Currents on Transmission of Excitation. 



493 



Table II. — Showing Effects of Heterodromous and Homodromous Currents 

 of Feeble Intensity on Period of Transmission through 15 mm. 



Number. 



Intensity of current 

 in microamperes. 



Period of hetero- 

 dromous transmission. 



Period of homo- 

 dromous transmission. 



1 

 2 



3 

 4 



1-4 

 1 4 

 1-6 

 1-7 



14 tenths of a second 



19 „ 

 12 „ 



16 tenths of a second 



.15 » 

 Arrest. 



14 tenths of a second. 



Having demonstrated the effect of direction of current on the velocity of 

 transmission, I shall next describe other methods by which induced variations 

 of conductivity may be exhibited. 



3. Determination of Variation of Conductivity by Method of Minimal Stimulus 



and Response. 



In this method we employ a minimal stimulus, the transmitted effect of 

 which under normal conditions gives rise to a feeble response. If the passage 

 of a current in a given direction enhances conductivity, then the intensity of 

 transmitted excitation will also be enhanced ; the minimal response will tend 

 to become maximal. Or excitation which had hitherto been ineffectively 

 transmitted will now become effectively transmitted. Conversely, depression 

 of conductivity will result in a diminution 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 in the secondary 

 coil can thus be rendered exactly equal and opposite. 



Experiment 2. — Working in this way, it is found that the transmitted 

 excitation becomes effective or enhanced under heterodromous current. The 

 homodromous current, on the other hand, diminishes the intensity of excitation 

 or blocks it altogether. 



4. 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 induced by the current, which assisted the propagation of the 



