effect of local severe injury intentionally caused, a comparatively strong current is obtained, and it is highly probable that the comparatively weak currents observed from a carefully-exposed muscle are due to local slight injuries incidental to exposure. The direction of current when a local injury is inflicted is best understood by comparing it with the case of the Daniell cell.

The injured muscle is thus an electromotive element, comparable with the Daniell cell, and just as in the latter the zinc-at which most chemical action takes place-is the positive element, and has connected with it the negative electrode, so in the former the injured part-at which most chemical action takes placeis a positive element, and has connected with it a negative electrode. It is usual to describe direction of current with reference to its passage through the galvanometer, and to say that B, the injured part, is negative to A, the normal part. The expression negativity of injury' is sometimes used to express this relation of B to A.

A precisely similar mode of explanation is applicable to the effect of local action. At the seat of action most chemical change takes place; the active part is thus analogous with the zine of the Daniell element, and the current in the galvanometer is from part at rest to part in action, in the muscle itself from part in action to part at rest. Describing direction of current with reference to its passage through the galvanometer, we say that B, the active part, is negative to A, the resting part. The expression negativity of action' is sometimes used to express this

C C

relation of B to A. These considerations apply equally to muscle and to nerve.

The negative variation of muscle (or of nerve).-If an injured muscle giving the injury current from A to B be tetanised by excitation of its nerve, tetanus will include its whole mass, but the change from rest to action will be greater at the uninjured part, A,

FIG. 179.-THE NEGATIVE VARIATION. (Frog's Gastrocnemius.)

Simultaneous record of a tetanic contraction (white line) and of the accompanying negative variation of a current of injury (black line). (a) The current of injury is normally subsiding; (b) it is suddenly diminished during tetanus (negative variation); (c) it subsequently increases (positive after-variation); and (d) it finally resumes its normal decline. The galvanometer deflection is recorded by a spot of light falling upon a sensitised cylinder revolving round a horizontal axis.

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than at the injured part, B, i.e. there will be an action current from B to A, which, while the muscle is tetanised, will diminish the injury current from A to B. This diminution is the negative variation. A single muscular contraction is sufficient to give a negative variation, but tetanic contraction will bring it into stronger evidence.

Seeing that a simple negative variation as above described depends upon greater action at an uninjured point, A, than at an injured point, B, it is easy to see that the negative variation will be greater with greater current of injury. The negative variation in muscle appears to precede the contraction to which it belongs, as may be best observed in the heart. (See fig. 182.) We have said' appears to precede,' because there can be no doubt that under ordinary conditions the mechanical event really begins before it becomes visible, and it is possible that it actually begins with, and not after, the electrical event. In the case of voluntary muscle no interval is demonstrable; both events have a latent period as short as sec. (B. Sanderson).

Characters of the negative variation.-The negative variation in nerve quickly reaches a maximum with increasing strength of stimulating current, is not reversed in direction with reversal in the direction of the stimulating current, does not vary in magnitude with increasing interval between the stimulated and ledoff regions of the nerve, and is abolished by ligature in that interval. By the first three of these signs it is distinguished from electrotonic currents; the last is common to both. According to Sanderson and Gotch, the negative variation of potential accompanying a muscular contraction may exceed the potential difference due to injury, e.g., 0-08 volt in the former case, as compared with 0.04 in the latter. The negative variation of muscle is greater with greater tension, agreeing thus with the yield of heat and of work, which, as has been stated, are likewise greater with greater tension.

The double or diphasic variation.-Whereas the presence of a strong current of injury is favourable to the demonstration of the simple negative variation, which is the sign of the unbalanced negativity of action in the uninjured tissue, it is unfavourable to the demonstration of the diphasic variation, which is a manifestation of the negativity of action, first at one part, then at another part, of uninjured tissue. The double variation depends upon the fact that action is not simultaneous throughout the whole mass. of a muscle, but occupies time in its transmission from a point of stimulation.

If A and B (fig. 180) be electrodes applied to an uninjured muscle, C the point of application of a stimulus the part at and near A will commence to act and become negative before the part at and near B; during the interval between the commencement of action at A, and that subsequently at B, the negativity of A is

unbalanced; this constitutes the first phase. The part at and near B will continue active and remain negative after the part at and near A has ceased to be active; during the interval between the

B

cessation of action at A and that subsequently at B, the negativity of B is unbalanced; this constitutes the second phase. Between the first and second phases, while A and B are both active and negative, the negativity of A more or less accurately balances that at B; this constitutes the isoelectric interval. Now it is obvious that the slower the transmission of the active negative state from A to B, the more opportunity there is for the manifestation of unbalanced negativity, at A when the process begins, and at B when it ends. Hence it is most easy to demonstrate on the frog's heart, in which the active state travels at the rate of meter per second, less easy on muscle, in which the rate of transmission is not less than 1 meter, most difficult on nerve, in which the rate of transmission is about 30 meters. The diphasic variation in cooled nerve is, however, easily observed, the rate of transmission being then reduced to about 1 meter per second.

FIG. 180.

We may illustrate the mechanism of a diphasic variation by the following analogy. A railway train (=a nervous impulse) 50 meters long, running at a speed of 50 meters per second, works two signals, A and B, at 10 meters from each other, which fall while the train is at points opposite them, but rise again when the train has passed. During a first period the train is at A, and not yet at B, and A is negative to B (1st or initial phase).

During the second period the train is opposite A and B, which are both negative (isoëlectric interval). During a third period the train is past A, but still at B, and B is negative to A (second or terminal phase). With the speed and distances given above the initial phase would last sec., the interval sec., and the terminal phase sec. Bearing in mind that in a nervous impulse there is no actual transport of matter, but only the propa

gation of a motion, this figment usefully represents the diphasic and some other of the electrical changes taking place in nerve or muscle-e.g., if there is an injury at B, B will be negative to A (current of injury); if a continuous or tetanic series of impulses passes when B is down, A only will fall (excitatory diminution of current of injury); if a continuous series passes when there is no injury, both A and B will be kept down; if impulses pass in an opposite direction, the direction of the phases will be reversed, &c.

Experiments on man.-Two different experiments relating to the electromotive action accompanying muscular contraction have been made upon the human subject-one by du BoisReymond relates to voluntary tetanic contraction, the other by Hermann to tetanic contraction caused by electrical stimulation.

Du Bois-Reymond's experiment.-The two hands are led off to a galvanometer by two vessels, into each of which a finger is dipped. The voluntary contraction of the muscles of either arm gives a deflection, which indicates the passage of a current through the galvanometer from the passive to the contracting arm, and through the body from the contracting to the passive arm-i.e. negativity of the active side. Objection has, however, been taken to the view that the negativity is due to muscular action, and it has been attributed to cutaneous secretory action; it has been found that on a curarised cat, excitation of the sciatic, while causing no muscular contraction, gives the current above described, while on an atropinised cat, excitation of the sciatic causes muscular contraction, but no action current.

Hermann's experiment. The forearm is led off to a galvanometer by two bracelet electrodes; muscular action is provoked by electrical excitation of the brachial plexus, and the rheotome is employed. A series of observations are taken, the rheotome being shifted from zero (simultaneous excitation and rheotome closure) onwards, which demonstrates the presence of a diphasic variation. The phases are such as to indicate-1, negativity of muscle proximal to the nerve-entrance; 2, negativity of muscle distal from such nerve-entrance. The phenomenon is far more rapid than on frogs' muscle, and time-measurements give for the rate of propagation of the excitatory state a value of about 12 meters per second.

Experiments on the heart.-The frog's heart. On the frog's heart the normal diphasic variation of its spontaneous systole is as follows:-first phase, base negative to apex; second phase, apex negative to base. If the heart be Stanniused and stimu