APPENDIX. 



substitute for the more convenient and accurate foreign instrument. 

 A word of caution may be added against regarding all forms of sliding 

 microtomes as equally efficient, since the satisfactory working of such 

 tools is largely dependent upon details of their construction and 

 workmanship. While theoretically more accurate, the beautifully- 

 made Thoma microtomes with the * ' Naples' ' holder are much less 

 convenient than the Schanze instruments, and are less desirable than 

 the latter for general use. Where much ribbon-cutting is carried on, 

 the Minot automatic microtome will be found a most valuable time- 

 and labor-saving device ; equally perfect ribbon-series can be pro- 

 duced on the Schanze, but with much less rapidity. 



Assuming that some satisfactory form of sliding microtome is at 

 command, and that the tissue has been embedded interstitially in 

 paraffin, the method employed in cutting will depend on whether 

 isolated sections or a series are desired. For very many purposes 

 the separate sections are all that is needed, their relative position and 

 sequence being preserved by systematic arrangement as they are 

 cut. In making such individual sections, the knife should be placed 

 obliquely to the tissue, the exact angle being such that the entire 

 length of the blade is successively brought into use. It will be found 

 necessary to adopt some means of preventing the rolling up of the 

 sections as they are cut, this tendency being especially pronounced 

 with the harder grades of paraffin ; after this has occurred, the sec- 

 tions are usually useless. The simplest and most effective means of 

 overcoming this difficulty is to hold a small red sable brush over the 

 edge of the knife, and, as the latter enters the block, lightly hold the 

 section as it is being cut from curling over and rolling up ; the manip- 

 ulation requires some little dexterity, but when once acquired sup- 

 plies a simple ' ' section smoother' ' equally as efficient as any of the 

 more elaborate mechanical devices. In cutting paraffin sections, no 

 fluid is needed, both block and knife being kept perfectly dry. The 

 knife should be wiped occasionally with a clean cloth, to remove any 

 particles of the embedding mass that may adhere ; especial attention 

 must be given to the edge and under surface of the blade, as some- 

 times a minute adherent fragment will cause cracks across the entire 

 surface of the subsequent sections. The forceps or a brush serve to 

 transfer the sections from the knife-blade to the adjacent tray lined 

 with perfectly clean paper, upon which the sections may remain for a 

 long time if properly guarded against high temperature and dust. 

 The average thickness of satisfactory paraffin sections is about .01 

 mm.; large sections are usually somewhat thicker, small delicate 

 objects, as embryos, readily yielding sections not much over half as 

 thick (.005 to .008 mm.) ; it should not be forgotten that a keen blade 

 and proper paraffin are essential to first-class results. 



