We come now to the electrical phenomena exhibited by a nerve when it is brought, at any point in its course, into the circuit of a constant battery-a condition to which Du Bois-Reymond has given the name of Electrotonus. This observer' found that, when we galvanize a piece of nerve by means of a current of uniform power, the original inherent nerve-current undergoes a change, and, indeed, is increased in power, when the artificial current has the same direction as the inherent current; on the contrary, the inherent current is weakened, or entirely overcome, when the artificial current flows in the contrary direction. These manifestations of the electro-tonic condition are designated in nerve physics by the following expressions: The piece of nerve between the poles of the battery is called the irritated, the piece in the circuit of the multiplicator is called the derivative. If the nerve-current is increased, it is said to be in the positive phase; if it is decreased, it is said to be in the negative phase. The increase takes place when the positive electrode (the anode) is nearest the transverse section, the decrease when the negative electrode (the cathode) is nearest. Since the nerve has a transverse section on both sides of the constant stream, the two conditions appear simultaneously on each nerve. On the side of the positive electrode the nerve-current through the electrotonus is increased (positive phase of the electrotonus, or anelectrotonus); on the side of the negative electrode the current is weakened (negative phase of the electrotonus, or catelectrotonus). The electrotonic condition appears simultaneously with the closing of the generating battery, continues so long as the battery remains closed, and disappears simultaneously with the opening. The degree of the electrotonic increase, as well as of the deflection of the needle caused by the increase, depends on a variety of circumstances. It increases, not only with the length of the galvanized piece of nerve, but also with its proximity to the 1 L. c., vol. ii., pp. 289-889.

piece of nerve introduced into the circuit of the multiplicator; it is disproportionably greater when the exciting current flows longitudinally than when it crosses the nerve at a right angle. The degree increases further with the density of the exciting current; it, however, soon reaches its maximum, beyond which no increase takes place. It reaches finally, under like conditions, in a fresh and vigorous nerve, a point from which it descends with the decrease of nervous vigor, disappearing when the physiological powers of the nerve cease. If a portion of a muscle be traversed by a constant current, it will also be placed in the electrotonic condition. The electrotonus of the muscles differs, however, from that of the nerves in the following particulars: 1. It continues to increase in strength after the electrotonic current has ceased. 2. It is confined to the portion through which the current flows, while the electrotonus of the nerve extends with diminished strength beyond this limit on both sides.

The illustration we imagined to explain the phenomena of the quiescent current-the current produced by a regular series of peripolar molecules embedded in a moist layerwill serve to illustrate the electrotonus produced by the dipolar-nerve molecules that are arranged in the manner of the Voltaic pile. In this case all the nerve-molecules lying between the electrodes would be so arranged, that they would turn their negative elements toward the positive and their positive elements toward the negative electrode. This is easily imagined if we suppose each peripolar molecule of the quiescent current to be composed of two dipolar molecules with their positive zones touching each other-such molecules as they are resolved into at the closing of the battery. This illustration explains all the phenomena of the electrotonic condition. On the one hand, it shows how opposite phases are exhibited at the ends of a nerve traversed by an electric current; on the other, it explains the degrees of the electrotonic condition incident to the density of the current, the proximity to the electrodes, and finally to the

angle at which the electrode is brought into contact with the

nerve.

If an uninterrupted electric current be made to pass through a nerve in any part of its course, whether in the same or in various directions, the nerve is said to be tetanized -an expression that is justified by the fact that a muscle. connected with a nerve treated in the manner just described responds, not only with one simple convulsive jerk, but with a series of twitches which terminate in a contraction of considerable duration, so that the muscle may be said to be in a tetanic state. If the tetanized currents flow in the same direction, the interruptions succeed each other at certain intervals; and, if the intensity of the current is inconsiderable, the action becomes similar to that of the continued current, and the phase of the electrotonic condition appears. If the tetanized currents flow in the same direction, appearing only momentarily, both phases soon follow in their usual form; the positive, however, is generally weaker. In this case the nerve-current quickly decreases, as in the use of Saxton's battery. If, finally, the tetanized currents flow in various directions, the interruptions follow each other in rapid succession, as with the Volta induction-apparatus with a Wagner hammer attached. Thus, under all circumstances, the negative-current oscillation is secured.

Of the action of quiescent electricity on the animal tissues on the parts or the whole of the animal body we know nothing. Furthermore, much as has been said of the effect of the electric tension of the atmosphere on the physical condition, etc., there have thus far been no experiments made which furnish data to justify the assertion that the free constant tension long continued exerts a demonstrable influence on any animal part whatever.

FOURTH SECTION.

THE ACTION OF THE ELECTRIC CURRENTS ON THE ORGANS AND TISSUES OF THE ANIMAL BODY.

A. The action of the electric current on the nerves and

muscles.

Ludwig's Lehrbuch der Physiologie, vol. i., pp. 102, seq. E. Du Bois-Reymond, Untersuchungen über thierische Electricität, vol. i., pp. 303-409-282, seq.,

I.

etc.

Mémoire sur l'emploi de l'Électricité en Méd. par le Dr. H. Valerius. Annales de la Société de Méd. de Gand, vol. xxix., pp. 115-154. A. Fick, Die Medicinische Physik, 1856, pp. 437, seq. H. Wundt, Lehrbuch der Physiologie des Menschen. Erlangen, 1865, pp. 430, seq. R. Heidenhayn, Physiologische Studien, Berlin, 1856, Art. III., p. 55. Uber Wiederherstellung der erloschenen Erregbarkeit durch constante galvanische Ströme. C. Eckhardt in Henle's und Pfeuffer's Zeitschrift, 1853, vol. iii., pp. 187, seq. Eckhardt, Beitrage zur Anatomie und Physiologie, Heft I. Giessen, 1855, Art. II. Ueber den Einfluss des constanten galvanischen Stromes auf die Erregbarkeit der motorischen Nerven. E. Pflüger in der Medicinischen Central-Zeitung vom 15. März und 16. Juli, 1856. Ueber die durch constante Ströme erzeugte Veränderung der motorischen Nerven. E. Pflüger, Ueber das Hemmungsnervensystem für die peristalischen Bewegangen der Gedärme, Berlin, 1857. R. Remak, Galvanotherapie des Nerven- und Muskelkrankheiten, Berlin, 1858. E. Pflüges, Untersuchungen über die Physiologie des Electrotonus, Berlin, 1859. A. von Bezold, Untersuchungen über die electrische Erregung der Nerven und Muskeln, Leipzig, 1861. H. Ziemssen, die Electricität in der Medicin, III. Auflage, 1866.

ACTION OF THE CURRENT ON MOTOR NERVES AND MUSCLES.

Ir a motor nerve be subjected to the action of only a moderately intense INTERMITTING CURRENT, there will follow, in all the muscles supplied by this nerve, a series of spasmodic contractions, so that, if the several closings and open

ings of the circuit follow one another slowly, clonic or intermitting spasms ensue, if they follow one another rapidly, in such manner that the new contraction begins before the preceding has ceased, rigid or tonic spasm results. If this condition has continued too long, or if the nerve originally possessed insufficient irritability, intermitting spasms will ensue. In the vigorous muscle of a frog, in the beginning at least, tonic spasms may appear, if even there are not more than two strokes in a second; a less number produce clonic spasms from the first.

The phenomenon, that a muscle contracts only at the closing and opening of the circuit, or only at the moment when the density of the current increases from nought to a certain degree, or descends again from this degree, but not between these two points, when the circuit remains closed and the density remains unchanged, finds its explanation in the fact that the contractions depend on the changing of the density of the current with the greatest possible rapidity. For this reason Du Bois-Reymond gave the following as the first law of the electric-irritation experiments: "It is not the absolute grade of the current density at each moment to which the motor nerve, by a contraction of its muscle, responds, but to the change that takes place in the grade of density from one moment to another. Thus the power of producing the contractions that follow these changes is increased the more rapid the changes are when alike in degree, or the greater they are in a given length of time."

If we have, therefore, a given quantity of electricity, and conduct it in a current of unvarying power through a muscle, the current intensity remains the same from the closing of the circuit to the opening; consequently, during this time, no contractions take place. The same quantity, conducted in an interrupted current, will produce tonic or clonic spasms, according as the interruptions follow one an

1 L. c., p. 258.