SECTION CUTTING. 49 



back of the blade, the back of the hand being directed upwards. 

 The razor is made to glide thrdugh the tissue, cutting as thin 

 sections as possible, which are placed in alcohol. The tissue to be 

 cut may be embedded in any of the ways already described, and it 

 is held in the left hand while being cut. 



A razor ought to be sharp and free from notches in its cutting 

 edge. A razor with notches in its edge causes striated bands in 

 the section. Notches can readily be detected by drawing the razor 

 across one's nail or over a piece of cardboard. In sharpening the 

 razor use a soft hone moistened with water, then lay the razor flat 

 on the hone and draw it diagonally from heel to point, with the 

 edge forwards. 



The razor must be frequently stropped to keep it sharp, and it 

 should not be hollow-ground ; it is better to be flat on one side. 



2. Valentin's Knife (fig. 32) consists of two parallel blades, 

 which can be placed at a greater or less distance from each other by 



a 



FIG. 32. Valentin's Knife. 



means of the screw (a). The blades are first set apart at the 

 required distance \ the thickness of the section depends on the 

 distance between them. The knife was formerly much used by 

 pathologists for making sections of fresh organs. It is now rarely 

 used. 



3. Microtomes. For many purposes some form of freezing 

 microtome will be found most convenient for teaching purposes ; 

 where a large number of sections is required it is indis- 

 pensable. 



Preparation of Hardened Tissues for Cutting by Freezing. 

 In order to secure the full advantages of cutting sections by freez- 

 ing, the tissues must be previously soaked in and saturated with 

 proper " freezing fluids." If the tissues be kept in alcohol, first 

 remove all the alcohol by soaking them for twenty-four hours in 

 running water. After this the tissue is soaked in gum mucilage, 

 or, what is preferable, a mixture of gum and syrup. The best 

 receipt is that of Hamilton. 



.Make a syrup of 28.5 grains of pure cane-sugar in 30 cc. of 

 water ; boil and saturate it with boracic acid. Allow it to cool and 

 filter. Place 45.6 gnuns of gum acacia in 2400 cc. of cold water, 

 allow it to "dissolve, saturate, it with boracic acid by boiling, and 

 filter. 



