METHYLEN BLUE. 81 



the nerves were still living at that time. But it seems more 

 natural to conclude with APATHY (Zeit. f. wiss. Mile., ix, 1, 

 1892, pp. 15 et seqq.) that nerve-tissue can be stained after 

 life has ceased. 



APATHY has directly experimented on this point, and sums 

 up the necessary conditions as follows : The tissue need not 

 be living, but must be fresh; nothing must have been ex- 

 tracted from it chemically, and its natural state must not have 

 been essentially changed by physical means. For example, 

 the tissue must not have been treated with even dilute 

 glycerin, nor with alcohol, though a treatment for a short 

 time with physiological salt solution is not very hurtful; 

 it must not have been coagulated by heat. 



Another common belief concerning the methylen-blue 

 nerve-reaction is that the presence of oxygen is necessary to 

 the reaction. It is therefore the usual practice to dissect out 

 the organ to be investigated after having exposed it to the 

 action of methylen blue by injection or immersion, and leave 

 it for some time exposed to the air. APATHY has also inves- 

 tigated this point, and finds (loc. cit., p. 25) that the practice is 

 in some cases correct, but the belief erroneous. It has been 

 explained above that shortly after a tissue has attained the 

 maximum degree of coloration of which it is susceptible it 

 begins to give up its colour again to the surrounding liquid. 

 The larger the volume of liquid with which the tissue is sur- 

 rounded, the faster will this washing-out process go on; and 

 in order that it may not go on with excessive rapidity, wash- 

 ing out the stain from the nerve-fibres as well as from the 

 earlier stained elements (which alone it is desired to wash 

 out, so as to leave a differentiated specific stain of nervous 

 elements), it is in many cases desirable to have the process 

 go on in presence of as little liquid as possible. Another 

 consideration that justifies the practice is that by exposure 

 to air the preparations take up a trace of ammonia (derived 

 from the ammonium carbonate of the air) ; and Apathy has 

 experimentally established that this is an important factor 

 in the sharpness of the stain. Oxygen has nothing to do 

 with it. 



116. Staining Nerve-tissue by Injection or Immersion. The 

 practice of the earlier workers at this subject was (following 



6 



