39 8 APPENDIX. 



sections. The knife is set at right angle to the microtome track and the 

 paraffin block so trimmed (the desired plane of section being first secured by 

 the microtome adjustments) that the two long sides of the rectangular block 

 are exactly parallel, not only with each other but also with the edge of the 

 knife. Since the mounted sections will be separated by twice the width of 

 the border of paraffin surrounding the object, the block should be trimmed 

 as close as possible to the tissue, a thin covering of paraffin, however, being 

 left over the upper surface of the object. 



Everything being in readiness, the object is raised to a height sufficient 

 to allow several sections of the overlying paraffin being made without in- 

 volving the object. After cutting a few sections that include the entire sur- 

 face of the block, the knife is returned to the beginning of the track, the 

 block elevated by the micrometer screw, and, without removing the section, 

 the blade drawn across the block for the next section. If the consistence of 

 the paraffin is proper, the last cut section pushes the preceding one before it 

 on the blade and, at the same time, adheres to it. The manipulation is re- 

 peated and, if all goes well, each succeeding section adheres to its prede- 

 cessor and advances the chain. For embryological work, in which ribbon- 

 cutting is particularly useful, the sections should be of uniform thickness, 

 .01 mm. being satisfactory for most purposes. Sections less than .005 mm. 

 in thickness seldom yield good ribbons. The same is true of very thick 

 ones. A small sable brush held in the left hand is useful in keeping the first 

 few sections flat, in brushing away shreds of paraffin, and in supporting the 

 ribbon after it has become too long to lie on the knife. Under favorable 

 conditions and with careful support, ribbons one or two feet in length, or indeed 

 much longer, may be cut readily. With the ordinary sliding microtome, it 

 is safer to remove the chains to an adjacent sheet of clean paper when 

 they reach a length of ten or twelve inches, care being taken that the 

 last cut two or three sections remain undisturbed on the knife to guide 

 the succeeding ones. The under surface and edge of the knife must be kept 

 free from particles of paraffin, otherwise breaks and fissures in the sections 

 may occur. 



As the cutting proceeds, it may become necessary to retrim the block, 

 since, unless the sides have been cut strictly vertical and not, as they often 

 are, slightly sloping, the border of paraffin becomes gradually excessive. 

 Then, too, the front and back edges of the block must be parallel, otherwise 

 the ribbon will curve instead of remaining straight. After the cutting is well 

 under way, any considerable change in the plane of section should be 

 avoided if it is important to secure a complete series, as such change usually 

 necessitates the loss of several sections. If the embedding mass be too hard, 

 the sections will not adhere and, of course, the ribbon not form ; if too soft, 

 the sections will wrinkle and compress and, perhaps, not advance. These 

 defects may often be overcome by appropriate changes in the temperature of 

 the room, or by judicious local application of cold or heat to the block it- 

 self. When other means fail, coating the block with paraffin of appropri- 

 ate consistence often affords relief. Now and then, especially when using 

 rather hard paraffin on a cold, bright, dry day, the atmospheric conditions 

 are such that sufficient electricity is liberated during sectioning to disturb 

 seriously the sections. The latter may break apart and shorter or longer 

 pieces of the ribbon attach themselves to one another or to the microtome 

 or knife. A repetition of this exasperating accident may usually be avoided 

 by charging the atmosphere with moisture, often conveniently accomplished 

 by liberating steam. 



