Vll] SECTION CUTTING AND IMBEDDING 59 



NOTES. 



Clearing agents. Cedar-wood oil and clove oil are the essential 

 oils chiefly used for displacing the alcohol before the piece of tissue 

 is placed in paraffin. The former is generally recommended, but 

 different specimens vary more in their power of taking up water than 

 do different specimens of clove oil, so that if tissues are passed 

 direct from 95 p.c. alcohol, it is safer to use clove oil. Clove oil makes 

 the tissues more brittle than does cedar-wood oil, so that it is less 

 good for a large piece of tissue, and it makes a somewhat longer stay 

 in paraffin necessary. (Cp. also p. 305.) 



When a piece of tissue is transferred from alcohol to an essential 

 oil, it rises to the surface, and a rapid evaporation of alcohol occurs; 

 to avoid this the tissue with absolute alcohol is placed in a small 

 bottle (or test tube) and the pipette containing the essential oil is 

 passed to the bottom of the bottle and the oil allowed to form a layer 

 under the alcohol. When the tissue has sunk into the oil, the alcohc 1 

 is sucked off. 



Xylol or turpentine are often used in the place of an essential oil, 

 since they allow a more rapid penetration of the paraffin. Tissues 

 warmed in these are very apt to become brittle, so that stay in these 

 fluids should be shortened as much as possible. Turpentine is 

 commonly said to cause more shrinking than the other clearing 

 agents, but I have not found that the particular clearing agent used 

 has any effect upon the appearance of sections of the great majority 

 of tissues, provided the stay of the tissue in it is short. 



The larger the spaces in a tissue, and the less it is hardened, the 

 more gradual should be the changes from one fluid to another. For 

 tissues much liable to distortion the following method may be used : 



The tissue is transferred from absolute alcohol to chloroform by 

 placing a layer of chloroform under the alcohol (cp. above), paraffin 



