52 



Paraffin Method (chap. 5) 



Figure 11. Mounting paraffin tissue block 

 on an object disc. 



slide) is held betAveen the paraffin on the object disc and the under 

 surface of the tissue block (Fig. 11). When both surfaces are melted, the 

 instrument is withdra^vn and the tissue block pressed firmly into the 



paraffin on the object disc. 

 After cooling, the block is 

 ready to be sectioned (Fig. 

 12). 



Set the Rotary micro- 

 tome for section thickness 

 (5 or 6 microns, etc.) . Raise 

 the tissue carrier and place 

 in it the object disc Avith 

 its mounted tissue block. 

 Tighten the clamp of the 

 tissue carrier onto the stem 

 of the object disc. If using 

 a wooden block, allow it to 

 extend about \ inch be- 

 yond the metal clamps; this 

 will prevent the paraffin 

 block from breaking loose 

 from the wooden block when the clamp is tightened. 



Insert the microtome knife and tighten its clamps. The knife must 

 be held in the clamps at a proper angle for optimal sectioning, produc- 

 ing a minimal amount of compression, and allowing the sections to 

 adhere to each other. Many suggestions can be made concerning angle 

 determination, but in most instances the technician finds the answer 

 after a process of trial and error. The cutting facets are small as deter- 

 mined by the back. When placed in the microtome, the knife must be 

 tilted just enough so the cutting facet next to the block and the surface 

 of the block clear each other. If the tilt is not sufficient (Fig. 13), the 

 surface of the block is pressed down with the wedging effect of the facet 

 and no section results. At the next stroke of the knife, this compression 

 increases, and finally a thick section is cut, a composite of the com- 

 pressed sections. Too great a tilt of the knife makes it scrape through 

 like a chisel, rather than cut through the tissue (Fig. 14). 



Turn the feed screw handle (left side of instrument), moving the 

 feed mechanism backward or for^vard until the face of the tissue block 

 barely touches the cutting edge of the knife. With an old knife start 

 trimming into the block luitil the desired area is reached. Change to a 

 good knife and readjust tissue carrier if necessary. Never touch the edge 



