GENERAL TECHNIC 15 



for holding the specimen and a microtome knife or razor. The two 

 are so arranged that when knife and specimen meet, a section of any 

 desired thickness may be cut. 



The technic of section cutting differs according to whether the 

 specimen is embedded in celloidin or in paraflin. 



In cutting celloidin sections the knife is so adjusted that it passes 

 obUquely through the specimen, as much as possible of the cutting 

 edge being used. The knife is kept flooded with 8o-per-cent. alcohol 

 and the specimens are removed by means of a camel' s-hair brush to a 

 dish of 8o-per-cent. alcohol, where they may be kept for some time 

 if desired. When celloidin sections tear or when very thin sections 

 are desired it is often of advantage to paint the surface of the block, 

 after cutting each section, with a coat of very thin celloidin. 



Celloidin sections are usually not thinner than lo//, although 

 under favorable conditions sections 5//^ or even 3/« in thickness may 

 be obtained. 



In cutting parafhn sections the knife is kept dry and is passed 

 not obliquely but straight through the specimen, only a small part of 

 the cutting edge being used. An exception is made in the case of very 

 large parafhn sections, where an obhque knife is used. Sections are 

 removed from the knife by a dry or slightly moistened brush. If not 

 desired for immediate use the sections may be conveniently kept for a 

 short time on a piece of smooth paper. If sections curl they may be 

 flattened by floating on warm 30-per-cent. alcohol or on warm 

 water. 



Parafhn sections may be so cut that the edges of the sections ad- 

 here. Long series or "ribbons" of sections may thus be secured. 

 This is of decided advantage when serial sections are desired. Fail- 

 ure of the sections to cut evenly or to adhere in ribbons is usually due 

 to the paraffin being too hard and brittle, which of course is due to its 

 low temperature. If much section cutting is to be done, the operator 

 will find himself amply repaid by having the room-temperature fairly 

 high. In case paraffin of a melting-point of 50° to 55° C. is used, a 

 room temperature of 73° to 75° F. will greatly facilitate the work. 

 Where it is not possible to have a high room temperature, recourse 

 may be had to coating the surface of the block with a paraffin of lower 

 melting-point than that used for the embedding. A similar effect 

 may be obtained by holding a heated metal plate or bar near the 



^ 11 =micromillimeter or micron =Taoo of ^ millimeter = microscopic unit of measure 

 = about jsioo 



