CUTTING SECTIONS WITH MICROTOME 



253 



from a different point of view, are necessary to an understand- 

 ing of the character and extent of the different tissues; these 

 are a cross-section, a longitudinal section parallel to a niedul- 

 lary ray, known as a longitudinal radial section, and a longi- 

 tudinal section at right angles to a medullary 

 ray, called a longitudinal tangential section 

 (Fig. 139). Good longitudinal sections are 

 more difficult to get than cross-sections, but 

 much of the difficulty is avoided if most of 

 the surface is pared down so that only a 

 small elevation is left to be sectioned, as 

 shown in Fig. 140. It is a good plan to 

 keep material that is to be sectioned in 

 equal parts of alcohol, glycerine, and 

 water; in this it may remain indefinitely, 

 but only after several weeks will its best 

 effects in softening the harder tissues and 

 toughening the weaker be produced. 



Cutting Sections with a Microtome. — 

 A simple form of microtome that can be 

 clamped to the laboratory table is often of 

 great advantage in cutting sections with a 

 razor (Fig. 141). If the material is hard 

 enough to bear the strain it may be clamped 

 directly in the jaws of the object holder by 

 means of the thumb-screw S; or it may first 

 be inclosed in elder pith, in velvet cork, 

 or even in soft wood, before clamping in. 

 The object is fed up a very little at a time 

 by turning the milled-head M of the micrometer feed-screw. 

 The section razor is laid flat on the plate glass ways PP and 

 pushed across the object with a long sliding motion from point 

 to heel of the razor as shown in Fig. 142. In doing this the 

 razor must be held firmly against the glass ways. After several 

 sections have accumulated on the razor, which is kept wet with 

 dilute alcohol, they may be swept with the finger into a dish of 



Fig. 139. — Showing the 

 planes in which sections 

 are cut, A, transversely; 



B, longitudinal radially ; 



C, longitudinal tangen- 

 tially. 



