This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
This includes both galvanism, strictly speaking, and the electricity developed by induction. Dynamic electricity differs, in its attendant physiological phenomena, materially from the static. It produces sensation, but in a much less degree, in proportion to other effects. Thus, a galvanic battery, capable of powerful chemical action, will give only a slight tingling sensation to the part at which the current enters; while the continuance of the current gives rise to effects which never proceed from a continuous current of machine electricity. The dynamic current causes also contraction of the muscles, and even energetic contraction, but without the violent shock to the system produced by the other form. Another important point of difference is that its influence can be directed to a certain part, and in a considerable degree limited to that part; so that a diseased muscle, for example, which may have lost its sensibility, and in which the electric stimulation may be strongly indicated, may be acted on by means of the galvanic or inductive current, with little or no disturbance or injury to neighbouring and more excitable tissues. This alone gives a vast superiority to this form of electricity over the static, as a therapeutic agent. For the methods of effectually localizing the action of galvanism, we are greatly indebted to M. Duchenne. Formerly no attempts of the kind were made, or quite ineffectually, until the method of acupuncture was applied by M. Sarlandiere to this object; but the necessity for this has been superseded by the methods of M. Duchenne, which, while less unpleasant, arc even more effectual.
In the physiological operation of galvanism, there are effects produced which cannot be completely separated, and which often interfere injuriously with one another when the agent is employed therapeutically. The current may be either continuous or intermittent; and the effects of the two modes of application differ materially.
The continuous current, while it produces sensation in the skin, exercises also an influence over the organic actions, giving rise to heat, irritation, and inflammation in various degrees, according to its power and continuance, sometimes ending in absolute cauterization. The most powerful current, if introduced into a muscle, occasions but slight, irregular, or partial contractions, while it causes a sense of burning heat, even in the depths of the tissues along its course.
In the intermittent current, the organic action of the battery, or its tendency to produce heat, inflammation, and disorganization, is diminished; while its power of exciting sensation and muscular contraction is greatly increased. This difference is readily explicable. At every breaking of the current, there are three physiological actions; one at tin-entrance of the current, one at its cessation, and a third in the interval. Sensation and muscular contraction are produced chiefly at the entrance, and much less at the moment of interruption, while in the interval, or during the continuance of the current, there is little comparative influence on the sensibility, none or scarcely any on muscular contractility, but a strong tendency to provoke inflammation or organic change. It follows, therefore, that if the current be frequently interrupted, sensation and contraction will be proportionably increased, and organic action diminished; and thus the intermittent current can be more effectively applied to the former purposes, the continuous to the latter.
There is one effect which, according to M. Duchenne, the galvanic current produces beyond all other modes of electric action. It has great influence on the organ of vision, and, if made to operate on the face or scalp, where the fifth pair of nerves, which are mainly sensitive, are distributed, it occasions dazzling luminous sensations. These flashes are produced strongly at the entrance of the current, feebly at its cessation, and very slightly, so as to be appreciable only in a dark room, during its continuance. By a rapidly recurring intermission of the current, there may be kept up a constant succession of the luminous phenomena. They are produced chiefly on the side to which the application is made, more powerfully as the median line is approached, and on both sides, when the excitors connected with the two poles are both placed at that line. An important inference is deducible from this fact, in determining the therapeutic application of the agency; namely, that galvanism is to be preferred when the indication is to excite the retina, and its application to the face avoided in the opposite indication.
I need scarcely state that the interruption of the galvanic current may be effected by withdrawing the excitors, and a succession of intermissions more or less rapid obtained, by their more or less rapid withdrawal and reapplication.
But there is much inconvenience in this manual operation; it is almost necessarily effected comparatively slowly and irregularly; and, even at best, the continuous current, while it lasts, is exercising its organic influence, and may at times be productive of great inconvenience. While, therefore, the galvanic battery is preferable in all cases in which the object is to excite inflammation or other organic change, it is highly desirable to obtain the means of exciting at will the nervous properties of sensibility and muscular contraction, without endangering the integrity of the tissues. Such means are supplied by the form of electricity developed by induction, whether through the electro-magnetic, or volta-electric instruments.
In the operation of the instruments above referred to, it is at the moments when the circle is closed, and when it is broken, that the effects are produced; no phenomena whatever being excited between these two points of time. While the circle remains closed, the electric force appears to be quite quiescent. This is the important point in which the induced electricity differs, in its physiological and therapeutical effects, from the galvanic. That the fact is as stated may be shown by a simple experiment. If a frog's muscle be placed in the electric circle, it contracts instantaneously when the circle is closed, then becomes perfectly quiescent, and continues so until the circle is broken, when it again contracts, and more strongly than at first But, though there are thus shown to be two actions, one at the closing, and the other at the breaking of the current, it is only the latter which is strong enough to be effective in the human subject; the one occurring at the closure of the circle being scarcely perceptible, though sufficiently powerful to produce contraction in the muscles of a frog. It is to this power of strongly exciting sensation and motion, without producing inflammation, that the inductive instruments owe their great superiority, as therapeutic agents, over other galvanic arrangements, for the general purposes which are aimed at in the use of electricity. However powerful their operation, or however long continued, though they may produce insupportable pain, and the most energetic muscular contraction, they never cause disorganizing inflammation; and, though a little erythe-matic redness of the skin may be produced, with erection of the papillae, the effect quickly subsides upon the cessation of the action.
It is obvious that, the more rapid the succession of the intermissions, the more rapid will also be the succession of the muscular contractions produced by them; and thus a method is offered of controlling the effect, to a considerable degree, by diminishing or increasing the number of intermissions. The contractions, however, are severally more powerful, when at long than short intervals; but, by their very frequent repetition, the muscle may be kept in an apparent state of steady contraction, similar to that produced under the influence of the will. There is a sort of vibratory movement in the fibres; but, to the touch, the muscle feels as though steadily contracting. An influence analogous to the healthful stimulus is thus obtained, which has a tonic effect on the muscle, and promotes its nutrition. Hence its application in cases of relaxation from debility, and in atrophy of the muscles. Though the contractions are more powerful at long intervals, yet, in relation to sensation, the more rapid the succession of intermissions, the greater is the effect. Hence, when it is important to awaken sensibility, as in cutaneous paralysis, the instrument should be made to act rapidly. On the contrary, a rapid movement is contraindicated in disease of the brain, upon which the pain may react injuriously; in cases of great natural susceptibility; and in operating on delicate organs, as the tympanum of the ear, different parts of the face, and the testicle.
But this is not all the merit of these machines. By varying their mode of application, different physiological effects are obtained, each susceptible of beneficial therapeutic use. Thus, the current of the two wires, the larger and smaller, or, as M. Duchenne designates them, the currents of the first and second order, differ materially in their effects. The machine with the double wire operates much more powerfully on the face and eyeballs in producing luminous phenomena than that with only one wire; and the effect, according to M. Duchenne, is much greater from the electro-magnetic than the volta-electric apparatus. Even with a feeble action of the former, considerable reaction ha produced upon the retina; while the latter operates in this way only when somewhat intensely excited, and when the excitors are applied to the emerging points of the fifth pair, or to the globe of the eye itself. The current from the first order (larger wire) of the electro-magnetic instrument produces no stronger an impression than the volta-electric. But the luminous phenomena, excited by these machines, even by the second current of the electro-magnetic, are much feebler than those which result from simple galvanism.
Moreover, the current of the first wire appears to have a special influence over muscular contractility, that of the second over cutaneous sensibility; that is, though both currents act on both properties, yet one produces a greater relative effect on the one, the other on the other. (Electrisation Localisee, pp. 15, 16.) Cutaneous insensibility will often yield promptly to the current of the second order (small wire), when that of the first has no effect whatever; but, in very susceptible persons, it will be advisable to have recourse to that of the first or larger wire, because less disturbing to the sensibility.