Need and Action of Reagents y 



The required differentiation, on tiie other hand, often cannot 

 be obtained without previous treatment with a certain reagent. 

 For example, the pictures of the neurone given by silver nitrate 

 (Golgi method), or those given by certain of the specific hsema- 

 toxylin methods for the myeline sheaths (Weigert's method), 

 have not been obtained without previous treatment with a mix- 

 ture containing chromic acid or one of its salts. The bichromate 

 of potassium, the salt more often used in fluids for these meth- 

 ods, is supposed to form distinct chemical compounds with the 

 special tissue elements. The compounds formed by its action 

 upon the nerve cell react, in the one case, selectively upon silver 

 nitrate or probably silver chromate, while that formed by its 

 action upon the medullary sheath is, in the other case, selectively 

 acted upon by the special haematoxylin mixture. 



Thus the fixing agents in more general use may be roughly 

 divided into two classes: 



1. Those which fix, coagulate, or precipitate without com- 

 bining chemically with the tissues. 



2. Those which in the process of fixation and hardening may 

 form chemical compounds with certain of the structural elements 

 of the tissue. 



A reagent of either class may exert a distinctive action, and 

 in the special histological methods advantage is taken of this 

 action. 



THE USES OF ALCOHOL. 



As a fixing agent. — Alcohol has, in the past, been much 

 used as a fixing agent, especially by physicians. Because of its 

 shrinking effect upon the tissue it is not altogether satisfactory, 

 but it is sometimes necessary, both because of its ready avail- 

 ability and because it is thought to merely coagulate or pre- 

 cipitate without otherwise altering the chemical composition of 

 the tissues. 



Alcohol mixes with water with a violent ionic action. Its 

 molecule is supposed to pass through the cell-wall less readily 

 than that of water, and the resultant shrinkage is therefore due 

 to the water being extracted from the cell. When used as a 



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