LABORATORY DIRECTIONS. 459 



grades 50%, 70%, 95%, and leave in 80%. When 

 ever picric acid forms a part of the fixing agent, 

 alcohol wash must be used. 



3. Dehydrating. 



(a) We dehydrate to preserve the tissue from 

 action of bacteria. 



(6) In order that imbedding fluids, which do not 

 mix with water, may penetrate the tissue. Alcohol, 

 the grades ending with absolute alcohol, is always 

 the agent used. 



4. Imbedding with Celloidin, Evaporation Method. 



(a) A bsolute A Icohol and Ether, Equal Parts. After 

 thorough dehydration in absolute alcohol, the tissue 

 is transferred to absolute alcohol and ether, equal 

 parts, where it is left for twenty-four hours. 



(b) Thin celloidin, 4%, made by dissolving cel- 

 loidin shreds in equal parts of absolute alcohol and 

 ether. The tissue may be left any length of time in 

 thin celloidin, the longer the better. 



(c) Thick celloidin, 10%; time, twenty-four hours 

 or longer. 



5. Mounting on Block and Evaporation of Ether. 



(a) Cover surface, of perfectly dry block, with thin 

 celloidin. 



(6) Remove the tissue from thick celloidin and place 

 upon block, in the proper position for cutting. 

 This is called orienting. A piece like a nerve may 

 have to be supported with needles, and these re- 

 moved later. 



(c) Add thick celloidin, from time to time, and open 

 any air bubble that may appear. Leave the speci- 

 men in air until the ether has evaporated so that the 



