intensity of transmitted excitation will also be enhanced ; the minimal response will tend to become maximal. Or excitation which had hitherto been inetfectively trans- mitted will now become effectively transmitted. Con- versely, depression of conductivity will result in a diminu- tion or abolition of response. We may use a single break-shock of sufficient intensity as the test stimulus. It is, however, better to employ the additive effect of a definite number of feeble make-and-break shocks.
We may again employ additive effect of a definite number of induction shocks, the alternating elements of which are exactly equal and opposite. This is secured by causing rapid reversals of the primary current by means of a rotating commutator. The successive induction shocks of the secondary coil can thus be rendered exactly equal and opposite.
Exjjeriment 42. — Working in this way, it is found that the transmitted excitation against the direction of current becomes effective or enhanced under 'up-hill' current. A current, flowing with the direction of transmission, on the other hand, diminishes the intensity of transmitted excitation or blocks it altogether.
Henceforth it would be convenient to distinguish currents in the two directions ; the current in the direction of trans- mission will be distinguished as Homodromons, and against the direction of transmission as Heterodromuus.
AFTER-EFFECTS OF HOMODROMOUS AND HETERODROMOUS CURRENTS.
The passage of a current through a conducting tissue in a given direction causes, as we have seen, an enhanced conductivity in an opposite direction. We may suppose this to be brought about by a particular molecular arrangement
