88 COMPARATIVE ELECTRO-PHYSIOLOGY 



substance at the time being. One of the most important 

 factors, then, in determining the character of response is the 

 molecular condition of the substance itself. The numerous 

 anomalies hitherto encountered in our interpretation of 

 responsive phenomena are all traceable to our failure to take 

 this factor of molecular condition into account. For a full 

 exposition of the modifying influence which it exercises on 

 response, however, though I shall here state some of the 

 principal conclusions which I have arrived at, the reader is 

 referred to Chapter XLII. 



From the fact, that every type of response is to be 

 obtained from inorganic matter, where chemical assimilation 

 and dissimilation are obviously out of the question, it is clear 

 that the fundamental phenomenon must be dependent on 

 physical or molecular, and not on such hypothetical chemical 

 changes. It must, however, be remembered that though re- 

 sponse phenomena and their modifications are undoubtedly 

 in the first place physical or molecular, yet in the borderland 

 between physics and chemistry there is no sharp line of 

 demarcation. For example, yellow phosphorus becomes 

 converted, under the stimulus of light, into the red, or 

 allotropic, variety. This molecular change, however,Js also 

 attended by a concomitant change in the chemical activity, 

 phosphorus in its allotropic condition being less active than 

 in the yellow. Under certain circumstances, further, it is 

 possible to have a secondary series of chemical events follow- 

 ing upon a condition of unequal molecular strain. A homo- 

 geneous living tissue, when unstimulated, is iso-electric. 

 When stimulated, however, an electro-motive difference is 

 induced, as between the stimulated and unstimulated parts of 

 the tissue. The result is an electrical current attended 

 by electro-chemical changes. As a consequence of such 

 volta-chemical action, when prolonged, by-products (fatigue 

 stuffs ?) may be accumulated, and these may have a de- 

 pressing effect on the activity of the tissue. Hence, just 

 as, after very prolonged activity of a voltaic combination, it 

 is necessary to renew the active element and change the 



