157 



more clearly than in preparations hardened in alcohol, zinc chloride, 

 or other commonly used reagents, and is, indeed, fully as pronounced 

 as in fresh brains. The only important defect that is noticeable in 

 such brains is a marked increase in volume. This swelling action of 

 formol, which in the sheep's brain produces an enlargement of some 

 40 7o of the original volume, has already been noted by Lanzillotti- 

 BuoNSANTi ('94, p. 274), Hoyer ('94, p. 237) and Hofer ('94, 

 p. 93). Although perhaps less of a disadvantage than the opposite 

 result, shrinking, it nevertheless distorts the natural relations of parts 

 to such an extent that its avoidance is desirable. 



With the object of correcting this defect we were led to experi- 

 ment with other hardening fluids. The reagents usually employed in 

 hardening brains — alcohol, zinc chloride, etc. — shrink rather than 

 swell the nervous tissue; hence we decided to try some combination 

 of these fluids with formol in such proportions that the swelling etfect 

 of the formol would be counterbalanced by the shrinking action of 

 the other fluid. Zinc chloride when mixed with formol tends to ob- 

 literate the distinction between the white and the gray substance and 

 was therefore abandoned. Alcohol and formol on the other hand pre- 

 serve this difierence as well as formol alone does, and, after some 

 experimenting with fluids mixed in different proportions, the following 

 combination was found to meet the requirements. 



Alcohol 95 Vo — 6 volumes, 

 FormoP) 2 „ — 4 



Owing to the rapidity with which this mixture penetrates tissues, 

 the hardening of large, freshly prepared brains necessitates little more 

 than simple immersion. This is best carried on in tightly stoppered 

 jars, for the formol loses strength rapidly on exposure to the air; 

 moreover, care should be taken that during the process the brain 

 rests on some soft material such as cotton-wool and not against the 

 resistant sides of the jar. In the case of sheep's brains from which 

 the dura mater had been removed, the process was completed in about 

 a week. If there is five times as much fluid as brain substance, a 

 renewal of the hardening mixture during the process is not necessary. 

 After the brain has been finally prepared by the removal of the pia 

 mater, etc., the fluid in which it was hardened, if filtered, seems to 



1) The formol employed was that sold by E. Meeck, Darmstadt, 

 Germany, as Formaldehyd 40 "/q. 



