By means of the faradic brush, and pointed electrodes closely approximated, the state of sensibility of the skin may be readily marked out. The skin must be carefully dried to prevent diffusion of the current, and then the brush or electrodes conveying the induced current must be carefully passed over the supposed anaesthetic and analgesic area, and the outlines of the affected surface thus ascertained.

The galvanic current by the polar method has been very successfully applied to determine the state of the auditory nerve. The pole intended for excitation is introduced through a suitable non-conducting speculum into the ear, previously filled with warm water, and the other pole is placed on the mastoid process, nape of the neck, or any indifferent point. Brenner, of St. Petersburg, who has contributed the most of the exact knowledge now in our possession regarding these auditory reactions, has invented resistance-coils to be utilized in these researches. A strong current, reduced to the necessary point for acting on the auditory nerve by the introduction of sufficient resistances, is passed through the organ, and the resulting sounds, subjective, and audible, of course, only to the patient, indicate the condition of the nerve. These sounds, whistling, singing, roaring, etc., have been reduced to formulated expressions. Although there are differences of opinion in regard to the real value of these auditory reactions, it seems to be now well established that Brenner's method and formulae are correct in the essential details.

The state of gustatory sensibility is most correctly ascertained by galvanic stimulation of the end-organs of the nerve. For this purpose a pointed electrode, the positive and negative as closely approximated as possible, is carefully passed over the area of distribution of the sense of taste, and its condition noted.

Feigned paralysis may sometimes be detected by faradic stimulation of the muscles, and feigned insensibility by a strong current. Hysterical paralysis is remarkable for the absence of electro-sensibility and the presence of electro-contractility.

Therapy

Manipulation.—Electrodes for the application of galvanism or faradism are of various shapes and materials. The best probably, is a disk of carbon covered with wash-leather. A metal disk covered with soft sponge is also much used. The size depends on the purpose to which applied. When large volume and high intensity are the qualities of the current, the electrode should be of large size, of soft sponge, well moistened. For application to small muscles, Duchenne's olive-shaped, curved electrodes are most suitable; and to individual nerves, metal buttons of various shapes covered with wash-leather. The instrument-makers are now supplied with all forms and kinds of electrodes, so that the operator can always obtain what form of electrode soever he may desire.

When it is intended to act on parts beneath the skin, the electrodes and skin should be well moistened, for the conductivity of the tissues is in direct ratio with the amount of water they contain. On the other hand, if the skin alone is to be acted on, it should be well dried to prevent diffusion of the current. When weak galvanic applications are made, but little tingling is felt by the patient, and hence he may suppose that no curative effect is produced. Under such circumstances, it were better to add a little salt to the water with which the electrodes are moistened.

Electrical applications should always be made to the affected part, and also to those parts in which decided symptoms are felt. The principle of localized electrization, as established by Duchenne, was a most important advance. Next, the polar method did much to give exactness to methodical applications. When the poles have a fixed position, and are not moved, the application is said to be stabile, and, when moved over the part operated on, labile. If the direction the galvanic current is taking is parallel to the nerve-current, or from the center toward the periphery, the application is said to be direct, or descending; if in the opposite direction, indirect, or ascending.

General electrization is a term used to indicate the application of either current to the whole surface of the body—one pole placed on the nape of the neck, or to the feet, and the other passed over the whole surface of the body. Central galvanization is a term invented by Beard and Rockwell to signify applications to the cervical sympathetic, to the pneumogastric, to the cervical and dorsal parts of the spinal cord, and to the solar plexus. If one pole be placed on the seventh cervical vertebra, and the other in the fossa behind the angle of the jaw; if the former be kept in this position, and the latter put on the epigastrium; and, lastly, if the first be moved down the spine to a point opposite the second—there will be brought into the circuit, successively, the ganglia of the cervical sympathetic and their cardiac branches, the pneumogastric, spinal accessory, phrenic, the semilunar ganglion and solar plexus, and the spinal cord.

In what mode soever applied, and at what point, more or less diffusion of the current takes place. Although the current flows from the higher to the lower potential by the most direct route, a greater or less deflection is caused (diffusion) by the resistance encountered on the circuit.