vm.J INJURY CURRENT AND BLAZE-CURRENT 143 



But we may hardly proceed further on these lines of 

 argument without further information concerning the exact 

 freezing points and the relative conductivities of various animal 

 and vegetable tissues, and to obtain such information means 

 engaging in a new investigation, for which new methods have to 

 be elaborated. 



83. A question. It has probably occurred to some of us to 

 ask what relation, if any, exists between the normal current and 

 a blaze-current. Is not the normal current itself a sign of life, 

 and in certain cases of obviously identical nature with a blaze ? 



To some extent these questions have been incidentally 

 answered in previous lectures, but not formally and explicitly. 



Indeed it is difficult to frame a formal and explicit answer, 

 and I should prefer to avoid giving such an answer otherwise 

 than with much reservation. For the facts of the case are by 

 no means as clear as they seem to be at first sight. As regards 

 the first question, we can make, with about equal probability, 

 the two apparently contradictory statements that in some cases 

 there is an evident relation between normal and blaze-currents, 

 in other cases there is evidently no relation at all between the 

 two currents. And on reflection it will appear that our first 

 question should logically be preceded by our second as to 

 whether normal current is or is not a sign of life, which amounts 

 to asking what view we take of the nature of normal currents. 



Now, I think you may take as granted that normal current 

 is always injury current or excitation current ; but I do not 

 think you may take as granted that injury current is always 

 excitation current. 



The question of the nature of injury current has largely 

 turned of late years upon the demarcation current of medul- 

 lated nerve ; in opposition to the view of Gotch, who assumes 

 that this current is wholly an excitation current, MacDonald 

 urges that it is a concentration current dependent upon the 

 electrolytes of nerve. He regards nerve as a concentration cell 

 in which the sheath is the dilute solution (= 0.9 per cent. KC1), 

 the axis-cylinder the concentrated solution (== 10 per cent. KC1), 

 and the current determined by the physico-physiological state 



