Need and Action of Reagents 3 



appears as a black insoluble precipitate, and thus the reagent 

 has a blackening effect upon the tissue which is often undesir- 

 able. Also, the staining qualities of the tissue fixed in'osmic 

 acid are often greatly impaired. It will be found, however, that 

 occasionally the browning or blackening action of osmic acid is 

 its virtue. Advantage is taken of it in some of the special 

 methods where the object is to differentiate just -those structures 

 which are most blackened by the reagent. One of the most 

 serious objections to osmic acid is its poor penetrating power. 

 While fixing rapidly the parts with which it comes in direct 

 contact, it penetrates the tissue so slowly that it can be used 

 alone successfully only upon very small pieces. 



The bichloride of mercury (corrosive sublimate) is another 

 example of a reagent which, when used alone, produces verv 

 good fixation. Mercuric chloride is also reduced by the tissue, 

 and the resulting, almost insoluble, albuminate of mercury forms 

 crystals in the tissue which are very objectionable. 



These objectionable results may be got rid of by some 

 special procedure, which, however, is often long and tedious, 

 and the effect of which upon the tissue may be injurious. Spe- 

 cimens blackened by osmic acid may be bleached by subjecting 

 them to hydrogen peroxide, which reoxidizes the reduced black 

 product. The crystals resulting from corrosive sublimate may 

 be finally removed by long washing in water and 70 per cent, 

 alcohol, or more rapidly by treating the tissue for twenty-four 

 hours or longer with a dilute solution of iodine. The iodine is 

 supposed to gradually convert the crystals into the more soluble 

 iodine compound (probably the biniodide of mercury). 



Both corrosive sublimate and osmic acid are frequently used 

 in mixtures, and the formula of some of the best fixing fluids 

 call for one or the other as the chief ingredient. When mixed 

 with chromic acid, as in Flemming' s fluid, for example, the black- 

 ening action of osmic acid is much reduced. 



The nature of fixation. — Owing to the fact that so little is 

 known of the chemical processes taking place within the 

 living tissues, both normally and in pathological conditions, very 



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