74 Nitrocellulose Method (chap. 7) 



70%, 80% or 95% alcohol, but for long storage add a little glycerine 

 to the alcohol. 



Sectioning 



Clamp the jaws of the tissue carrier tightly against the lower two- 

 thirds of the mounting block. If the jaws are clamped against the upper 

 portion, the resulting compression in the block can loosen the nitro- 

 cellulose. 



If the knife has a concave surface, clamp that surface uppermost in 

 the knife holder. The knife holder must be adjusted so the knife is 

 pushed back on the instrument far enough to clear the tissue before the 

 tissue is elevated in preparation for the next section. An automatic 

 lever on a small slide (sliding surface) on the back side of the knife 

 block can be moved until the correct knife position is determined. If 

 the knife does not clear the tissue before the latter is elevated, the press- 

 ing of the knife can injure the tissue and frequently alters the thickness 

 of the next section. 



The maximum thickness of a section as controlled by the automatic 

 feed lever may be only 30 or 40 microns on most microtomes. Thicker 

 sections however can be cut by moving the hand feed counterclockwise 

 and then releasing it. The hand feed is a large round knob (American 

 Optical sliding microtome) mounted at the base of the tissue carrier 

 and near the micrometer screw. Each movement of the hand feed will 

 equal the number of microns indicated by the automatic feed. 



Keep the surface of the slide for the knife block well oiled with the 

 oil provided with the microtome (or Pike Oil, Norton Co.). A dry slide 

 produces a jerky knife movement and rough, irregular sections. 



Always maintain a pool of 70% alcohol on the edge of the knife and 

 the top of the tissue block. Otherwise the sections may shrivel instead of 

 slicing in a smooth sheet. The following methods are suggested for sec- 

 tioning. 



Method 1. Knife and block are wet with alcohol. Draw the knife 

 through the block until a section is almost, but not quite, cut to com- 

 pletion, making certain that the cut has cleared the tissue in the block. 

 If the cut does not clear the tissue, a striation will appear in the tissue 

 at the point where the knife stopped or hesitated. The section rolls as 

 it is cut. While the section is still attached at one corner, unroll it 

 against the knife, and then finish the cut. Squares of paper cut to a 

 size a little larger than the section are moistened with 70% alcohol. 



