44 Microtomes and Microtome Knives (chap. 4) 



and required for some processes when certain cell components must be 

 retained, such as fat and enzymes, also for immediate diagnosis. 



4, The ultra-thin sectioning microtome for sections thinner than 1 micron, 

 electron microscopy. Only for special technics and not commonly found in 

 laboratories. 



5. The base sledge microtome for exceedingly large tissues (brains) and hard 

 blocks of tissues. Only for special technics and not commonly found in 

 laboratories. 



Microtomes should always be kept well oiled to prevent parts from 

 wearing unnecessarily or sticking. Either of the latter faults can cause 

 imperfect sectioning — sections of variable thickness. Obtain advice 

 from an expert concerning the parts to be kept oiled, and consult the 

 booklet accompanying the instrument. The best oil recommended for 

 this purpose is Pike Oil, manufactured by Behr-Manning, Troy, N.Y., a 

 Division of Norton Co., Abrasives. 



Microtome Knives 



There are three familiar types of microtome knives: 



1. The plane-edge for frozen sections and paraffin ribbons. 



2. The biconcave used sometimes for paraffin work. 



3. The plane-concave for celloidin, sometimes for paraffin. 



Because knives seem to demand hours of attention, they often become 

 the technician's nightmare, and the task of keeping them in optimimi 

 condition presents problems. 



Theoretically, a perfect cutting edge is the juncture of two plane 

 smooth surfaces meeting at as small an angle as is feasible — ideally 14° 

 as suggested by Dr. Lorimer Rutty [Krajian and Gradwohl, 1952, page 

 28). These cutting surfaces are called the ciuting facets. The cutting 

 edge of a very sharp knife, when examined by reflected light under 100 

 magnifications, appears as a fine discontinuous line. It may vary slightly 

 in width, but it should show only a slight reflection, a narrow, straight 

 bright line. At a higher magnification of about 500 times, the edge will 

 have a finely serrated appearance. The fineness or coarseness of these 

 serrations depends on the degree of success in sharpening the knife. The 

 facets are determined by the "back" which is slipped on the knife during 

 sharpening to raise it just enough to form these facets (Fig. 4). Every 

 knife must have its own back. Never interchange backs. The back must 



