99 



the sides at least being as nearly parallel as possible. Clamp the 

 block of paraffin in the holder of the microtome so that the tissue 

 will be at the proper level for cutting, being careful to have the par- 

 allel sides also parallel to the edge of the knife. If a ribbon micro- 

 tome is used, heat the holder and melt the end of the block upon it. 

 Cool and place the holder in its place in the microtome, again hav- 

 ing the parallel sides and the edge of the knife parallel. Use a vers^ 

 sharp, dry razor for cutting the sections. Clamp it in the (Minot) 

 microtome slightly inclined to the cutting surface of the tissue. If 

 the temperature of the room is right for the paraffin used, the sec- 

 tions will remain flat, and if the directions given above for trimming 

 and arranging the block be observed, they will adhere and thus form 

 a ribbon. If the room is too cold or the paraffin too hard, the sec- 

 tions will roll ; if it is too warm, the sections will crush or be imper- 

 fect. If a microtome in which the knife is not fixed, is used, make 

 the sections with a rapid straight cut as in planing. Do not try to 

 section with a drawing cut as used in collodion sectioning, lofi will 

 be found the most convenient thickness for the sections, though in 

 special cases they should be thinner or even thicker. 



Remember to have the paraffin block trimmed with two sides 

 parallel and the knife edge parallel to these. Also, do not attempt 

 to cut if the temperature of the room is too high, — above 21" C. 



§ 49. Resume of the method. To obtain as good results as 

 possible with a certain organ fixed and hardened in a certain way, 

 the steps must be carefully and exactly followed. Let the dehydra- 

 tion be complete, clearing thorough, infiltration sufficient ; imbed, 

 carefully observing the three cautions mentioned ; and in cutting, 

 remember to have a sharp knife, a cool room, and the imbedding 

 block properly trimmed. Success also depends largely on the pre- 

 vious treatment in the fixer and on the care with which the fixer is 

 washed out. 



Properly employed, the paraffin method is widely serviceable, 

 being only useless where the tissue is very large, very hard, hard- 

 ened or injured by heat, or where the exact form of a large organ is 

 important. 



THB COLLODION METHOD. 



§ 50. A comparison with the paraffin method has already been 

 given (§40); there may be emphasized here three points: (i) 



