected with the galvanometer, or, still better, with the capillary electrometer, and that we stimulate at the base, there is a contraction, the base becomes negative to the apex, and the next instant the apex becomes negative to the base.

This is what occurs with a normal beat. On the other hand, if the apex is stimulated, the apex becomes first negative to the base, and then the base negative to the apex. Evidently, then, in a normal beat, the contraction commences at the base and travels to the apex. But the electrical change does not occur in the same phase throughout the heart at one moment; on the contrary, the wave of negativity travels to the apex. An instant afterward, however, the contraction change

at the base has disappeared, while it still remains at the apex. At this moment the apex is negative to the base. There are thus two phases with each contraction, and the phenomenon is termed a diphasic variation. Hence the swinging of the needle of the galvanometer. It is driven alternately in opposite directions. Similar phenomena have been noticed in the isolated mammalian heart. By far the most beautiful demonstration of this kind, how ever, has been recently given by Dr. Augustus Waller, of St. Mary's Hospital, London. Using the capillary electrometer, he has succeeded in showing electrical variations in man, without the necessity of making even an abrasion of the epidermis. Deeply placed as the heart is in the chest, full as it is of blood, and surrounded by the chest walls composed of bone, muscle, and skin, yet by placing one electrode, say in the mouth, and the other on the left foot, or even by placing them on opposite sides of the heart on the chest wall, the electrical variations with each beat can be demonstrated. Each beat of the human heart shows different electrical potentials if two points are connected with the capillary electrometer, one on each side of an axis passing, roughly speaking, from the left shoulder obliquely downward to the right side. Thus the body may be divided into two asymmetrical electrical districts, so far as the beat of the heart is concerned, one including the head and right upper extremity, and the other the three remaining extremities.

We have seen that electrical variations occur in connection with muscular contraction, and at once the question arises of whether any such changes can be demonstrated in the human being. Suppose we have a very sensitive galvanometer. Take two shallow vulcanite troughs, and fill them two-thirds full with a per cent. solution of common salt. Dip a perfectly clean slip of platinum into each trough, and lead wires from the strips to the galvanometer. Connect the two troughs with a strip of clean white blotting-paper wet with the salt solution. As a rule, if precautions have been taken to have everything absolutely clean, no current will pass through the galvanometer. Then wash the hands thoroughly and place one in each vulcanite trough. At first there is usually a swing of the galvanometer, but it soon comes to rest. Then contract

powerfully the muscles of the right arm. There will be a swing in one direction, say to the right. Next throw the muscles of the left arm into contraction. The needle of the galvanometer will now swing in the opposite direction. By alternately contracting the muscles of the right and left arms the needle of the galvanometer can be caused to swing rhythmically. This experiment, first made by du Bois Reymond, demonstrating what he calls the man-current, is of great interest. Careful examination shows that when the muscles of the right arm are contracted an electrical change passes through the body from the right to the left arm, out from the left arm to the galvanometer, and back from the galvanometer to the right arm. When the muscles of the left arm are contracted the reverse occurs; or, in other words, a current passes through the body from the contracting to the passive arm, and through the galvanometer from the passive to the contracting arm. Some have supposed that this is a skin-current, or rather a current due to a change in the cutaneous secretions, and it has been stated that it will not occur if the secretory nerves have first been paralyzed by atropine. As excitation of secretory nerves gives a positive variation, it is difficult to account in this way for the negativity that occurs in the actively contracting muscles, while the remarkable uniformity in the results that one, by careful experiment, obtains by alternately and rapidly contracting the muscles of the two arms, is in favor of the view that the man-current is due to electrical changes occurring in the muscles themselves.

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Enough has been written to show that in all probability all vital phenomena are associated with electrical changes. Up to the present time, however, there is no absolute proof that these changes are caused simply by the chemical phenomena happening in the tissues, and on which it is usually assumed the phenomena of life depend. It is possible that the electrical changes may be of a different order, and that what we call vitality is dependent, not only on physico-chemical changes, but also on those more subtile phenomena which we call electrical. Electricity, in its essence, is just as mysterious as life, and we are yet far from being able to correlate the two classes of phenomena. We may be helped toward this consummation