The passage of a current, therefore, induces opposing effects on the conductivity of the nerve and the excitability of the muscle, the resulting response being due to their differential actions. Under heterodromous current a more intense excitation is transmitted along the nerve, on account of induced enhancement of conductivity. But this intense excitation finds the responding muscle in a state of depressed excitability. In spite of this the resulting response is enhanced (Fig. 50). The enhancement of conduction under heterodromous current is, in reality, much greater than is indicated in the record. Similarly, under homodromous current the depression of conduction in the nerve may be so great as to cause even an abolition of response, in spite of the enhanced excitability of the muscle (Fig. 51). The actual effects of current on conductivity are, thus, far in excess of what are indicated in the records. AFTER-EFFECTS OF HETERODROMOUS AND HOMODROMOUS CURRENTS. On the cessation of a current there is induced in the plant-tissue a transient conductivity change of opposite sign to that induced by the direct current (_cf. Expt. 43_). The same I find to be the case as regards the after-effect of current on conductivity change in animal nerve. Of this I only give a typical experiment of the direct and after-effect of homodromous current on salt-tetanus. [Illustration: FIG. 51.--Direct and after-effect of homodromous current. Transmitted excitation (salt-tetanus T,) arrested under homodromous current denoted by up-pointing arrow; on cessation of current represented by dotted line there is a transient enhancement above the normal.] _Experiment 46._--In this experiment sufficient length of time was allowed to elapse after the application of the salt on the nerve, so that the muscle, in response to the transmitted excitation, exhibited an incomplete tetanus T. The homodromous current was next applied, with the result of inducing a complete block of conduction, with the concomitant disappearance of tetanus. The homodromous current was gradually reduced to zero by the appropriate movement of the potentiometer slide. The after-effect of homodromous current is now seen in the transient enhancement of transmitted excitation, which lasted for nearly 40 seconds. After this the normal conductivity was restored. Repetition of the experiment gave similar results (Fig. 51). The results that have been given are only typical of a very large number, which invariably supported the characteristic phenomena that have been described. It will thus be seen that with feeble or moderate current, conductivity is enhanced against the direction of the current and depressed or blocked with the direction of the current. Under strong current the normal effect is liable to undergo a reversal.