f h. XI] PREPARATIONS BY THE PARAFFIN METHOD 



379 



meters or more of paraffin over the tissue at the end as well as on 

 the sides. The block is trimmed in a pyramidal form, so that it 

 will be rigid. Take particular pains that the opposite faces at the 

 end of the block are parallel and all the corners right angles. 



In some laboratories, Dr. McClung's for 

 example, a cubical block of metal attached 

 to a rod is placed in the knife holder of the 

 microtome and the four sides of the im- 

 bedding mass trimmed with great exactness 

 by the use of a straight-edged scalpel, or 

 better by a small chisel, the cube of metal 

 serving as a guide. As the metal cube can* 

 be slid along in the knife holder, and the 

 imbedded tissue can be raised and lowered 

 by turning the wheel of the microtome, inw 

 bedding masses of large and small sizes can 

 be trimmed by the same metal guide. This 

 guide for trimming is a great help in get- 

 ting straight ribbons, and consequently 

 good series. 



§ 615. Making paraffin sections. — Put 

 the paraffin block or the metal holder in 

 the clamp of the microtome. Arrange the 

 block so that one side of the pyramidal 

 end is parallel with the edge of the knife; 

 then tighten the clamp; and if an automatic 

 microtome is used, make sure that the sec- 

 tion knife is also tightly clamped by the 



proper set screws. It is well to have the knife lean slightly toward 

 the paraffin blocks. 



The knife edge meets the paraffin squarely, as in planing. The 

 thickness of section is provided for in the automatic microtome by 

 the indicator, which may be set for any desired thickness, or one 

 can turn up the screw by hand in the table microtome. The par- 

 affin and its contained tissue are cut in a thin shaving. If the 

 tissue was stained in toto with eosin, as suggested in § 611 A, it is 



1 



Fig. 223. 



2 3 



Scalpel Blades. 



1, 2 with curved edges 

 for cutting ribbons; j, with 

 straight edge for trimming 

 paraffin blocks. 



