ON CUTTING SECTIONS xxvii 



getting inside the rim which carries the lens, may lead to the^ 

 separation of the two lower lenses from each other, rendering 

 the objective useless. 



VIII. ON CUTTING SECTIONS. 



Many points in anatomy and histology may be best made 

 out by examination- of microscopical sections, that is, slices cut 

 so thin as to allow of their being examined under the micro- 

 scope by transmitted light. 



The hardened object, supported, if necessary, between tw& 

 pieces of carrot, may be held between the fingers and thumb 

 and sections shced off it freehand with a sharp razor. 



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 simplest form 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 the 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. By 

 means of a Threlfall- Caldwell microtome, sections may be 

 obtained cohering at their edges to form a continuous ribbon 

 at the rate of two or three hundred a minute or more. 



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 purposes for which 

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



