276 



TECHNIQUE 



means of a warm scalpel blade, leaving plenty of paraffin around 

 the tissue. To fasten this paraffin block to metal disc or wood 

 block, an old scalpel blade is heated so that it will easily melt 

 paraffin. The blade is then held against the metal disc or paraffined 

 end of a wood block, and then the paraffin piece is pressed against 

 the other surface of the blade. The blade is withdrawn and the 

 melted paraffin thus produced hardens and seals the paraffin block 

 to the metal or wood holder. It is advisable to place mounted 

 tissue in cold water for a minute or two to completely harden the 

 paraffin. 



The next step is to trim the paraffin block containing the tissue. 

 Some of the extra paraffin about the object is cut away, so that the 



piece is in the shape of a truncated 

 pyramid, with the base toward the 

 wood block or metal. The paraffin 

 at the free end is trimmed down until 

 little is left between the end face and 

 the tissue. The free face should be 

 rectangular or square in form, and 

 the angles between adjacent edges 

 should be right angles. (Fig. 164.) 



It is an easy matter to prepare a 



considerable number of small wood 



blocks, and these can be furnished to 



all the students who can proceed to 



mount embedded material and have it 



ready for sectioning. Or if one has a 



good many paraffin blocks to section, 



it is more efficient to mount all on 



wood blocks at one time and store 



them in boxes properly- labelled for 



sectioning later. Storing paraffin blocks in water a week or more 



before cutting makes cutting easier and prevents brittleness and 



cracking of the tissue. 



The clamp holding the mounted ])araffin-wood block is attached 

 to a movable part of the microtome that is connected with a finely 

 threaded screw. The turn of the wheel tiu-ns the screw and ad- 

 vances the part holding the paraffin block. It will be noted that 

 in this ])rocess the paraffin block rises and falls. Each machine 

 has a setting de\'ice by which the thickness of the sections can be 



Fig. 164. — Diagram illustrat- 

 ing paraffin block ready for sec- 

 tioning. 



