Currents on Transmission of Excitation. 



499 



The upper record, denoted by the left-handed arrow, shows the action of a 

 heterodromous current in shortening the period of transmission and thus 

 enhancing the velocity above the normal rate. The lower record, denoted by 

 the right-handed arrow, exhibits the effect of a homodromous current in retard- 

 ing the velocity below the normal rate. I find that a very feeble heterodromous 

 current is enough to induce a considerable increase of velocity, which soon 

 reaches a limit. For inducing retardation of velocity, a relatively strong 

 homodromous current is necessary. I give below a table showing the results 

 of several experiments. 



Table V. — Effect of Heterodromous and Homodromous Currents of Feeble 

 Intensity on Velocity of Transmission. 



Specimen. 



Intensity of 

 heterodromous 

 current. 



, , .. Intensity of 

 Acceleration , , J 

 , , nomodromous 

 above normal. , 

 current. 



1 



Retardation 

 below normal. 



1 



2 

 3 

 4 

 5 

 6 



microampere. 

 0-35 

 0-7 

 0-8 



0- 8 



1- 

 1 -5 



per cent. 

 16 

 13 

 18 

 11 

 18 

 15 



microamperes. 

 1 



1 -5 



2 -0 

 2-0 



2- 5 



3- 



per cent. 

 20 

 19 

 14 

 13 

 12 

 40 



3. Variation of Intensity of Transmitted Excitation under Heterodromous and 



Homodromous Currents. 



In the next method of investigation, the induced variation of intensity of 

 transmitted excitation is inferred from the varying amplitude of response of 

 the terminal muscle. Testing stimulus of sub-maximal intensity is applied at 

 the middle of the nerve, where the polarising current induces no variation of 

 excitability. Stimulation is effected either by a single break-shock or by the 

 summated effects of a definite number of equi-alternating shocks, or by 

 chemical stimulation. 



Experiment 6. — Under the action of feeble heterodromous current the 

 transmitted excitation was always enhanced, whatever be the form of stimu- 

 lation. Homodromous current on the other hand inhibited or blocked 

 excitation (figs. 8, 9). 



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 



VOL. LXXXVIII. — B. 2 R 



