736 HISTOLOGIC TECHNIC 



hours. (For relatively large pieces of tissue cedar-wood oil or pure 

 anilm oil may be substituted for the xylol). 



4. Place in melted paraffin containing a little xylol (or cedar oil) ; 

 that which has been previously used for embedding does very well. 



5. Transfer to pure melted paraffin. 



6. Transfer to a second dish of pure melted paraffin (with melting 

 point of about 54). The object of these changes is to replace the 

 xylol (or oil) with pure paraffin. If the xylol is not completely re- 

 moved the tissue will contain bubbles and satisfactory sections cannot 

 be made. 



7. Embed in a paper box or a watch glass. If glass is used the 

 surface should be smeared with the least trace of glycerin to prevent 

 adhesion. The box should be filled with pure melted paraffin, the tissue 

 handled with warmed forceps, and placed with proper orientation so 

 that it is completely covered with the melted paraffin. The paraffin is 

 now rapidly cooled by immersion in cold water; in summer months 

 ice water must be Used. If a paper box is used it can be left to float 

 on the water until the paraffin is thoroughly congealed. The manner 

 of preparing these boxes is shown in Fig. 594. 



Considerable depends upon the choice of a proper grade of paraffin. 

 That which melts at 58 to 65 is most desirable for use in temperate 

 climates during the warmer mouths; during the winter months paraffin 

 of 54 to 56 is preferable. If too hard, the paraffin cracks; if too soft, 

 it fails to retain its form during sectioning. The former condition may 

 be improved, if necessary, by the proximity of a small flame during the 

 sectioning process, or by breathing upon the knife blade and tissue block ; 

 the latter fault may be remedied by placing the tissue for a short time 

 in the refrigerator, just prior to cutting. 



SECTIONING 



The cutting of free-hand sections is so simple an operation as to 

 scarcely require description. A small, inexpensive hand microtome and 

 a sharp razor whose surfaces are ground flat, not concave, are all that is 

 necessary. 



For more precise sectioning a stationary microtome is a necessity. 

 Many types of these instruments are on the market. The Thoma type 

 of instrument is specially adapted for celloidin work, but may also be 

 used for paraffin sections. The Schanze instrument is very useful for 



