458 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



rapidly killing the cell and preserving its constitu- 

 ents, nucleus and cytoplasm, before disintegration 

 can take place. Most fixing agents coagulate the 

 protoplasm and cell contents. 



(a) Heat. Cover-glass preparations may be fixed 

 by heating them in an oven or over a gas flame to 

 100 or even 150 C. 



(b) Fluids. 



(1) Acids osmic, i%; chromic, i%; ni- 



tric, 10%; etc. 



(2) Salts mercuric chloride saturated, 



potassium bichromate, 3%; etc. 



(3) Alcohol, 95%, or absolute. 



(4) Formalin, 5%. 



Acids and salts may be used separately as above, 

 but as a rule different combinations are used, 

 formulae of which are given on another page. 



(c) Precautions. 



(1) Fix living tissues, if possible. 



(2) Fix small pieces, so that fluids may 



readily penetrate. 



(3) Heat hastens the penetration. 



(4) Change as often as the fluid becomes 



cloudy. 



(5) Use a large quanitity of fluid (50 to 100 



times the volume of the tissue) . 



2. Washing. 



(a) Water. If there is a chemical change between 

 the fixing agent and the tissue, use water. This is 

 most frequently the case. The specimens should be 

 very thoroughly washed, preferably in running 

 water, for twenty-four hours or more. 



(6) Alcohol. When alcohol is indicated, use the 



