652 TECHNIQUE 



6. Transfer to a second dish of pure melted parafin. The 

 object of these changes is to replace the xylol (or oil) with pure 

 parafin. If the xylol is not completely removed the tissue will con- 

 tain 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 pre- 

 vent adhesion. The box should be filled with pure melted parafin, 

 the tissue handled with warmed forceps, and placed with proper 

 orientation so that it is completely covered with the melted parafin. 

 The parafin 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 parafin is thoroughly 

 congealed. The manner of preparing these boxes is shown in 

 Fig. 462. 



Considerable depends upon the choice of a proper grade of 

 parafin. That which melts at 58 to 65 is most desirable for use 

 in temperate climates during the warmer months ; during the 

 winter months parafin of 54 to 56 is preferable. If too hard, the 

 parafin cracks; if too soft, it fails 'to retain its form during sec- 

 tioning. 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 micro- 

 tome and a sharp razor whose surfaces are ground flat, not con- 

 cave, 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 parafin sections. The Schanze instrument is very 

 useful for celloidin sections and may also be used for small celloid- 

 in sections. The Minot rotary microtome is specially adapted for 

 the production of serial sections in parafin. These instruments of 

 themselves suggest the manner in which they are to be used, and 

 the technique is easily acquired. Like all delicate instruments, 

 they must be kept well cleaned and properly oiled, to do good 

 service. 



