452 ELECTRO-PHYSIOLOGY CHAP. 



of closure of the current, we should anticipate that induced 

 currents (which are of such brief duration) would be peculiarly 

 appropriate to the production (with homodromous stimulation) of 

 a positive after-effect in the same direction. 



The part played by duration of closure with the constant 

 currents also, is shown by the following table from Gotch : 



Instantaneous closure (7 Groves) homodromous ; galvanometer deflection 



+ 50 (homodromous). 

 1 second closure (7 Groves) homodromous ; galvanometer deflection 



- 52 (heterodromous). 

 Instantaneous closure (7 Groves) homodromous ; galvanometer deflection 



+ 30 (homodromous). 

 1 second closure (7 Groves) homodromous ; galvanometer deflection 



40 (heterodromous). 



By means of a contrivance to be described below, Gotch sent 

 a break induction-shock, first in one and then in the other direc- 

 tion, through an organ -preparation, in a galvanometer circuit 

 which also included the secondary coil and a resistance of 10,000 

 ohms. On opening the primary circuit, the full after-current in 

 the preparation due to the induced current went through the 

 galvanometer. With a strip of organ 1 6 mm. long, 7 mm. broad, 

 and 2 mm. in diameter, the deflection (with three Groves in 

 primary circuit and 5 cm. distance of coil) = 



Break shock heterodromous 150 (homodromous) 

 homodromous 650 



heterodromous 180 



homodromous 780 



The excitation effect of the homodromous current is thus 

 much stronger than that in the heterodromous direction. Yet, 

 as Gotch pointed out, this is not invariably the case. At times 

 no difference can be detected in the (exciting) action of the two 

 currents, or the heterodromous current may even be stronger than 

 the homodromous. Gotch is inclined to bring this into relation 

 with the facts observed by Eckhardt, to the effect that descending 

 induction-currents via the nerve excite more strongly, since the 

 homodromous current passes through the majority of the finer 

 branches of the nerve in a descending direction. The exceptions 

 are no doubt due to the fact that the larger nerve-trunks some- 

 times lie in the organ-preparation, so that they are traversed by 

 descending heterodromous currents, and produce an effective 

 excitation. 



