104 Neurological Technique 



specimens may remain in it indefinitely without injury to the 

 specimen and consequent inconveniences to the operator. 



Also the preservation should be of such a character that 

 during the dissection tne material may, without injury, be so 

 treated that it may be handled freely without discomfort to the 

 hands. 



In case of the central nervous system especially, the above 

 conditions are more nearly attained by the use of formalin as 

 the fixing and preserving agent than by the use of any other 

 agent known. Alcohol, for example, decolorizes, shrinks, and 

 distorts the specimen, and also renders it brittle. Chromic 

 acid and the bichromates do not shrink the specimen ; rather 

 they produce an increase in its_vOlume, which, for dissecting 

 purposes, is not very objectionable. But, on the other hand, 

 they do produce a brittleness of the tissue which, especially 

 after a long period of action, renders satisfactory dissection 

 impossible. They also color the specimen either black or 

 dark brown. It may be said, though, that the salts of chromic 

 acid, especially the bichromate of potassium, give rise to 

 fewer objectionable qualities than do any of the other salts 

 usually employed for fixation. Again, brains fixed in acids 

 (nitric or acetic usually) become greatly swollen and discolored, 

 and, while possessing a fair degree of toughness, they soften in 

 a very objectionable way when placed in water, and repeated 

 washing in water is necessary that the specimen may be handled 

 without injury to the hands. 



Commercial formalin is a 40 per cent, aqueous solution of the 

 ^■as,, formic aldehyde or formaldehyde . Formaldehyde(CH20) is the 

 intermediate product in the oxidation of methyl alcohol 

 (CH^O) into formic acid (CHgOg). Methyl alcohol when 

 used as a fixing agent produces a violent shrinkage of the 

 tissue, while formic acid, by oausing the tissue to take up water, 

 produces a greater swelling than is produced by any of the - 

 acids used in histological technique. Theoretisally, therefore, 

 formalin, the median product between methyl alcohol and 

 formic acid, should produce a median effect upon the tissues. 



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