When the paralysis is limited to a group of muscles, to one member, or to one side, the behavior of the paralyzed muscles is compared with the normal. If, however, as is comparatively infrequent, symmetrical muscles are paralyzed, their condition must be contrasted with that of another healthy subject.

The paralysis of muscles may be complete, and yet they react in a normal manner. In other cases, there may be merely quantitative changes; that is, there is a mere increase or diminution of electric excitability, the polar reactions conforming to the normal, in order. In the third group, the paralyzed muscles are found to be changed, not only quantitatively but qualitatively, in respect to their responses to the electrical excitation. If the muscles respond in a normal manner to both forms of current, it may be concluded that the spinal cord connected by nerve-fibers to the paralyzed part is free from disease.

The changes in respect to the electric excitability, in some cases of paralysis, consist merely in a quantitative increase to both currents: the muscles react to a less strength of current, or more energetically to the same strength. In the latter, while the normal formula is developed by acting on the healthy muscles, on the paralyzed side the reaction is more decided, as indicated by a cathodal closing contraction (Ka S Z) occurring with a very weak current, or Ka S Z becoming, on the application of the same current, a tetanus (Ka S Te); also indicated by the early appearance of an anodal opening contraction (An O Z) and the prompt appearance of a cathodal opening contraction (Ka O Z). A hemiplegia of recent occurrence, some cases of spinal paralysis at a very early stage, and very rarely the earliest manifestations of paralysis from nerve-lesions, are illustrative of this state.

The usual condition for which the electrical currents are employed in diagnosis is diminution or loss of electric excitability. Spinal, motor-nerve, and muscular lesions are the most important. "Reactions of degeneration" is the happy term employed by Erb to signify the changes in the electrical reactions. Owing to disease of the cord, or of the motor-nerve trunks, degenerations of tissue ensue, and hence the term. For a very brief period, in some cases, there is an increase of electrical excitability, but a decline then quickly ensues. As regards faradism, the strength of current necessary to cause a contraction of the affected muscles must be constantly increased, and in a short time no strength of current will cause the least movement. The normal formula for the galvanic current is changed, pari passu, with the decline of faradic excitability. First, the cathodal closing tetanus ceases (Ka S Te), then anodal closing contraction (An S Z), and finally cathodal closing (Ka S Z) can be excited only by the strongest current. These changes represent a gradually increasing atrophy of the muscles, and the final cessation of the cathodal closing contraction signifies an extreme degree of atrophy, and the disappearance of the muscular elements.

In the condition known as the reactions of degeneration, it is important to distinguish between the reactions of the motor nerve and of the muscles. It has been already pointed out that, whether the motor nerve supplying it, or the muscle itself is acted on, muscular contractions take place. In the changes which ensue in cases of paralysis, the state of the nerve is separable from that of muscle. If the paralysis is due to a lesion of the nerve-trunk—to an inflammation of the nerve, for example—there may be a brief period when, as above stated, the electric excitability is heightened; but, as the nerve undergoes degenerative atrophy, there ensues a quantitative decline in the response to electrical excitation, to both faradic and galvanic, and by the twelfth day, sometimes earlier, it has usually entirely disappeared, if the lesions have proved destructive and irremediable. On the other hand, if the injury done is less, and is remediable, the electric excitability is modified only, and not lost. When recovery from the injury, or inflammation of the affected motor nerves takes place, the muscles innervated will respond to the impulses of the will, long before they react to faradic or galvanic stimulation.

When the spinal cord is the seat of disease, as in infantile paralysis, glossolabiolaryngeal paralysis, progressive muscular atrophy, etc., the paralyzed nerves and muscles exhibit most characteristic electrical reactions. As regards the motor, nerves, in two or three days, usually, after the paralysis has manifested itself, a regular and steady quantitative decline in excitability to both forms of current takes place, and by the end of the second week, usually, and sometimes by the end of the first week, no strength of current applied to the nerve will cause muscular contractions. At first, cathodal closing ceases, then anodal closing, and finally anodal opening.

The muscular reactions are much changed from the normal. The muscles, in about a week after the paralysis appears, begin to decline in their excitability to the faradic current, and, at the end of two weeks, it is totally lost, and they cease to respond to any strength of application. If regeneration of the diseased nerve-tissue can be effected, restoration of the faradic excitability may be accomplished, but to a less extent than before.

The phenomena connected with the galvanic excitability are very different. For the first week of the paralysis the response of the muscles to the galvanic current declines, as it does to the faradic; but, after the second week, a remarkable change ensues: then the galvanic excitability begins to increase, and with this there occur qualitative changes in the order and mode of muscular contractions. These consist in a gradual increase of the anodal closing contraction, which soon equals if it does not surpass the cathodal closing, and the cathodal opening contraction declines in the same measure. In other words, an actual reversal takes place of the normal formulae. If the degenerations continue, and the muscular elements are finally destroyed, the reactions ultimately cease, the last to disappear being a very feeble anodal closing contraction. Such are the reactions of degeneration. They occur in cases of spinal paralysis, when the disease in the cord is in direct anatomical association with the paralyzed parts, and in cases of peripheral paralysis when due to injury or disease of nerve-trunks. The so-called infantile paralysis is an illustration of the former, and facial paralysis of the latter. When the disease is situated in the cord above the point from which nerves are given off to the paralyzed members, there is no change in the law of muscular contraction. When, for example, a transverse myelitis exists entirely above the dorso-lumbar enlargement of the cord, the muscles of the lower extremities, although paralyzed, react normally to the faradic and galvanic currents. Again, in disseminated myelitis there are groups of muscles that react in accordance with the law of normal contraction, and other groups that manifest the reactions of degeneration— the former being in anatomical connection with a healthy part of the cord; the latter with a diseased area. It follows, therefore, that a proper electrical examination should be made as a means of diagnosis in the diseases of the nerve-centers.