$42 COMPARATIVE ELECTRO-PHYSIOLOGY 



not give us those two opposed effects, positive and negative, 

 which we have already seen to be exhibited by living tissues 

 in other forms of response whether mechanical or electro- 

 motive. Of these, again, supposing them to occur, it will 

 also be necessary to determine whether it is the increase or 

 decrease of resistance which corresponds to the negative and 

 positive mechanical and electromotive responses respectively ; 

 and finally it must be determined what are the effects of the 

 various physiological modifications, induced by different 

 agencies, on the response by resistivity variation. 



In this investigation many serious experimental difficulties 

 have first to be overcome. These will be dealt with in series 

 in the detailed description of the method to be employed. 

 It will be well, however, to see in what important respects 

 the conditions for the obtaining of response here are unlike 

 those of the electromotive variation. In the latter case, it 

 we employ an isotropic tissue, diffuse stimulation will induce 

 similar excitatory electromotive variations in every part of 

 the tissue. The differential electromotive variation, there- 

 fore, on which the recording of response depends, will, under 

 these circumstances, be impossible. In this case, therefore, 

 it is necessary to injure or kill the tissue at one of the two 

 contacts. Such artificial induction of anisotropy would not, 

 however, be necessary under an experimental method which 

 was hot dependent on any differential action. Thus an 

 isotropic tissue would give response by longitudinal con- 

 traction when the stimulus was diffuse. Similarly, though 

 an isotropic tissue fails to give an electromotive response 

 under diffuse stimulation, yet we may expect it to exhibit 

 response by variation of resistance. The recording of 

 excitatory response by resistivity variation has thus one 

 advantage over that of the electromotive variation, inas- 

 much as the record is not affected by complications due to 

 differential action, but is the expression of the direct effect 

 of excitation. The question which we have next to deter- 

 mine, then, is whether or not the excitatory variation of the 

 living tissue is attended by any variation of its resistance, 



