16 Mr. J. S. Macdonald. [July 31, 



provided only that the portion tested be first injured previous to the applica- 

 tion of the test. The circumstance of injury reveals the presence of a highly 

 concentrated solution of potassium salt within the intramyelin material, 

 which makes an immediate appearance at every injured point. The bizarre 

 appearances produced by this method of mapping out injured points had been 

 observed by Macallum, but was left completely unexplained by him. Their 

 relation to injury is, however, unmistakable when sought for. Since these 

 points of injury can be multiplied indefinitely so as to include the whole 

 length of the nerve-fibre, it follows that potassium really exists everywhere 

 within the intramyelin material in some manner hidden from the action of 

 the reagent, and that the processes attendant upon injury are capable of 

 presenting it in a new condition ready to take part in the reaction. I have 

 discussed the probable meaning of this fact in a previous communication, and 

 the important conclusions which can be drawn from it as to the nature of 

 nerve-function. 



A slight modification in the method of use of this reagent brings out further 

 points of considerable interest. For if nerve-fibres are teased in " normal 

 saline " and allowed to lie in this solution for some time before being tested 

 with Macallum's solution, the potassium solution is found much more widely 

 distributed. It is now found in all those regions of the fibre, in which 

 toluidine blue can be used to reveal its own characteristic effects. Under 

 these conditions potassium is discoverable at each site of injury, and in a 

 neighbouring tract to which it has diffused from this site. Potassium is also 

 now detected in all those regions in which I have described the occurrence 

 of coagulative change. We are, therefore, introduced to a new and very 

 important conclusion. Potassium salts are also discoverable in regions 

 of the fibre distant from points of injury, provided that time is given for 

 their appearance. 



Although not situated at injured points this more distal region of the 

 fibre has a secondary relation to injury of a most interesting kind. This 

 relation is at once made evident on reference to Bethe's observations upon 

 the staining capacity of polarised nerve-fibres. 



Bethe has made use of an ingenious experimental method, whereby the 

 " fixation " of a nerve is secured during the traverse of a polarising current. 

 The nerve is then prepared for examination under the microscope, and is 

 stained in toluidine blue or in some other similar basic dye. Prepared in 

 this way the fibres are found to exhibit a remarkable distribution of staining 

 capacity. The anodal region is pallid, the indifference point is studded with 

 granules, and the kathodal region is intensely stained. It will be seen that 

 this set of appearances is exactly similar to that which I have observed in 



