In a living muscle electric currents may be detected, having a definite direction, and certain relations to the vitality of the tissue. As they seem to be invariably present in a passive muscle, they have been called natural muscle currents.

They are generally studied in the muscles of cold-blooded animals after removal from the body. The muscle is spoken of as if it were a cylinder, with longitudinal and transverse surfaces corresponding to its natural surface and its cut extremities. In such a block of frog's muscle the measurement of the electric currents requires considerable care, because they are so difficult to detect that a most sensitive galvanometer must be used; and such an instrument can easily be disturbed by currents due to bringing metal electrodes into contact with the moist saline tissues. Specially constructed electrodes must be used to avoid these currents of polarization taking place in the terminals touching the muscle. These are called non-polarizable electrodes, and may be made on the following plan: Some innocuous material moistened in saline solution (.65 per cent.) is brought into direct contact with the muscle, and, by means of saturated solution of zinc sulphate, into electrical connection with amalgamated zinc terminals from the galvanometer. Thus the muscle is not injured, and the zinc solution prevents the metal terminals from producing adventitious currents.

Non polarizable Electrodes.

Fig. 181. Non-polarizable Electrodes. The glass tubes (a a) contain sulphate of zinc solution {z. s.), into which well amalgamated zinc rods dip. The lower extremity is plugged with china clay {ch. c), which protrudes at (c') the point. The tubes can be moved in the holders [h h), so as to be brought accurately into contact with the muscle. {Foster).

Small glass tubes drawn to a point, the opening of which is plugged with china clay moistened with salt solution, make a suitable receptacle for the zinc solution. If a pair of such electrodes be applied to the middle of the longitudinal surface at (e) (Fig. 182), and of the transverse surface at (p), respectively, and then be brought into connection with a delicate galvanometer, it is found that a current passes through the galvanometer from the longitudinal to the transverse surface. A current in this direction can be detected in any piece of muscle, no matter how much it be divided longitudinally, and probably would be found in a single fibre, had we the means of examining it. The nearer to the centre of the longitudinal and transverse sections the electrodes are placed, the stronger will be the current received by them. If both the electrodes be placed on the longitudinal section or on the transverse surfaces, a current will pass through the galvanometer from that electrode nearer the middle of the longitudinal section (called the equator of the muscle cylinder) to the electrode nearer the centre of the transverse section (pole of muscle cylinder). If the electrodes be placed equidistant from the poles or from the equator no current can be detected.

The central part of the longitudinal surface of a piece of muscle is then positive, compared with the central part of the extremities or transverse sections. And between these parts - the equator and poles of the muscle cylinder, where the difference is most marked - are various gradations, so that any point near the equator is positive when compared with one near the poles.

There is, then, a current passing through the substance of the piece of muscle from both the transverse sections or extremities of the muscle block to the middle of the longitudinal surface, whether it be a cut surface (longitudinal section) or the natural surface of the muscle. This is called the muscle current, or sometimes natural muscle current.

If the cylinder in the accompanying figure be taken to represent a block of muscle, e would correspond to the equator, and p to the poles, and the arrow heads show the direction of the currents passing through the galvanometer, the thickness of the lines indicating their force. The dotted lines o are connected with points where the electro-motive force is equal, and, therefore, no current exists.

Diagram to illustrate the currents in muscle.

Fig. 182. Diagram to illustrate the currents in muscle.

(e) Equator, corresponds to the centre of the muscle cylinder. (p) Polar regions of cylinder representing the extremities of the muscle. The arrow heads show the direction of the surface currents, and the thickness of lines indicates the. strength of the currents. {After Fick).

The electro-motive force of the muscle current in a frog's gracilis has been estimated to be about.05-.08 of a Daniell cell. It gradually diminishes as the muscle loses its vital properties, and is also reduced by fatigue. The electro-motive force rises with the temperature from 50 C. until a maximum is reached at about the body temperature of mammals.

These muscle currents are very weak if the uninjured muscle be examined in situ, the tendon being used as the transverse section; they soon become more marked after the exposure of the muscle, and if the tendon be injured they appear at once in almost full force. In animals quite inactive from cold the muscles naturally are but slowly altered by exposure, etc., and the muscle currents do not appear for a considerable time, which is shortened on elevating the temperature. It has, therefore, been supposed that in the perfectly normal state of a living animal there are no muscle currents so long as the muscle remains in the passive state.