120 



COMPARATIVE ELECTRO-PHYSIOLOGY 



record of these contrasted effects. In the first part of the 

 curve we observe a sudden movement of the record upwards 

 corresponding to the sudden rise of temperature. This is so 

 great as to carry the curve out of the photographic field. We 

 have here, then, a sudden excitatory diminution of the natural 



current. In the next stage, while 

 the temperature is steadily ascending, 

 we find a reversal of the curve, and 

 the natural current is enhanced above 

 the normal. On now allowing the 

 chamber to cool down to the original 

 temperature of the room, the natural 

 current was found to return more or 

 less to its normal value. 



We shall next study the effect 

 of chemical agents on the natural 

 current. The mode of procedure 

 is to apply the given agent on both 

 the contacts at the same time. If 

 the substance be liquid, it can be 

 applied by a pipette. If it be gaseous, 

 the specimen is placed in a chamber 

 through which the gas or vapour is 

 allowed to stream. In observing 

 the effects of various agents we 

 obtain results which are at first 

 sight very perplexing. For example, 

 certain substances will be found to 

 induce a diminution of the natural 

 current, and others an increase. The 

 effect, moreover, is found to be modi- 

 fied by the strength of the dose. Thus an agent which, 

 in a given strength, will cause a diminution of the natural 

 current, may often be found to cause an increase, when 

 sufficiently diluted. This inquiry is of great importance, 

 since it is directly connected with many equally obscure 

 problems in medical practice, where the effect of a drug 



FIG. 85. Photographic 

 Record showing effect of 

 Sudden, followed by 

 steady Rise of Tem- 

 perature on Natural Cur- 

 tent, |, in Musa 



During sudden variation of 

 temperature an excitatory 

 negative variation of 

 natural current takes 

 place, as shown by first 

 up curve ; when rise of 

 temperature becomes 

 steady there is a positive 

 variation, as shown by 

 the down curve ; on re- 

 turn to surrounding tem- 

 perature, the normal cur- 

 rent is restored to its 

 original value. 



