Making Sections 125 



it is necessary only to take a sharp knife and cut a thin slice from the ob- 

 ject under examination. Very few materials, however, are suitable for 

 this, and this procedure does not produce sections of the same thickness. 

 It is therefore customary to use an instrument known as a "microtome," 

 which is a device for advancing a block of tissue a given amount, cutting 

 a slice from it, readvancing it the same amount, and repeating the process. 

 Another objection to the mere cutting of slices from an object is the 

 nature of biological specimens themselves. Very few of these are stiff 

 enough to withstand the action of the knife without bending, and many 

 contain cavities that would be crushed out of recognition as the section 

 was taken. It is customary, therefore, for most biological work to surround 

 and support the object to be cut with some material that will impregnate 

 its whole substance. The medium most commonly used to support struc- 

 tures is wax. The technique for cutting wax sections is described later. 

 There are, however, a number of materials that may be cut without 

 either complicated microtomes or the support of impregnating substances. 

 Sections which are so cut are known as "free," or "freehand," sections. 



FREE SECTIONS 



Microtome for Free Sections. Even if the material itself is of the correct 

 consistency to withstand the action of the knife, it is still necessary to have 

 some mechanism that will allow the pro- 

 duction of sections of known thickness. 

 The type of microtome usually employed 

 in hand sectioning is shown in Fig. 88 and 

 consists essentially of a disc, usually of 

 highly polished plate glass, supported on 

 a cylinder, which is gripped in the hand. 

 For holding specimens within this cylin- 

 der there is a mechanism that terminates 

 at its lower end on a micrometer screw. 

 When this screw is turned, therefore, the 

 object in the holder is pushed above the 

 surface of the glass plate. The collar of 

 the micrometer screw is graduated, some- 

 times in thousandths of an inch, but more 

 usually in hundredths of a millimeter. The unit commonly employed to 

 describe the thickness of a section is a "micron" (fi), which is one- 

 thousandth of a millimeter, but hand sections are rarely cut less than 

 ten fx thick and are usually better at two or three times this. 



Methods of Holding Material. The material, although it may be suit- 

 able for cutting, is rarely of a size and shape that may be gripped in the 



Fig. 88. Hand microtome. 



