VI. On the modification of the excitability of motor nerves produced by injury

§ 1. If the gastrocnemius of a frog be placed in a horizontal direction on non-polarizable electrodes with its convex surface uppermost, one may generally observe that the muscle is somewhat more sensitive to minimal stimulation, supplied by closure of the constant current, when the femoral end rests on the kathode, than when this end rests on the anode. Conversely, under similar circumstances the gastrocnemius is more sensitive to minimal stimulation, supplied by opening of the constant current, when the femoral end rests on the anode, than when this end rests on the kathode. In view of the other facts of electrotonus, the present ones are of interest; because, as the sciatic nerve enters the gastrocnemius near the femoral end of the latter, and then spreads out its peripheral ramifications as it advances, in the experiments just mentioned one electrode is in almost immediate contact with the nerve-trunk where it enters the muscle, while the other electrode supports the part of the muscle that contains only peripheral nervous elements. It is therefore to be expected, upon the theory of electrotonus, that the muscle under these conditions should prove itself most sensitive to the closing shock when the nerve-trunk rests on the kathode, and most sensitive to the opening shock when the nerve-trunk rests on the anode. It is to be observed, however, that although this expectation is in most cases fulfilled, it is not so invariably. Different gastrocnemius muscles, though treated as far as possible in exactly the same way, manifest considerable differences, both in their general sensitiveness to electrical stimulation, and in their relative sensitiveness to interruptions of the ascending and of the descending currents. Even the same muscle, if rapidly prepared, will generally be found to undergo fluctuations in these respects from minute to minute. Attributing this fact to the unnatural conditions which the experiment imposed on the process of nutrition, I conducted some observations on muscles while they were still attached to the body of the frog; but the results yielded by this method were not more uniform than those which I had previously obtained by the method of rapidly preparing and observing excised muscles.