In the experimental arrangement employed, the hetro- dromous current is obtained by making the electrode on the spine cathode and that on the tendon anode. The depressing influence of the anode in this case may be expected to lower, to a certain extent, the normal excita- bility of the responding muscle. Conversely, with homo, dromous current, the tendon is made the cathode and under its influenc3 the muscle might have its excitability laised above the normal. These anticipations are fully supported by results of experiments. Sub-maximal stimulus of equi- alternating induction shock was directly applied to the muscle and records taken of (i) response under normal condi- tion without any current, (2) response under heterodromous current, the tendon being the anode, and {3) response under homodromous current, the tendon being now made the cathode. It was thus found that under heterodromous current the excitability of the muscle was depressed, and under homodromous current the excitability was enhanced.
The effect of current on response to direct stimulation is thus opposite to that on response to transmitted excita- tion, as will be seen in the following Table.
TABLE VIII.— INFLUENCE OF DIRECTION OF CURRENT ON DIRECT AND TRANSMITTED EFFECTS OF STIMULATJON.
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Direction of current.
Transmitted excitation.
Direct stimulation.
Heterodromous current
Homodromous current
Enhanced response
Depressed response
Depressed response
Enhanced responpe.
The passage of a current, therefore, induces opposing effects on the conductivity of the nerve and the excitabil- ity 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
