4 Neurological Technique 



little can be said as to the exact action upon the tissue of any 

 fixing fluid. If these processes were known, or even the 

 chemical changes taking place which result in the death of the 

 organism, then there might be formulated what, at least, would 

 more nearly be a working hypothesis as to the action of the 

 various reagents, and a method for the study of some special 

 structure could be devised on a more logical basis. 



The procedure has usually been to totally disregard the chem- 

 ical constitution of the cell, but to use reagents the action of 

 which is to produce death changes modifying as little as possible 

 the normal structure of the cell. The investigator, desiring a 

 method for a specific purpose, quite often has obtained that 

 method either by chance or only after repeated and laborious 

 experimentation. And too often has it been true that his pub- 

 lished observations had more to do with what his method gave 

 him than with the normal structures of the tissue he described. 

 A reagent should be proved by comparing its results as far as 

 possible with the appearances of the fresh tissues and also with 

 the results obtained after various other fluids. 



Opinion is somewhat divided as to the nature of the process 

 of fixation, or what the exact action of the reagent is upon the 

 tissue. It is held by some' that fixation consists in the coagula- 

 tion of the albuminoids, etc., of the tissue, or in rendering more 

 rigid and insoluble certain of the tissue constituents otherwise 

 fluid, semifluid, or soluble ; and that, in doing this, the fixing 

 agents in the majority of cases enter into chemical combination 

 with the tissues or with certain of the tissue elements. Many of 

 the various tissue elements are of different chemical composition, 

 and thus reagents acting upon them may produce different chemi- 

 cal compounds. On this basis, in the methods devised for the 

 special differentiation of particular elements, the elective actions 

 of the special mordants and stains must depend upon the chemi- 

 cal action of the likewise special fixing agent which precedes. 

 This may be called a chemical theory. 



'See Lee, The Microtomist's Vade-mecum. Fifth edition. Philadelphia, 1900. 



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