8 



Making Sections 



Nature of the Process. A section is a thin sHce cut from biological mate- 

 rials with a view to studying either the cells themselves or their arrangement, 

 neither of which can be made out from a wholemount. 



Though sections may be cut at any angle, they are usually taken through 

 any one of three planes (see Fig. 16) which are known as "transverse," "sag- 

 ittal," and "frontal" planes. The purpose of this orientation of the material is 

 to enable one to realize better the structure of the whole from an examination 

 of the section. Theoretically, to produce a section, it is necessary only to take 

 a sharp knife and cut a thin slice from the object under examination. Very 

 few materials, however, are suitable for this, and this procedure does not pro- 

 duce sections of the same thickness. Therefore, it is customary to employ 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 with- 

 stand the action of the knife without bending, and many contain cavities 

 which 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 which will impregnate its whole sub- 

 stance. The medium most commonly used to support structures is wax. The 

 technique for cutting wax sections is described on page 66. There are, however, 

 a number of materials which may be cut without either complicated micro- 

 tomes or the support of impregnating substances. Sections which are so cut 

 are known as "free" or "freehand" 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 which will permit one to produce sections of known thick- 

 ness. The type of microtome usually employed in hand sectioning is shown in 

 Fig. 17 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 cylinder there is a mechanism which 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 microm- 

 eter screw is graduated, sometimes in thousandths of an inch but more 

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



52 



