arrangement is termed "electro-magnetic," the latter "magneto-electric;" in the one, electricity is developed from chemical decomposition, in the other, from magnetism; but in the former-inasmuch as magnetism is induced by the initial current-there is in addition to the "primary induced current," that order of induction which exists alone in the latter, and the addition of this is one mode of augmenting the intensity of the current. Thus, then, the initial current developes magnetism in the bars of soft iron which are inserted into the hollow of its helix, and the presence of magnetism in these bars, at the moment of its induction, developes an electrical current in the copper wires; the intensity of the latter induced current, being, cæteris paribus, in proportion to the size of the temporary magnet, and determined or regulated by the length to which these soft bars are inserted in the helix. The tension, therefore, of the induced current depends upon that of the initial current, upon the size of the wire, upon its length-i.e., upon the number of turns in the spiral-and upon the force of magnetism temporarily developed in the bars of soft iron. Whatever form of current is employed, the nature and degree of its physiologic effects-i.e., of its power to occasion vital phenomena, as distinct from chemical and thermal-are determined mainly by differences in this quality of tension. Generally speaking, a weak current produces feeble contractions of the muscles, and slight effects upon the organs of sensation; whereas a powerful current produces strong contractions and violent sensations. Both sensory and motor phenomena may be occasioned by the application of any one of these currents, but their variations in intensity render some more useful for one class of effects, and others for a second class. Thus, Duchenne has drawn considerable attention to the fact that the "extra-current" acts very readily on the muscles, and that the "induced current" affects more powerfully than the extra-current, the skin, nerves and retina. This difference of action he refers to a special elective power on the part of the two currents respectively; but Becquerel has proved that, in reality, it is merely dependent upon the difference of their intensity, the induced current having much greater tension than the extra-current. M. Becquerel has shown, by a simple experiment, in which he modifies the arrangement of the wires, that the effects which Duchenne attributes to the one current may be obtained from the other, and vice versa, p. 90. In proportion to the intensity of the current employed, electricity has the power of evoking the ordinary physiologic action of a nerve or muscle; of occasioning excessive and perverted action; of exhausting the functional activity for a time; or of destroying it altogether. In the first degree there is sensation or motion, each of these being within the limits of physiologic function; thus, luminous appearances, gentle sounds, gustatory effects, &c., on the one hand, and slight muscular contraction on the other, contraction so slight as merely to exhibit the persistence of muscular contractility, and not to test its power, are the results of applying an electric current of low intensity. If a stronger current is employed, the impressions upon the sensory organs become excessive in degree and painful in character; while, in the place of gentle muscular contraction, there is distressing cramp, or arrested (inhibited) action in certain organs. A still more violent current exhausts both nerve and muscle; and here sensation and contraction, though for a time withdrawn, are capable of being restored by repose, or by the inverted current; whereas the electricity may be so powerful as at once to put an end to the vitality of the tissues-i.e., to kill the nerve, limb, or individual through which it passes. It is owing to these different effects of variations in intensity that electricity may be employed both physiologically and therapeutically for so many different purposes. As a test of irritability, or a gentle stimulus of weakened sensibility and contractility, the current of low intensity may be employed. For the sake of displaying the inhibiting influence of the vagi and the splanchnic nerves, or for awakening the torpid nervous centres of an individual poisoned by opium or alcohol, a more powerful current is required. Whereas for the relief of excessive mus cular contraction, or of neuralgia, a still more intense current, one that shall temporarily exhaust the nervous function, may be employed. Besides the quantity and tension of a current, the mode of its transmission exerts a notable influence upon its physiologic effects. Under this head we place the different actions of the continuous and interrupted currents; and with regard to the former, the changes produced by altering their direction; and with regard to the latter, their convection by means of moist or dry conductors, the rapidity or slowness of their interruption, and the degree of pressure with which the conductors are applied. The most general differences between the effects of the continuous and interrupted current, are displayed very simply by an arrangement of M. Claude Bernard's, in which there are introduced into the same current from a small Cruickshank's battery; 1st, the nerve of a frog's leg, and 2nd, a delicate voltameter; the apparatus being so constructed that the current may be either continuous or intermittent. By this arrangement, says M. Bernard, it is shown that— "so long as the current is continuous, chemical effects are produced, and the physiological effects are 'nuls,' or at all events inappreciable. The facts are, that the water in the voltameter is decomposed by the current, whilst the limb of the frog remains perfectly motionless. But immediately that, by means of the interrupter, the current is rendered intermittent, everything is changed; the decomposition of water ceases in the voltameter, and the frog's limb becomes violently convulsed." 11 But this experiment, although it illustrates very aptly the broadly marked difference between the effects of the continuous and intermittent current, by no means exhausts the subject of that difference, nor does it accurately represent all the facts. For the continuous current is not devoid of physiologic action, nor is the interrupted, under all circumstances, incapable of acting chemically. True, there is no visible contraction of the frog's leg, but under certain conditions the irritability of the nerve is exhausted, and under others it becomes increased. True, there is no sign of sensation in an amputated frog's leg, but the continuous current can produce sensory effects; for the proof of which let any one pass a continuous current through his tongue, or eyeballs; or, as Purkinje did, through the ears. And, further, it is quite easy to produce permanent, i.e. tonic contraction of a muscle or group of muscles, as we have often done, by a current of this kind; and there is evidence to show that not only persistent contraction of muscles may be relaxed by such influence, but that hyperesthesia may be reduced.§ Here then we have evidence of four kinds of physiologic action due to the continuous current, viz., the production of sensory effects, and also of motor, as well as the relaxation of spasm, and the reduction of hyperesthesia, the different manner in which the current acts being mainly due to its intensity. Other circumstances, however, influence the quality and degree of action exerted, viz., the direction of the current. Generally speaking, the transmission of a continuous current through a nerve, in the direction from the centre to the periphery, exhausts the vital property of the nerve; whereas, a current passed in the opposite direction, i.e. from the periphery towards the centre, increases the vital property. The former is termed "direct," the latter "inverse." Again, the direct current acts more energetically than the inverse in producing muscular contractions. This we have often witnessed, when employing, for the purpose of experiment or therapeutic application, an ordinary Cruickshank's battery, and so making use of the initial current that its intensity could be regulated and measured by varying the number of plates employed. Not only is the muscular contraction produced by transmitting a current from twenty plates, much stronger when this current is direct than when it is inverse, but a current of such low intensity as to cause no appreciable contraction when transmitted in the latter direction (inverse), will occasion very evident action when passed in the former (direct). Leçons sur la Physiologie et la Pathologie du Système Nerveux, 1858, tome 1. p. 151. + Rust's Magazin, bd, xxiii. p. 297. Remak, Medical Times and Gazette, May 8, 1858. § Becquerel, p, 97. Thus the difference between these currents must be remembered in testing irritability, as well as in testing power. It is sometimes a source of fallacy in physiologic experiments; as, for example, in examining the irritability of muscles in a paralysed limb, by passing the current from one arm to the other. In this case, it is, of course, direct in one arm, and inverse in the other; and we have frequently seen the difference between the irritability of the muscles on the paralysed and nonparalysed sides so slight as merely to equal, or even fall below that which exists between the action of the inverse and direct current respectively. When such is the case, the irritability appears greater in that limb through which the direct current passes, whereas it may be really less. Two conditions affect the result of applying the interrupted current; one of these being the rapidity of intermittence, and the other the degree of contact which is ensured. Dr. Lawrance states that "if we cause a paralysed muscle (whose irritability is normal), for instance, the flexor communis digitorum, to contract alternately with quick and slow intermissions, hanging weights at the same time to the fingers acted on by this muscle, we shall find that a rapidly intermittent current does not enable the muscle to raise a heavy weight so readily as one which intermits slowly."* When the intermittence is extremely rapid, the effects resemble, pro tanto, those of the continuous current, viz., exhaustion of motor and sensory functions; whereas in proportion to the integrity of contact, there is, cæteris paribus, a relative depth of effect. Thus, if dry conductors are placed upon the dry skin, the skin alone is irritated, whereas when moist conductors are applied with pressure the underlying muscles are affected. To these differences attention has already been directed in an earlier number of this journal. The physiologic effects of galvanism are in part determined by the organism, and this in two ways, 1st, by the special property or function of the organ (nerve or muscle), and 2nd, by its condition at the time of application. With regard to the first of these, let it be observed, that there are different kinds of action, and degrees of action, in the same organs, and that the effect of galvanism is, in relation to its intensity, to elicit these different degrees of action. Thus, a nerve may have the power of either causing, increasing, diminishing, or arresting activity in certain muscles; and any one of these effects may be produced by galvanism, the particular effect which follows its application being determined by its intensity. The experiment of Eduard Webert upon the cardiac branches of the vagus is familiar to every one; the subsequent researches of Pflügers have shown that an influence upon the intestinal movements is exerted by the splanchnic nerve, similar to that which the pneumogastric exhibits upon the action of the heart; irritation of the splanchnic nerves producing almost immediate arrest (erzeugt fast augenblicklichen Stillstand) of the peristaltic movements of the small intestines. (p. 66.) But the conclusion at which Pflüger arrived, viz., that there is a certain set of nerve fibres, the "inhibitory system," whose peculiar function it is to arrest or diminish action, is, we think, ably refuted by Mr. Lister,|| who has shown that the same nerves may either increase or diminish muscular contraction, according to the degree to which they are stimulated by galvanism. Mr. Lister thus "sums up" that portion of his paper which refers to the intestines: "It appears that the intestines possess an intrinsic ganglionic apparatus, which is in all cases essential to the peristaltic movements, and which, capable of independent action is liable to be stimulated or checked by other parts of the nervous system; the inhibiting influence being apparently due to the energetic operation of the same nerve fibres which, when working more mildly, produce increase of function." (p. 372.) On Localized Galvanism, p. 44. + Becquerel, p. 97. Wagner's Handwörterbuch. Art. Muskelbewegung. Uebur das Hemmungs-Nervensystem für die peristaltischen Bewegungen der Gedärme. Preliminary Account of an Inquiry into the Functions of the Visceral Nerves, with special reference to the socalled "Inhibitory System." Proceedings of the Royal Society of London, vol, ix. No. 82. A similar difference of effect, in dependence upon the degree of stimulation, has been demonstrated by Mr. Lister to exist in regard of the influence not only of the vagi, but also of "the sympathetic branches connecting the cord with the cardiac ganglia" (p. 378); and he has, as we think, very ably shown the error of the conclusion to which Professor Schiff arrived-viz., that the "inhibiting influence depends upon nervous exhaustion." (p. 379.) As the result of all these researches we may conclude that the different powers are inherent in the nerves, and that galvanic stimulation does but call one or the other of them into exercise, in the same manner that other "stimuli," or "occasions of action," are known to operate; for example, emotion, which may either accelerate, retard, or even arrest the action of the heart. Whatever, then, may be the properties of a nerve; whether they are sensory or motor; in relation to the particular properties of matter (light, taste, sound), or to peculiar conditions of the organism (fatigue, exhaustion, excitement); whether their function is to increase or to repress muscular activity; to occasion slight contraction, or persistent spasm; whatever these nervous properties may be, they can be called into operation by means of galvanism. But the special property which is elicited stands in definite relation to a certain amount of galvanic stimulus; and just as electric irritation of the retina produces the sensation of light, while a similar irritation of the crural nerve occasions muscular contraction, so a definitely proportioned galvanic stimulus of the splanchnic or pneumo-gastric nerves will elicit their property of increasing muscular movement in the intestines or the heart; and the same stimulus, differently proportioned, will call forth their inhibiting influence, and arrest the rhythmic or peristaltic action which they are destined to control. But further, the condition of the organism at the time of its exhibition very materially influences the effect of electricity. To this we have already partially referred in detailing the different results of the continuous current. A nerve in a state of hyperesthesia may be reduced in sensibility; an enfeebled nerve may have its dormant faculties aroused; a motor nerve, half-paralysed from inaction, may be stirred up to healthy exercise; while a similar nerve, so irritated as to induce tonic spasm, may have its augmented irritability brought down to the average standard. But besides these results, there are some due to the action of electricity which should be borne in mind. The nerve may be so affected by a galvanic current that it becomes partially or completely "exhausted;" and at the different stages of its exhaustion there are different phenomena. Thus Bernard has shown, that when first operating upon a motor nerve, there is a simple contraction in the muscles it supplies at the entrance of the current, whether the latter be direct or inverse. That after a time there is contraction at both the exit and the entrance of either current. That subsequently there is contraction only at the entrance of the direct current, and at the exit of the inverse; and that, finally, there is contraction only at the entrance of the former. These four phases he terms respectively-1, unique; 2, double; 3, alterne; and 4, ultime ;* and they represent different conditions of the nerve-function. In the first, there is the physiological result; in the second, there is, as we take it, somewhat augmented irritability; in the third, diminished irritability, or commencing exhaustion; and in the fourth, exhaustion carried to a further degree. It appears, however, probable that many of the positions hitherto considered to be established with regard to the irritability of nerves will be found to require considerable modification. Eckhard has recently shown that if a constant current is transmitted upwards through a motor nerve (i.e., inversely), that the irritability of the whole nerve is diminished; but that if it is directed downwards (i.e., directly) through a portion of the nerve-trunk, diminution of irritability is found only in those parts through which, and above which, the current passes, while below the negative electrode the irritability is augmented.† * Leçons cit. p. 185, + Schmidt's Jahrbücher, 1857, 11. p. 266. Another difference of effect, in reality dependent upon the part of the organism to which electricity is applied, but practically determined by the mode or locality of administration, is that which Duchenne pointed out between what he termed "direct and indirect faradisation." This Remak and Ziemssen agree in referring to the excitation of the muscular nerves, without or within the muscles themselves, and which Remak terms "extra and intra-muscular galvanisation." Ziemssen further shows that the value of extra-muscular irritation is this, that a feeble current will cause a whole muscle to contract, whereas with intra-muscular excitation a much more powerful current is required. (p. 6.) Much that has been said and written upon the subject of modern or "localised galvanism," as compared with the older method of applying that agent, rests simply upon the fact that whereas, years ago, the different tissues (skin, muscles, and nerves) were indiscriminately affected, now the action of electricity can be limited with considerable precision to either one of them. This we owe in great measure to Duchenne,* who discovered, clinically, that there were certain points of the surface, applied to which the electric currents acted more vigorously than when directed upon other points. Further experience has shown that, in the main, Duchenne was right; but a somewhat curious and unworthy controversy has arisen as to whom the credit belongs for having systematised and explained these facts. Dr. Robert Remakt claims for himself the discovery that these "boasted points of election" (diese berühmten Wahlpunkte) are simply those at which the muscular nerves make their entrance. But Duchenne replies, that he had already, in 1852, exhibited his modus operandi, and the effects thereof, to Dr. Remak, and that he had deemed it quite unnecessary to give an anatomical dissertation to those distinguished men who attended his demonstrations, and among whom was Dr. Remak himself; and that now, instead of a great discovery having been made by his former visitor, and present critic, all that he can suppose is that in 1852 Dr. Remak was wanting in that anatomical knowledge which in 1855 he appears to have acquired. Notwithstanding this reply by Duchenne, so lately as May, 1858,§ Remak thus describes his share in the investigation: "I was not a little curious to know the nature of these mystical points, and on directing my attention to this subject I soon found that they corresponded with the points of entrance of the muscular nerves, and that the degree of contraction of a muscle was proportioned exactly to the number of motory nerve-fibres embraced by the current at its point of application." gave a The truth of the matter appears to be that the practical discovery was Duchenne's; and that, whether or not he understood his own discovery, he did not distinctly explain it. Remak accomplished this part of the and process, theory to account for the result; and this theory has, we think, been most ably proved to be correct by the laborious investigation of Ziemssen, who conducted two series of inquiries, in one of which he determined clinically the precise localities of these "points of election," and marked them upon the skin; in the other he examined post-mortem the course of the nerves, especially their motor branches, and noted accurately their points of entrance into the muscles; and, upon quent comparison of the two series, he found that they agreed perfectly with each other. (p. 3.) subse In Dr. Lawrance's book some useful practical directions are given with regard to the points through which certain nerves and muscles may be reached most readily. (pp. 62 et seq.) Ziemssen has also added to our definite knowledge of the influence exerted by the organism upon electric application two further facts, one that the conductibility of the tissues is in direct proportion to the quantity of water they * Brit. and For. Med.-Chir. Rev., No. xxix. + Ueber methodische Electrisirung gelähmter Muskeln. Berlin, 1855. Medical Times and Gazette, No. 410, p. 479. |