276 COMPARATIVE ELECTRO-PHYSIOLOGY 



galvanometer-circuit. The manner in which this is done will 

 be understood from fig. 170. We have a highly insulating 

 electrical key, K, of ebonite. P and Q are connected with two 

 points A and B of the tissue, whose relative excitabilities are 

 to be determined. A spiral spring keeps the key down, con- 

 necting the two points in the specimen with the galvanometer. 

 Any existing difference of potential, as between the two points 



FIG. 170. Experimental Arrangement for Determination of Excitatory 

 After-effect of Equi-alternating Electrical Shocks 



M, electrical motor working PohPs commutator for alternate reversal of 

 current in primary, P. Note that the connecting-rod, A, works simply 

 up and down, causing reversals of current ; s, secondary coil ; K, 

 ebonite key kept down by elastic spring, the two surfaces of the 

 specimen being thus in circuit with galvanometer, G. When key is 

 pressed, these are put in circuit with exciting coil, s. When key is 

 released, after-effect of excitation on specimen exhibited by galvano- 

 meter deflection, c, the compensator. 



is balanced by the compensating potentiometer c. Under these 

 circumstances, the galvanometer spot of light would remain 

 steady, whether the key was up or down. By pressing the key 

 K, the galvanometer-circuit is broken, and the tissue is put in 

 series with the exciting circuit S, which forms the secondary 

 of the induction-coil. When the key is again released, the 

 galvanometer-circuit is rapidly made, at a definite short 

 interval after the cessation of the exciting shock, and the 

 resulting deflection of the galvanometer indicates the differ' 



