THE MICROTOME AND SECTIONING. 



35 



Beneath the frame which supports the knife are two horizontal ways, upon 

 which runs the sliding carriage supporting an adjustable object-carrier. 

 The object is raised by a micrometer screw, fed automatically by a large- 

 toothed wheel attached to the bottom of the screw. Both paraffin and 

 celloidin sections may be cut with this instrument. The other type of mi- 

 crotome is known as the ''New Rotary Microtome." In this instrument 

 (see Fig. 6) the knife is carried by two upright standards which can be 

 adjusted as to their distance from the object. The object, which needs 

 to be imbedded in paraffin, is fixed to an object -carrier, which may be 

 adjusted to any plane, and which is fixed to a vertical carriage, held by 

 adjustable gibs against the vertical ways, and which is raised or lowered 

 by a crank, working in a slide, and attached to an axle turned by the 

 wheel. The vertical carriage also carries the micrometer screw, to which 

 is attached a toothed wheel ; this is turned by a pawl which acts upon it. 

 This instrument may be most highly recommended for the cutting of 

 serial sections. 



In cutting paraffin sections with the sliding microtome the 

 knife is placed at an angle of about 35 to 40 to the horizontal plate of 

 the microtome. Sections are cut more easily with the knife in this posi- 

 tion than when the knife is placed at right angles to the microtome, as is 

 often recommended, and it does not seem that the tissues suffer materially 

 from distortion when they are cut with the knife at an angle, as is some- 

 times claimed. 



Before fastening the paraffin blocks into the clamp on the microtome, 

 preparatory to cutting sections, the paraffin is trimmed with a sharp knife 

 from the end of the paraffin block until the tissue is nearly exposed, care 

 being taken, however, to leave a flat surface. The top of the paraffin 

 block is then beveled off on three sides to within a very short distance 

 of the tissue. The fourth side, that which faces the knife when the block 

 is clamped in the microtome, should be trimmed only to within about y% of 

 an inch of the tissue. This edge of paraffin is made use of, as will be seen 

 in a moment, for preventing the sections from curling while they are being 

 cut. The paraffin block is now ready to be clamped in the microtome. 

 This is done in such a way that the paraffin block just escapes the knife 

 when drawn over it. A number of rather thick sections (20 to 40 

 microns) are cut by moving the micrometer screw from right to left 4 

 to 8 notches every time the knife has been drawn over the paraffin 

 block and has been brought back again, until it is noticed that the knife 

 touches all parts of the top of the paraffin block, or until the tissue is 

 fairly exposed. (In this description reference is made to the simple labora- 

 tory microtome shown in Fig. 4. ) The succeeding sections may now be 

 kept. It may perhaps be well to state that it is better not to try to cut 

 very thin sections at the beginning; sections 15 to 20 microns in thickness 

 will answer very well. To begin with, then, the milled head of the mi- 

 crometer screw is turned 4 notches from left to right, and the knife is 

 drawn over the block with a steady, even pull, and without using undue 

 pressure. Usually the sections will curl up as they are being severed from 

 the paraffin block. This may very readily be prevented by holding the 

 tip of a camel's-hair brush, which has been pointed by drawing it between 

 the lips, against the edge of the section as soon as it begins to curl. A 

 little practice will enable one to do this almost automatically. The 

 sections are transferred to paper by means of the camel's-hair brush, 

 which process is facilitated if the brush has been slightly moistened with 

 saliva, as the section will then adhere lightly to the brush. 



