132 COMPARATIVE ELECTRO-PHYSIOLOGY 



normal responses, shown in fig. 91, is seen a very interesting 

 instance of alternating fatigue. 



In order to bring out clearly the main phenomena, I 

 have postponed till now the consideration of a point of some 

 difficulty. To determine the influence of a reagent in 

 modifying the excitability of a tissue, we rely upon its effect 

 in exalting or depressing the responsive E.M. Variation, and 

 we read this effect by means of changes induced in the 

 galvanometric deflection. Now as long as the resistance of 

 the circuit remains constant, an increase or decrease of 

 galvanometric deflection will accurately indicate a heightened 

 or depressed E.M. Variation, due to augmented or lowered 



Before f After 

 FIG. 91. Photographic Record showing Action of Formalin (Radish) 



excitability, induced by the reagent in the tissue. But by 

 the introduction of the chemical reagent the resistance of 

 the tissue may undergo a change, and, owing to this cause, 

 modification of response, as read by the galvanometer, may 

 be induced without any E.M. Variation. The observed 

 variation of response may thus be partly owing to some 

 unknown change of resistance, as well as to that of the 

 E.M. Variation. 



This difficulty may, however, be obviated by interposing 

 a very large and constant resistance in the external circuit. 

 The variation in the tissue then becomes negligible, the 

 galvanometric deflections being now proportional to the 

 electro-motive variation. An actual experiment will make 



