SECTION CUTTING 211 



The ground-work is stained very pale violet-blue and the organisms deep 

 blue. 



SECTION II.^HISTOLOGICAL PREPARATIONS. 



For the demonstration of micro-organisms in situ in tissues very thin 

 sections (0'05 mm.) must be cut. Hand-cut sections are not sufficiently 

 thin for purposes of bacteriological investigation, so that the tissue must 

 be cut with a microtome, which involves the embedding of the tissue first of 

 all in some suitable material. 



The materials ordinarily used in histology for embedding tissues (gum, 

 wax, soap, celloidin and collodion) do not lend themselves to the cutting of 

 very thin sections, so that for bacteriological purposes the tissue is either 

 frozen or embedded in paraffin. 



1. Instruments. 



Microtomes. Most of the mechanically- worked microtomes are suitable 

 for cutting the thin sections required in bacteriological work. For paraffin 

 sections, Minot's, Radais' and the Cambridge " rocking " microtome (fig. 155) 

 are among those in most frequent use. 



FIG. 155. Cambridge " rocking " microtome. 



It will be unnecessary here to discuss the construction of the different 

 forms of microtome and the method of working them, for a careful examina- 

 tion of the instrument itself will be of far more assistance than any detailed 

 description. 



It will suffice to say that microtomes being instruments of precision must 

 be carefully handled ; that they must be cleaned every time after use, and be 

 protected from dust and damp by being kept under a bell jar or in a wooden 

 box. 



Microtome razors. A good razor is indispensable for the cutting of satis- 

 factory sections. One surface of the razor must be flat (the one in contact 

 with the paraffin block). The cutting edge must be sufficiently sharp to 

 sever an hair held between the finger and thumb or one of the fine hairs on 

 the back of the hand. 



Always strop the razor before using it, first on the prepared surface of 

 the strop and then on the dry surface, remembering to strop with the back 

 foremost and to pass from heel to tip, stropping each side of the razor 

 alternately. 



It is useful also to ensure satisfactory results and to avoid having to send 

 it frequently to the instrument-maker to know how to sharpen a razor on a 

 stone. The razor must be passed with the edge foremost from heel to tip ; 



