Making Sections 163 



rial that would be dissolved out by the reagents used prior to impregnat- 

 ing; and, second, when speed is of primary importance, as in the pro- 

 duction of quick sections from tumors for diagnostic purposes. In both 

 these cases, recourse may be had to the method of frozen sections, in 

 which material is rapidly frozen until it is of a consistency to cut. Frozen 

 sections should not, however, be employed on any occasion when the 

 normal processes of embedding can be used. 



Fig. 127. Spencer clinical microtome fitted for freezing. 



Choice of a Microtome. The type of microtome shown in Fig. 127 is 

 so universally employed that it will be taken as the basis for the present 

 discussion. It is essential in cutting frozen sections that the knife slice, 

 rather than push, through the tissue. This slicing effect is produced by 

 mounting the knife to swing through the object when the handle on top 

 is turned. These microtomes are not so accurate, either as to the thickness 

 of section cut or the repetition of this thickness, as is the big Schantz 

 shown in Fig. 104, but it is to be presumed that no one would cut frozen 

 sections, in any case, if thickness and reproducibility were primary ob- 

 jectives. The method of freezing the object will be discussed after we 

 have dealt with the question of supporting the material in a suitable 

 medium. If there are available even a few moments in excess of the 

 absolute minimum time required to cut without embedding, better re- 

 sults will be obtained if the object is smothered in several layers of: 



