muscle, a muscular contraction is set up by the faradic current, the same as that produced by stimulating the nerve-trunk. If the current be not too strong and the electrode not too large, the effect may be limited to the given muscle, or to a portion of a muscle. Every muscle may also be actuated by the galvanic current by applying the active pole on the point corresponding anatomically to the entrance of its motor nerve. A familiarity with motor points is of practical importance. They are shown in the accompanying diagrams. Slight variations in their location

It is

are common, and several trials are often required to find them. well to mark with a lead-pencil the necessary points, if repeated tests are to be employed. The galvanic polar responses are, in healthy muscles, identical with those obtained through the nerve-trunks.

In Disease.-Diseases changing the nutrition and structure of the lower neuron, which consists of the spinal cell-body and its peripheral projection and termination in muscular tissue, alter not only the trophic conditions over which the neuron presides, but also interfere with its

electric properties. If the spinal cell be destroyed, or its peripheral filament divided, degeneration follows below the lesion, in both the nerve and muscle, with loss of voluntary control. The reactions of such a nerve and muscle to electricity are modified in a highly characteristic manner. They present the reaction of degeneration (R. D.), the presence of which is an indubitable sign of structural change. At the end of about a week, during which there has been a gradual decrease of electric excitability in the muscle and nerve to both faradic and galvanic shocks, four differences become manifest:

First. The muscle responds weakly, sluggishly, and deliberately to faradism, and shows a tendency to maintain the contraction after the current is withdrawn. This is the modal change.

Second. The nerve-trunk loses progressively and equally its responsiveness to both galvanism and faradism-a quantitative change. Third. The muscle becomes much more excitable by galvanism and much less excitable by faradism, which latter reaction with the nerve-trunk responses is completely lost after two or three weeks. This is the qualitative change.

Fourth. A polar change appears in the muscle about the second week when directly stimulated by galvanism. It is now found not only that the muscle is more readily caused to contract by the constant

=

current, but that the normal mastering strength of the negative closing contraction over the positive has disappeared and that the positive closing contraction is equal to or greater than the negative. Expressed thus: A. C. C. or > C. C. C. Anodal tetanus (A. T.) is often obtained. This polar difference continues until the nerve either regenerates and regains its normal status, or, failing to recover, gradually the galvanic irritability subsides. It is entirely abolished with all other electrical responses in complete degeneration.

Should recovery take place, the restitution is marked ordinarily, first, by a return of voluntary control, then, by the appearance of galvanic and faradic excitability in the nerve, and, last of all, by faradic excitability of muscle. The hyperexcitability of the muscle to galvanism and

Fig. 14.-1, Paralysis with early return of motion (modified from Erb). 2, Incurable paralysis with complete atrophy and degeneration (modified from Erb).

the polar variations from the normal gradually right themselves at the same time. These changes and the pathological process in nerve and muscle are shown in the diagrams (Fig. 14) modified from Erb.

In some cases we find a partial reaction of degeneration, the nervetrunk response being only slightly modified. The inverted polar formula is not necessarily present. The most essential element in the reaction of degeneration is the modal change, the slowly appearing muscular response, so at variance with the instantaneous effect produced in health. Next in importance is the lessened faradic control of the muscle, and, last, the polar modifications. The variations in the galvanic and faradic responses in disease are due principally to the fact that currents of a certain strength and duration are required to actuate the changed sarcode cells, and the rapidly interrupted faradic current thereby becomes

inoperative. The galvanic current, if interrupted with sufficient rapidity, is equally powerless.

In some conditions is found a simple increase or decrease of electrical excitability, constituting a pure quantitative change. In those diseases where the muscles seem permanently affected, as in the progressive atrophic myopathies, the responses are reduced, but are proportionate to the amount of healthy muscular fibers remaining. Some spasmodic conditions, like facial tic, present an increased activity to electric stimulation. There is increased electrical excitability in tetany, Graves' disease, and myotonia.

The use of electricity to test cutaneous sensation is open to so many errors and attended by so much difficulty that simpler means, always at hand, are to be preferred. In those feigning complete cutaneous anesthesia the surprise of a strong, painful, faradic current from a dry, metal electrode will be likely to unmask the deceit.

When the active electrode is placed over the closed eye or at the temple, the make or break of constant currents is attended by a subjective sensation of light, and this test serves to show the activity of the retina and optic nerve. By placing the indifferent electrode on the same side of the neck, one eye at a time may be examined, and for this purpose only small currents-from two or three cells or of as many milliamperes -should be used. In the same way either auditory nerve may be tested by placing the active electrode over the mastoid or before the tragus, the negative pole producing much the louder sound.

For testing the nerves of taste the galvanic current is a prompt and reliable agent. By those of keen perceptions a difference between the positive and negative pole can be distinguished. For the ordinary purpose of taste testing, electrodes ending in probe-pointed metal terminals, or even the metal ends of conducting cords held to the tongue or pharynx, at once give rise to an acid or salty taste, which is quickly appreciated by the patient. One or two cells are all that is required, and one side of the tongue can be easily compared with the other in lateral lesions, or the tongue of a healthy individual will furnish a standard when needed.

For the purpose of localizing the various muscular areas in the sensori-motor zone of the cerebral cortex, when it is exposed surgically or otherwise, a mild faradic current is used. It is applied to the brain surface by two probe poles, preferably mounted on a single handle, and so arranged that their extremities may be fixed at any distance apart. Such a current thus applied causes, in the related muscular periphery, similar responses to those produced by faradizing the nerve-trunk, but the movement has more of a purposive or gesticulatory character. These responses also follow an extradural application of the electrodes.

CHAPTER VI.

SENSORY CONDITIONS.

MODIFICATIONS of sensation are among the commonest conditions attending nerve-lesions and functional disturbance of the neural apparatus. It is well to have a prefatory understanding of the fact that sensation, of whatever variety or quality, is due to the appreciation of temporary motion and contact, or, in simpler terms, that all sensations are modifications of touch. It is the impact of luminous and soundwaves on properly qualified nerve expansions that gives rise to sensations of sight and hearing. The contact of odorous and sapid particles stimulates the sensations of smell and taste. The recognition of the various qualities of objects in contact with the skin gives us information of size, shape, temperature, hardness, smoothness, and a thousand physical properties of the external world.

Another general consideration is that any overstimulation of sensory nerves is painful. That which is so readily called the pain sense is but the subjective recognition of this overstimulation coupled by instinct and experience with the concept of harm. Thus the contact of a sharp point is at first so recognized and the sense of pain comes when the pressure is increased and the stimulation intensified. It can not be doubted, however, that pain and thermic sensations have courses in the spinal cord separate from tactile impressions. Extremes of heat, cold, pressure, noise, light, odors, and tastes give rise to pain or to discomfort analogous to pain. They cause automatic or conscious efforts at avoidance and self-preservation. It is also a matter of common experience that sensations, except of an extremely painful character, if long maintained, are more or less completely ignored. Therefore, to elicit sensation the motion or contact must be temporary or repeated after distinct intervals. Clothing to which we are habituated gives rise to little or no conscious sensation. With the long persistence of an odor there is a gradually diminishing perception of it. On the other hand, the sudden stopping of a clock or other familiar and disregarded sound is almost startling.

In considering the various qualities and modifications of the sense of touch it is requisite to bear in mind that keenness of sensibility varies greatly not only in different cutaneous areas in a given subject, but considerably in different individuals. The more keen, active, and intelligent the make-up of the subject, the quicker and more precise are the responses. To all tests of sensation the disadvantage pertains that we are dependent upon the responses of the of the person tested; so that such tests are only partly objective. Sudden extreme pain, however, is usually attended by certain recognizable signs, such as change of countenance, quickened pulse, dilated pupil, pallor, and even perspiration. To carry out the needful tests we require the intelligent coöperation of the patient, and to this end he must understand exactly what is sought. In order