A faradic current, whatever its direction, stimulating a motor nerve, causes contraction of the muscles innervated by this nerve. If the interruptions in the primary circuit are slow, the muscles have time to contract and relax; but when the interruptions are rapid, the muscles are kept in a condition of tonic contraction, or are tetanized. Long-continued, powerful faradic stimulation of a motor nerve exhausts its irritability. Muscles directly acted on by a faradic current contract energetically, but repeated over-stimulation will fatigue and ultimately destroy their contractility. When, however, muscles are in this condition of fatigue and exhaustion, from faradic stimulation, their proper functional state may be restored by passing a continuous galvanic current through them. If not too long applied or too powerful in action, faradic applications improve the nutrition of muscles, and through this increased activity of the circulation there occurs a rise of temperature, generally.

The Polar Method as applied to the Reactions of Motor Nerves and Muscles.—This important method of examining the condition of motor nerves and muscles must be understood, to rightly appreciate the modern mode of electro-diagnosis. As certain terms, having technical meanings, are much used in this connection, it is necessary to define them before proceeding to describe the phenomena. Anode is the term applied to the positive pole, cathode to the negative pole, and the adjectives, anodal, relating to the anode, and cathodal, relating to the cathode. As the polar method had its origin in Germany, the symbols now used to indicate electrical reactions are taken from German words. Charcot in France, Buzzard in England, and the American writers in general, have adopted the German formulae, so now their use is universal.

The polar method consists in exciting nerve or muscle with one pole, while the other rests on some indifferent spot, as the sternum, as advised by Erb. We are here concerned with the normal or physiological state; hereafter, the pathological conditions will be given. The normal formulae consist in the reactions which ensue on polar stimulation, nerves and muscles being healthy. When the current is closed by applying the cathode to the nerve or muscle to be excited, the symbols are Ka S (Ka, Kathode; S, Schliessung, closing). When the current is opened or broken by removing or disconnecting the cathode, the symbols are Ka O {Oeffnung, opening). Corresponding symbols are used in respect to the anode. Thus, the words anodal closing are An S, and anodal opening, An O. To express muscular contraction the symbol is Z (Zuckung); a strong contraction is Z'; for a weak contraction, z, and for a tetanic contraction, Te. In the physiological state, the muscular contractions as induced by the polar method are characteristic, and can be expressed in the symbolic language now employed for the purpose. It is to be observed, in the first place, that the cathode or negative pole has more power to induce muscular action than the anode or positive pole.

The cathode has more effect on closing the circuit; whereas the anode acts more powerfully on opening or breaking the circuit.

The normal formulae of muscular contractions induced by the polar method may be arranged in three grades:

In the first grade, the weakest current having power to induce any contraction causes it on cathodal closing — whence the formula Ka S Z; and no action takes place from the anode.

In the second or intermediate grade, the strength of current is sufficient to cause strong cathodal closing contraction (Ka S Z'), but no opening contraction; whereas, on anodal opening and closing there are feeble contractions—whence An S z, and An O z.

In the third grade, which is the highest, the current causes on cathodal closing a tetanic contraction, Ka S Te, and a feeble contraction on cathodal opening Ka O z ; whereas, there occur decided contractions on anodal opening, An O Z, and on anodal closing, An S Z.

Such are the normal formulae, such is the normal behavior of muscles; but in disease, as we shall subsequently learn, these formulae may be much changed, may be even entirely reversed.