624 COMPARATIVE ELECTRO-PHYSIOLOGY 



by recovery. This is exactly parallel to the sudden 

 enhancement of response in the retina on the cessation of 

 tetanising light, or to the enhancement of response in 

 the nerve when the tetanising electrical shock is suddenly 

 withdrawn (pp. 428, 536). From the present experiment it 

 will be seen that the suggested explanation of the pheno- 

 menon, as due to anabolic or katabolic changes, is gratuitous. 

 In responding substances, where the persistence of after- 

 effect is relatively great, the successive shocks for the obtaining 

 of the tetanic curve need not be repeated so quickly as where 

 recovery is rapid. The shifting of the base-line of a series 

 of even such responses as indicate incomplete tetanus, will 

 give an indication of the form of the characteristic curve. 

 The progressive molecular modification of a substance may 

 thus be gauged, as already pointed out, in either of the two 

 different ways by progressive changes in the character of 

 the response, or by means of the characteristic curve of the 

 substance. And if both these, again, represent correctly the 

 molecular condition of the material, we shall further find 

 that definite parts of the characteristic curve have each their 

 peculiar responsive features. In order to take these records 

 of the characteristic curve and corresponding responses of 

 a substance, moreover, we may adopt any method that is 

 convenient mechanical, electro-motive, magnetic, or that of 

 the resistivity variation. The feasibility of such records is 

 obviously a matter of the extent of the change induced and 

 the sensitiveness of the recording apparatus. Of the various 

 methods here mentioned, it may be said that there are no 

 particular sources of uncertainty to be guarded against in 

 regard to variations of resistance, of magnetisation, or of 

 length. But in the method of electro-motive variation, as the 

 change to be recorded is relative, being measured against a 

 neutral or indifferent point, some difficulty occurs in securing a 

 point which is invariable. This may be done more or less per- 

 fectly, however, by choosing an injured or killed point on the 

 tissue for the second contact, in order that it may be subject 

 to as little variation as possible from environmental changes. 



