Microtomes 



SECTIONS OF FREE MATERIAL 



89 



can be made by cutting a thin slice from 

 the object with a sharp knife. Few mate- 

 rials, however, are suitable for this, nor 

 does this procedure yield sections of the 

 same thickness. It is, therefore, customary 

 to use an instrument known as a micro- 

 tome: a device for advancing a block of 

 tissue a given amount, cutting a slice from 

 it, and then re-advancing it for the same 

 amount, and so on. A full account of all 

 the types of mechanism by which this re- 

 sult can be produced is to be found in 

 Richards 1949 and need not be repeated 

 here. 



Another objection to the mere cutting 

 of slices from an object is the nature of 

 biological specimens themselves. Few of 

 these are stiff enough to withstand the ac- 

 tion of the knife without bending, and 

 many contain cavities which would be 

 crushed out of recognition as the section 

 was taken. This makes it necessary, for 

 most biological work, to surround and sup- 

 port the object to be cut with some mate- 

 rial which will impregnate it. The medium 

 most commonly used is wax. The tech- 

 nique for cutting wax sections is described 

 fully in the next chapter. Nitrocellulose is 

 also employed and is described in Chapter 

 13. Another method of stiffening the ma- 

 terial, so that sections may be cut from it 

 without crushing, is to freeze the speci- 

 men. This techniciue is described in Chap- 

 ter 15. There are however, materials which 

 may be cut without either the complicated 

 microtomes described in these chapters or 

 the support of impregnating substances. 

 Sections which are so cut are known as 

 free or freehand sections. These form the 

 subject of the present chapter. 



Microtomes for Free Sections 



Even though the material itself is of the 

 correct consistency to withstand the ac- 

 tion of the knife, it is still necessary to 

 have some mechanism which will produce 

 sections of known thickness. The tj^pe of 

 microtome usually employed in liard so(;- 

 tioning is sliown in Fig. 34 and consists 

 essentially of a disk, usually of polished 

 plate glass, supported on a cylinder grip- 

 ped in the hand. Within this cylinder there 

 is some mechanism for holding specimens 

 Avhich terminates at its lower end on a 



micrometer screw. When this screw is 

 turned, the object in the holder is pushed 

 above the surface of the glass plate. The 

 collar of the micrometer screw is gradu- 

 ated, sometimes in thousandths of an 

 inch, but more usually in hundredths of a 

 millimeter. The unit commonly used to 

 describe the thickness of a section is a 



Fig. 34. Hand microtome. 



micron which is one-thousandth of a milli- 

 meter; but hand sections are very rarely 

 cut of less than 10-micron thickness and 

 are usually better at two or three times 



this. 



Methods of Holding the Material 



Though the material itself may be suit- 

 able for cutting, it is rarely of a size and 

 shape which may be gripped in the holder 

 of the hand microtome without additional 

 support. It must, therefore, be held in 

 some substance which will itself cut read- 

 ily and which may be easily shaped to sup- 

 port what is being cut. It is possible to 

 embed the material either in wax or nitro- 

 cellulose before cutting a hand section, but 

 if one is to go to this amount of trouble, 

 it is usually better to use a comi)lex micro- 

 tome of the type described in Chapters 12 

 and 13. Vegetable tissues are usually em- 

 ployed to support objects for hand section- 

 ing and the two best known are elder i)ith 

 and carrots. Klder pith has tlie advantage 

 that it may be stored indefinitely and cuts 

 with a clean crisp action. Unfortunately 

 the pith of the American elderberry (Sam- 

 bucus caiuidensis) does not ai)pear to he as 

 suitable for the purpose as the pith of the 



