30 PRACTICAL HISTOLOGY. 



tions of pure chromic acid do not penetrate well into tissues, 

 therefore the pieces of tissue must be small. In the case of tissues 

 placed in Mailer's fluid or potassic bichromate, they are hardened 

 very slowly. In the case of most organs 3-4 weeks suffice, but in 

 the case of the brain and spinal cord it takes sever il months to 

 harden tissues and organs in these fluids. The fluid must be 

 frequently changed, and it must be large in amount. The forma- 

 tion of fungi on its surface may be prevented by adding a piece of 

 camphor, thymol, or naphthaline to the fluid. The hardening 

 process is accelerated when the fluid and tissue are kept at 3o-4o 

 C., but the result is not so satisfactory as by the slower cooler 

 process. 



Except for special purposes, e.g., Weigert's method for the cen- 

 tral nervous system, the chromic acid salts must be thoroughly 

 washed out of the hardened tissue or organ. This is done by 

 leaving them for many hours or days in a stream of running water. 

 In all cases it is well to keep the fluids and tissues in a cool place,, 

 and in some cases in the dark, e.g., 3 and 6. 



In connection with the hardening of tissues, especially those of 

 the central nervous system, it is, with right, insisted upon that the 

 hardening fluid should be frequently changed. I find, however, 

 and my observations arc borne out by other observers, that after 

 the first change of fluid a very satisfactory result is obtained by 

 placing the organ to be hardened in a very large volume of the 

 hardening fluid, e.g., the spinal cord of a cat or dog in a litre of the 

 hardening reagent, and leaving it under proper conditions until the 

 cord is hardened. Stir the fluid from time to time. 



Chromic acid seems to form a compound with the tissues, so 

 that it is not easily removed from them. Tissues hardened in 

 chromic acid are not readily stained by carmine, so that for this 

 purpose it is better to use hsematoxylin or safranin. 



C. Acids and Acid Mixtures. 



1. Picric Acid. Make a cold saturated solution of picric acid in 

 water. There should always be a large excess of crystals on the 

 bottom of the bottle. Place a small piece of tissue in the solution 

 for 6-24 hours. The tissues are to be afterwards w T ashed in 70 per 

 cent, alcohol not water and transferred to 95 per cent, alcohol. 



2. Kleinenberg's Picro-Sulphuric Acid. To 100 cc. of a cold 

 saturated watery solution of picric acid add 2 cc. of concentrated 

 sulphuric acid. This causes a copious precipitate. After twcnt} r 

 hours filter, and to the filtrate add 300 cc. of distilled water. 



Tissues are placed in this fluid for a comparatively short space of 

 time -from a few minutes to two or six hours. The time should 

 never exceed six hours. The liquid must be changed if it becomes 



