ON CUTTING SECTIONS. xxxi 



The microtome or section-cutting machine, however, gives 

 better results. It consists essentially of three parts : a clamp 

 or other contrivance for holding the object to be cut, means 

 of guiding the razor, and a ' feeding ' arrangement. These 

 parts are so arranged that the razor can be drawn smoothly 

 across the object to be cut, taking off a section at each stroke, 

 the 'feeding' arrangement bringing the object into the path 

 of the razor, and the amount of ' feed ' determining the thick- 

 ness of the section. 



In the simplestform of microtome the razor is drawn by 

 hand over a smooth plate, the object to be cut being pushed 

 up through a hole in the centre of ihe plate by means of a 

 screw or other ' feeding ' arrangement placed below. 



More complicated microtomes have been devised to obviate 

 the wearing of the edge of the razor by friction upon the 

 plate ; to render it possible to cut the object in any desired 

 direction ; to cut more evenly, more easily, and more rapidly ; 

 and to yield sections in a string or ribbon, so as to simplify 

 and expedite the mounting of large series of sections. 



The preparation of an object for cutting in a microtome 

 depends to some extent upon the nature of the object and the 

 number of sections required, as well as the purpose for which 

 they are intended. The object has in any ease to be supported 

 by impregnating it with some homogeneous substance which 

 is firm enough without being brittle or too hard. The freezing 

 method is the simplest of all, but it is not easily applicable 

 when a large number of sections are required mounted in 

 consecutive series, nor when very thin sections are required. 

 The tissue to be cut is first soaked in gum-water, then placed 

 on the freezing-plate of the microtome and cooled by an 

 ether-spray or a freezing mixture underneath. The frozen 

 mass is cut when just thawing. 



The imbedding method is more compli6ated, but has many 

 advantages. The specimen, which may be stained previously, 

 is dehydrated by soaking in absolute alcohol ; the alcohol is 

 then removed by soaking in turpentine, xylol, or better, oil of 

 cedar-wood, and the object is then transferred to paraffin 

 kept just melted in a water-oven, the temperature not being 



