214 HISTOLOGICAL PREPARATIONS 



being careful that the tissue is embedded to a depth of several millimetres 

 to allow for the contraction which will take place during cooling. 



As soon as the paraffin has set sufficiently to hold the tissue the needle 

 which was used to retain the latter in position should be taken away. The 

 paraffin should be cooled rapidly by plunging the mould into cold water, 

 being careful first to moisten the bottom and not to immerse the mould com- 

 pletely before the paraffin has cooled sufficiently to allow of the formation of 

 a crust on the surface, otherwise of course the water would penetrate into 

 the paraffin and destroy the homogeneity of the mass. 



6. When the paraffin is firmly set, take it out of the mould and the tissue 

 is ready for cutting. 



Paraffin moulds. 1. The simplest mould is one made out of paper in the 

 following manner : Select a cork which loosely fits the carrier on the micro- 

 tome, and roll round it a strip of filter paper which may be fastened by 

 pinning it to the cork so as to form an hollow cylinder 2 or 3 cm. deep, the 

 bottom being formed by the upper surface of the cork. This surface may, 

 with advantage, be scored with a few small grooves cut with a scalpel to 

 ensure the paraffin adhering more firmly to it. Oil the inner surface of the 

 paper with a brush avoiding the surface of the cork at the bottom of the 

 cylinder. 



Pour the melted paraffin into this cylinder and when it has set take out 

 the pin and unroll the paper ; the paraffin with the tissue embedded in it 

 will remain attached to the cork. Trim up the surface of the paraffin and 

 fix the cork into the carrier of the microtome. The block is then ready for 

 cutting. 



2. The lead capsules used for covering the corks of bottles serve the same 

 purpose excellently. When the paraffin has set the capsule is torn off, leaving 

 a solid block of paraffin which can be trimmed up at leisure with a slightly 

 heated scalpel. Blocks cast in such a mould can be easily fitted to the carrier 

 of the microtome. In using a Minot microtome it is only necessary to heat 

 gently the grooved metal carrier and to press the lower surface of the paraffin 

 block lightly against it. To fix the block to the wooden cube or cylinder used 

 with other microtomes apply the blade of a lightly heated scalpel to the lower 

 surface of the block, and while the paraffin is still soft press it on to the wood 

 block ; or if preferred a little melted paraffin may be poured on to the latter 

 and the paraffin block pressed on to it. 



In the same way small cardboard or wooden boxes, cover-glass boxes for 

 example, make very good moulds ; these must be painted on the inside with 

 glycerin or oil to prevent the paraffin adhering to the sides. 



3. By using Leuckart's moulds blocks of various sizes with perfectly smooth 

 and parallel sides are obtained. These moulds consist of two pieces of brass, 

 which can be placed together in such a way that they form a rectangular 

 box (fig. 157). The surfaces of the two pieces of metal are smeared with 



FIG. 157. Paraffin moulds. 



glycerin and laid on a piece of glass which has also been smeared with glycerin 

 and they are then arranged so as to form a box of the size required. The 

 melted paraffin is poured into the box and when it has set the two pieces 



