APPENDIX. 



397 



atory to sectioning. This should be done with a sharp knife, care being 

 taken that in shaping the block the slices of paraffin are not too thick, lest 

 the enclosed object be subjected to undue pressure. While all super- 

 fluous embedding mass should be cut away, the trimming must not be too 

 close, a margin of paraffin, one or two millimeters broad, being left around 

 the object. 



For ordinary purposes, the knife, scrupulously clean, is set at angle and 

 carried obliquely through the tissue. For routine examinations, sections 

 .ooy-.oio mm. thick will be sufficiently thin, particularly when of some size. 

 If, however, cell-structure and other details requiring the use of high ampli- 

 fication are in view, the sections can not be too thin. Under the most favor- 

 able conditions, it is possible to obtain sections of small size which are not 

 over .001 mm. in thickness. 



If the consistence of the paraffin and tissue be just right and the sec- 

 tioned surface not too large, the sections will lie flat and smooth on the knife, 

 from which they are removed as cut and placed on a clean sheet of writing 

 paper. Protected from dust and excessive temperature, they may be put 

 aside for mounting at some future time. If, however, they be allowed to lie 

 too long, particularly in a warm temperature, there is danger of their stick- 

 ing to the paper; early mounting is, therefore, to be recommended. After 

 the sections desired at the time have been cut, the block may be put aside 

 for subsequent use. If the paraffin is hard the sections will roll up on 

 the knife. This annoying feature may be prevented by lightly holding 

 down the edge of the section with a small sable brush in the left hand as 

 the knife is drawn through the tissue. In this manner flat sections may 

 often be secured when, without the manipulation, they would be tightly and 

 hopelessly rolled. 



When the paraffin is too hard, still contains chloroform, or lacks homo- 

 geneity, it often is brittle and crumbling, so that the sections break before 

 completed. Too little consistence of the embedding mass is also unfavorable 

 for satisfactory cutting, since, if the paraffin be too soft, insufficient support 

 is given the sections, which then come off more or less wrinkled and com- 

 pressed with corresponding distortion of the object. If the compression is 

 not excessive, the sections may often be restored to their normal form by 

 floating them on judiciously warmed water, where they expand. Obviously 

 care must be taken that the temperature of the fluid is not sufficiently high 

 to dissolve the supporting paraffin. The temporary reduction of the tempera- 

 ture of the room, by opening an adjacent window, frequently serves to cor- 

 rect undue softness of the embedding mass. Similarly, bringing a gas-flame, 

 or other source of heat, into the vicinity of the microtome sometimes enables 

 satisfactory sections to be made with over-hard paraffin. Care must be 

 observed to keep, by an occasional touch with a dry cloth, the knife clean 

 and free from adherent particles of paraffin, especially the under surface of 

 the cutting edge. If this precaution be neglected, an attached fragment of 

 paraffin may ruin important sections by causing fissures and breaks. A 

 properly ground and really sharp knife is an indispensable requisite for the 

 production of successful preparations. 



Serial sections are most easily cut in the form of "ribbons." While, 

 of course, their sequence may be preserved by carefully arranging isolated 

 sections in the order in which they are cut, much time and trouble are saved 

 by adopting the ribbon-method, whereby the sections are caused to adhere 

 and form a long chain. The method is particularly suitable for small and 

 delicate objects stained in toto, as embryos, which are to be entirely cut into 



