THE ELECTROMOTIVE PHENOMENA 301 



cross-section. When the cross-section is made, 

 the tissue next the section passes through the 

 series of catabolic changes which- constitute 

 muscle death ; carbon dioxide is given off, lac- 

 tic acid is developed, a soluble proteid is con- 

 verted to a less soluble form, etc. The contact 

 of this dying layer with the uninjured tissue is 

 believed to create a difference of potential. The 

 potential difference, therefore, appears at the de- 

 marcation between dying and uninjured tissue, 

 hence the term " demarcation current." The 

 action current finds its explanation in the chemi- 

 cal changes accompanying contraction. It would 

 be interesting to consider here the parallel be- 

 tween the chemical transformations in contrac- 

 tion and those which usher in the death of the 

 muscle, but we must be content with mentioning 

 the apparently close relationship. In its most 

 general form, the alteration hypothesis rests on 

 the fact that living substance is everywhere the 

 seat of constant constructive and destructive 

 changes. Where these are nearly in equilibrium, 

 as, for example, in the resting uninjured muscle, 

 the tissue is equipotential ; where, on the con- 

 trary, either form of chemical change has the 

 upper hand, as in the explosion which we term 

 contraction, and in dying muscle, it is assumed 

 that a difference of potential is created. 



