388 GENERAL MICROSCOPIC TECHNIQUE. 



sary to save time (e. g., in diagnosis during a surgical operation), 

 and where methods of maceration or digestion are to bo 

 employed in the study of the sections. 



Where finer and more minute study is necessary, the sec- 

 tions must be fixed and hardened in various fluids, and 

 impregnated with paraffin or celloidin. 



(c) Fixation of Tissues. 



By this term we mean the killing and fixing of the fresh 

 tissue by means of various fixing fluids in such a way that the 

 structure remains unchanged. The object of this is to prevent 

 subsequent shrinkage of the tissue. 



Certain general rules with regard to the use of fixing fluids 

 may be mentioned : 



(a) Fixing fluids should always be used in large quantities 

 a volume fifty to a hundred times as great as that of the 

 tissue. This is to prevent its being too much diluted by the 

 fluids of the tissue. 



(b) The pieces of tissue used should be as small as possible. 

 Where a large section is necessary, the piece of tissue can be 

 made thin and the section cut from its flat surface. 



(c) The tissue should be taken perfectly fresh from the 

 animal's body. 



(d) Fixing fluids should be prepared freshly. 



(e) In the bottom of the flask in which the tissues are fixed, 

 a small amount of cotton wool should be placed, to prevent the 

 soft tissue being distorted by lying against the bottom. 



The most useful fixing agents are the following : 



(1) Absolute alcohol is a good fixing medium for very small 

 pieces of tissue. It should be changed often, and should be 

 allowed to act for six to twenty-four hours. 



(2) Perosmic acid (0.5-1 per cent, solution) can be used 

 only for very small pieces of tissue. It has small powers of 

 penetration. Its maximum action is obtained in six to twenty- 

 four hours. It should be kept in the dark in a glass-stoppered 

 bottle. It is sometimes better to add a drop of nitric acid to 

 the fluid, to prevent its reduction. The vapor of osmic acid is 



