[Illustration: FIG. 50.--Ineffectively transmitted salt-tetanus becoming effective under heterodromous current, denoted by down-pointing arrow.] _Complication due to variation of Excitability of Muscle_.--In experiments with the plant, there was the unusual advantage in having both the point of stimulation and the responding motile organ in the middle or indifferent region. Unfortunately this ideally perfect condition cannot be secured in experiments with the nerve-and-muscle preparation of the frog. It is true that the point of stimulation in this case is chosen to lie on the nerve at the middle or indifferent region. But the responding muscle is at one end, not very distant from the electrode applied on the tendon. It is, therefore, necessary to find out by separate experiments any variation of excitability that might be induced in the muscle by the proximity of either the anode or the cathode, and make allowance for such variation in interpreting the results obtained from investigations on variation of conductivity. In the experimental arrangement employed, the heterodromous 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 excitability of the responding muscle. Conversely, with homodromous current, the tendon is made the cathode and under its influence the muscle might have its excitability raised 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 (_1_) response under normal condition 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 excitation, as will be seen in the following Table. TABLE VIII.--INFLUENCE OF DIRECTION OF CURRENT ON DIRECT AND TRANSMITTED EFFECTS OF STIMULATION. +---------------------+-----------------------+-------------------+ |Direction of current.|Transmitted excitation.|Direct stimulation.| +---------------------+-----------------------+-------------------+ |Heterodromous current|Enhanced response |Depressed response | |Homodromous current |Depressed response |Enhanced response | +---------------------+-----------------------+-------------------+