98 



tration paraffin, — 50° C. paraffin (42° paraffin i part, 54° paraffin 2 

 parts), answers well in a room of 19° to 20° C. , and will be gener- 

 ally used. If the cutting is to be done in a room of lower tempera- 

 ture, a softer grade of paraffin may be used for imbedding ; if at a 

 higher temperature, a harder paraffin should be chosen ; 54° C. par- 

 affin giving good results when summer work is necessary. 



Make a small paper box, fill it with the melted imbedding 

 paraffin ; transfer to it the tissue from the paraffin oven, arrange it 

 carefully in the box in the way you wish it for cutting, and cool the 

 mass by floating the box on a dish of cold water. 



§ 47. In imbedding in paraffin observe the following rules ; 

 (i) Take no more paraffin (no larger box) than is needed to form a 

 mass of convenient size around the specimen. The aim is to have 

 as homogeneous a mass as possible ; paraffin tends to crystallize if 

 it cools slowly, hence the smaller the mass the more rapidly may it 

 be cooled. (2) Let the imbedding paraffin when poured into the 

 box be several degrees above its melting point, and the tissue like- 

 wise should have an equal temperature. Should the imbedding 

 paraffin (or the tissue) be too cool it will not set well around the 

 specimen, and a film of air may be enclosed. On the other hand, 

 take care that the paraffin is not hot enough to "cook" the tissue, 

 thereby shrinking it and rendering it hard and tough or ruining it 

 altogether. (3) Cool hy floating on cold water. Paraffin in cooling 

 must contract greatly if it does not crystallize ; the more homo- 

 geneous it is the more it must contract, and if it is cooled on all 

 sides it will crystallize in the center of the mass, because it cannot 

 shrink. 



§ 48. Cutting the sections. The essentials for good paraffin 

 sectioning are (i) well-imbedded tissue, (2) a sharp knife (or sec- 

 tion razor), (3) a room of the proper temperature, and (4) the par- 

 affin block properly trimmed and arranged in the microtome. Fur- 

 thermore, tissue fixed and hardened in different ways cuts very dif- 

 ferently. Tissue fixed in Hermann's, Flemming's, Miiller's, or 

 Zenker's fluid cuts well ; picric acid and mercuric chlorid tissue is 

 more apt to be tough or hard, etc. The different organs and tissues 

 have of course very different adaptabilities to the method. 



After the imbedding mass is well cooled, remove the paper box 

 and trim the part containing the tissue in a pyramidal form, two of 



