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.
In describing the instruments for the development of static and galvanic electricity, and the effects of these two modes of electrical excitement, I have probably said as much as may be necessary in relation to their method of application. But the following observations upon the application of induced electricity, derived almost exclusively from the work of M. Duchenne, appear to be necessary, to place the reader on a level with the state to which therapeutical electrization has been brought by that indefatigable investigator.
Faradisation of the Muscles (Duchenne).* - To affect the muscles, we may operate either through the nervous plexuses and trunks, or directly on the muscles themselves. In the former case, we necessarily influence at the same time several associated muscles; in the latter, we may limit the action to a single muscle.
* The term faradisation was invented by M. Duchenne, and applied in honour of the distinguished chemist, Faraday, who has done so much for electrical science. As the name of electricity is given to the static form of this agency, and galvanism to that developed by contact, with chemical action, M. Duchenne considered himself authorized to give that of faradism to the induced form of dynamic electricity, and faradisation to its application. (Note to the second edition).
The two excitors, or terminations of the directing conductors, should be placed near to each other, at distances varying from one to four inches. If the skin is well moistened, the electric influence penetrates readily through it to the parts beneath. When it is required to operate on the larger muscles, as those of the trunk for example, the cylinder with the moist sponge (Fig. 1, page 509) should be used. For small muscles, as those of the face or intercostals, or for deep-seated muscles, the conical excitors (Fig. 2, page 509), covered with thoroughly moistened leather, as the finger of a glove, are to be preferred. The latter also are preferable when it is desirable to excite the muscles through a nerve. The moist skin is a better conductor than the wet sponge; and, when a powerful effect is demanded, the conical excitors may be sometimes advantageously used even for large muscles, being in this case moved from point to point. To apply the instrument efficiently, the practitioner must of course have made himself acquainted with the precise situation of the muscles, and course of the nerves supplying them. Their depth below the skin must also be well understood. The influence is never to be directed to the tendons. To act on a muscle duly, its whole surface must be covered; and consequently, if this is large, the excitors must be moved from point to point successively until the purpose has been accomplished. The muscle is known to be contracting by its firmness or hardness under the fingers; and it often happens that one part of a single muscle will be relaxed, while another contracts. The thicker the muscle, the more intense must be the current applied. In the face, it is difficult to avoid affecting the nerves so abundantly distributed over it. One of these is known to be touched when the contraction extends to several muscles simultaneously. Should this happen, the excitors should be moved a very short distance from the point, until the effect is no longer produced.*
When a muscle, on account of its depth, cannot be reached directly by the electric influence, it may be excited by means of its supplying nerve. The excitement of a nerve, or of a muscle, always produces in the healthy state both a sensation and contraction. But the susceptibility of different nerves and muscles is very different in degree; and a force which will affect one but slightly, will on another act with great energy. Again, while one part is unusually excitable in relation to contractility, another is more so in relation to sensibility. It is against the latter that the operator must be particularly on his guard, prepared to withhold his hand, or diminish the force of the instrument, when the influence becomes excessive. Sometimes the sensibility to pain is so great, as to preclude this method of electrization. It is apt to be particularly strong in the muscles of the face, supplied by the fifth pair. The excitor should never be placed over the points corresponding with the suborbitar or mental nerve; and the excitation of the frontal nerves produces severe pain, which radiates through the head. The muscles of the eyelids, of the alae nasi, and of the upper and lower lips, are peculiarly susceptible. Of the muscles of the neck, the platysma myoides, the upper half of the sterno-mastoid, and the external edge of the upper half of the trapezius, are much more excitable than the remainder. The great pectoral and the muscles of the subspinal fossa are rather sensitive; the deltoid and the muscles of the arms somewhat less, the anterior being much more so than the posterior. The long dorsal and the sacro-lumbar are but slightly sensitive. The gluteal and fascia lata muscles are very much so, compared with those on the outer and posterior parts of the thigh; those of the internal crural region more so than those of the external. The posterior muscles of the leg are but slightly sensitive compared with the anterior and external.
* M. Duchenne found that the muscles contract most readily if certain points are touched by the excitors. Dr. R. Remak, of Berlin, ascertained that these points correspond with the points at which the nerves enter the muscles; and that the degree of contraction produced is exactly proportionate to the number of motory nerve fibres embraced by the current at its place of entrance. (Med. Times and Gaz., May, 1858, p. 479.) - Note to the second edition.
At the moment of contact, even when the surface is moist, severe pain is sometimes felt in the skin, which soon ceases. In such cases, in order to obviate the effect, the excitors should be brought into contact before application, so as to restore the equilibrium, and then gradually separated to the necessary distance.