

HISTOLOGICAL METHODS. 155 



such organs as the liver, kidney, intestine, etc., when 

 these have been previously partially hardened by the agents 

 just mentioned. 



279. Chromic Acid and Alcohol A solution of 



chromic acid in dilute alcohol is prepared thus : 



Chromic acid, i gramme. 



Water, 2oCC. 



Dissolve, and slowly add rectified spirit, iSoCC. 

 This fluid, devised by my former assistant Dr. Pritchard, 

 is excellent for hardening the retina, membranes of the coch- 

 lea, skin, and possibly some other organs, (pp. 4, 5, and 6). 



280. Sections of organs hardened in the above sub- 

 stances generally require to be rendered transparent. Gly- 

 cerine, or its diluted form Farrants' solution, clove oil, or 

 turpentine, are commonly employed, and the tissues are 

 preserved in glycerine, Farrants' solution, or dammar. 



281. Osmic Acid (Os 04) is a very valuable hardening 

 agent introduced by Max Schultze. It occurs in commerce 

 in small vitreous-like masses kept in sealed tubes. It is 

 most conveniently kept as a i per cent solution in distilled 

 water, preserved in well-stoppered bottles protected from the 

 light. It is excessively poisonous, and its disagreeable, 

 heavy, pungent vapour is dangerously irritating to the 

 conjunctiva and nostrils. Employed in solutions of T V to i 

 per cent it is of great service for hardening embryonic 

 tissues, nerve fibres, retina, connective tissue corpuscles, 

 epithelium, and epidermis, testis, etc. It fixes the tissue 

 elements without producing a granular precipitate or causing 

 shrivelling. It blackens fat and the medullary matter of 

 white nerve fibres, and it also darkens, though to a much 

 less extent, albuminous tissues. From ten minutes to two 

 days are required. The pieces of tissue should always be 

 small. After hardening they are washed in distilled water, 

 and are then placed in rectified spirit, or sections are made 

 and mounted in a saturated solution of potassium acetate, 

 or in Farrants' solution. The former is the best (Schultze), 

 for glycerine renders them too transparent. 



28 1 A. Silver Nitrate, Gold Chloride, Palladium 

 Chloride, are also used for hardening tissues, but they will 



