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 

 FIG. 85. Photographic is to apply the given agent on both 

 Record showing effect of the conta cts at the same time. If 



Sudden, followed by 



steady Rise of Tem- the substance be liquid, it can be 



renf ^ in n ^a ral ^ applied by a pipette. I f it be gaseous, 



During'sudden variation of the specimen is placed in a chamber 



temperature an excitatory through which the gas Or vapour is 

 negative variation of 



natural current takes allowed to stream. In observing 

 u^Turve-^lren b nse"of *^ e effects of various agents we 

 temperature becomes obtain results which are at first 



steady there is a positive . , , * -^ , 



variation, as shown by S1 g ht very perplexing. For example, 

 certain substances will be found to 



the down curve ; on re- 

 turn to surrounding tem- 

 perature, the normal cur- 



induce a diminution of the natural 

 rent is restored to its curren t, and others an increase. The 



original value. 



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 



