2 Neurological Technique 



begun. It must kill or fix and render permanent, without dis- 

 torting the normal shape, size, and position of the tissue ele- 

 ments. It should also either itself have a differentiative action, 

 or its action should be such as to produce, enhance, or allow the 

 best action of the special differentiating reagents (stains, etc.) 

 used in the after-treatment. 



As to the fixing value of the various reagents employed, 

 experience has taught that certain qualities must be avoided : 



1. A fixing fluid must not be alkaline in reaction. All the 

 bases, and also the carbonates of the alkali metals, kill quickly 

 enough, but tend to dissolve certain of the structures rather 

 than fix them. 



2. The fluid may be neutral, but would best be slightly 

 acid in reaction. Most acids, however, when present in too 

 great proportion, produce a swelling more or less distorting, by 

 causing the tissue to take up water. The swelling action of 

 some acids is much more marked than that of others, and there- 

 fore the proportion of an acid in a fluid must be considered 

 individually. 



3. On the other hand, certain reagents, while fixing rapidly 

 and permanently, cause a violent shrinkage of the tissue. 

 Ordinary alcohol and some of the more neutral salts cause an 

 unbalanced exosmosis of the more fluid cell contents, and the 

 consequent shrinkage or collapse of the cell is decidedly 

 objectionable. Alcohol as a fixing agent will be touched upon 

 later. 



The ideal fixing fluid would be one in which the ingredients 

 are mixed in such proportions that the swelling tendency of the 

 one will counteract the shrinking tendency of the other, and 

 that the fixing efficiencies of all would be combined. 



There are a few reagents which, when used alone, produce 

 an excellent fixation, and it is sometimes best that they be used 

 alone. But for most purposes these also have their drawbacks. 



Osmic acid (the tetroxide of osmium) is one of the best of 

 these. It fixes quickly and well, but is easily reduced by the 

 tissue, especially where there is fat. The reduction product 



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