32 THE MICROSCOPIC PREPARATION. 



The tissues may be imbedded by pouring the thick celloidin, to- 

 gether with the objects, into a small box made of paper. The surface of the 

 celloidin hardens in about an hour (preliminary hardening), after which 

 the whole is transferred to 80% alcohol, in which the final hardening 

 takes place. The paper is then removed, the block of celloidin trimmed 

 to a convenient size and fastened on a block. 



While being cut, celloidin preparations are kept moistened with 80% 

 alcohol. Organs consisting of tissues of varying consistency, as well 

 as very dense objects, can be cut with better results in celloidin than in 

 paraffin. On the other hand, celloidin sections can never be cut as thin 

 as paraffin sections, and the after-treatment (see below), fixation on the 

 slide, etc., are much more complicated than in the case of paraffin sec- 

 tions. 



The following is a diagram showing the process of infiltration 

 and imbedding in celloidin. 



90^ alcohol 



t 

 Abs. alcohol 



t 

 Abs. alcohol and ether (in equal parts) 



t 

 Thin celloidin solution 



t 

 Thick celloidin solution 



t 

 Imbedding 



t 

 So% alcohol 



3. CELLO1D1N-PARAFFIN. 



To combine the advantages which infiltration in celloidin and in 

 paraffin offer, a method of celloidin-paraffin infiltration is recommended. 

 Preparations that have been imbedded in celloidin or photoxylin and 

 hardened in So c / c alcohol are placed for about twelve hours in 90% alco- 

 hol, from which they are transferred to a mixture of equal parts of oil of 

 origanum and 90% alcohol. They are then immersed for a short time in 

 pure origanum oil, then in a mixture of equal parts of origanum oil and 

 xylol, and finally in pure xylol. From this point the regular method 

 of infiltrating with paraffin is followed, care being taken that the pieces re- 

 main for as short a time as possible in the different fluids, in order that 

 the celloidin may not become brittle. 



Very thin sections may be obtained by painting the cut surface with 

 a thin layer of a very dilute celloidin solution. This hardens and gives 

 the tissue a greater consistency. This treatment is useful in the combined 

 celloidin-paraffin method, as well as when paraffin alone is used. 



THE MICROTOME AND SECTIONING. 



Instruments known as microtomes have been devised in order that 

 section cutting may be rendered as independent as possible of the skill 

 of the individual, but more especially to obtain series of sections of uni- 

 form thickness. Their construction varies greatly. Some of these in- 



