Knives 



PARAFFIN SECTIONS 



109 



much care has l^een taken in the prepara- 

 tion of the block or liow complex a micro- 

 tome is used, if the knife-edge is not per- 

 fect there is no chance of securing a perfect 

 section. Ordinary razors are not satisfac- 

 tory for the production of fine sections, 

 and it is necessary to secure a microtome 

 knife, preferably from the manufacturer 

 of the microtome. Another type of micro- 

 tome knife employs the edge of a safety- 

 razor blade in a special holder; these do 

 not, in the writer's hands, give such good 

 results as a sohd blade. 



Three types of solid blade are available : 

 first those which are square-ground, that 

 is, in which the main portion of the knife 

 is a straight wedge; second, those which 

 are hollow-ground, that is, in which both 

 sides of the knife have been ground away 

 to a concave surface, which results in a 

 relatively long region of thin metal to- 

 wards the edge; third, knives which are 

 half-ground, that is, knives of which one 

 side is square- or flat-ground and the other 

 side hollow-ground. This last type of knife, 

 which the writer prefers, is a compromise. 

 There is no doubt that a square-ground 

 knife is sturdier than a hollow-ground 

 knife, a point of some importance when 

 cutting large areas of relatively hard tis- 

 sues; but there is equallj^ no doubt that a 

 hollow-ground knife can be brought more 

 readih' to a fine edge. Microtome knives 

 must be sharpened frequenth^; but it is 

 necessary, before discussing how to do 

 this, to give a clear understanding of the 

 nature of the cutting edge itself. 



If a wedge of hardened steel were to be 

 ground continuously to a fine edge, as in 

 Fig. 58, it would be utterly worthless for 

 cutting. After only a few strokes the fine 

 feather-edge, which would be produced by 

 this type of grinding, would break down 

 into a series of jagged saw teeth. A micro- 

 tome knife, or for that matter any other 

 cutting tool, requires to have ground on 

 its cutting edge a facet of a relatively ob- 

 tuse angle, whether it be a square-ground 

 knife, as in Fig. 59, or a hollow-ground 

 knife as Fig. 60. The process of applying 

 this cutting facet to the tip is known as 

 setting; it is an exceedingly difficult opera- 

 tion to conduct, but one which must be 

 learned by every user of a microtome 



knife. The actual grinding of the blade 

 itself to the correct angle, or to the correct 

 degree of hoUowness, cannot be done in a 

 laboratory; the knife must be returned to 

 the manufacturer or to some scientific sup- 

 j)ly house adetiuatcl}^ equipped with the 

 special machinery necessary. The cutting 

 facet, however, must be set at least once 

 a day if the blade is in continuous use. 

 The nature and purpose of this cutting 

 facet is best explained by reference to the 

 mechanism of cutting shown in Fig. 61. 

 Notice first that the knife blade itself 

 must be inclined at" such an angle to the 

 block that the cutting facet is not quite 

 parallel to the face of the block. There 

 must be left a clearance angle to prevent 

 the knife from scraping the surface every 

 time that it removes a section. This clear- 

 ance angle should, in cutting wax, be as 

 little as possible, and it is for this reason 

 that the blade holder of a microtome is 

 furnished with a device for setting the 

 knife angle. The knife angle should not be 

 set with reference to any theoretical con- 

 sideration, but with regard only to secur- 

 ing this small clearance angle. The only 

 way to judge whether or not a satisfactory 

 clearance angle has been obtained is to 

 observe the sections as they come from 

 the knife. If the clearance angle is too 

 large, so that the section is not being cut 

 from the block but is being scraped from 

 it, the section will have a wrinkled appear- 

 ance and will also usually roll up into a 

 small cylinder. If the clearance angle is too 

 small, so that the lower angle of the facet 

 is scraping the block after the tip has 

 passed, the whole ribbon of sections will 

 be picked up on the top of the block, which 

 will itself crack off when the knife point 

 reaches it. It is obvious that the knife 

 angle will be changed as the angle of 

 the cutting facet is changed, so that it 

 is desirable to maintain the cutting facet 

 of as uniform an angle as possible. This 

 angle is set onto the knife in the man- 

 ner shown in Fig. 62. Notice that the knife 

 has been furnished with a handle and also 

 that a small spHt cyhnder of steel has been 

 sUpped over the back of the blade. This 

 spht cylinder rests flat on the stone, as 

 does the edge of the blade, so that when 

 the knife is pushed forward (the figure 



