96 MICROSCOPIC METHODS 



1. Pieces of tissue, however hardened, are placed in fresh absolute 

 alcohol for twenty-four hours in order to their complete dehydration. 



2. Transfer now to a mixture of equal parts of absolute alcohol and 

 chloroform for twenty-four hours. 



3. Transfer to pure chloroform for twenty-four hours or longer. At 

 the end of this time the tissues should sink or float heavily. 



4. Transfer now to, a mixture of equal parts of chloroform and paraffin 

 and place on the top of the oven for from twelve to twenty-four hours. 

 If the temperature there is not sufficient to keep the mixture melted 

 then the tissues must be put inside the oven. 



5. Place in pure melted paraffin in the oven for twenty-four hours. 

 For holding the paraffin containing the tissues, small tin dishes such as 

 are used by pastry-cooks will be found very suitable. There must be a 

 considerable excess of paraffin over the bulk of tissue present, otherwise 

 sufficient chloroform will be present to vitiate the final result and not 

 give the perfectly hard block obtained with pure paraffin. With ex- 

 perience, the persistence of the slightest trace of chloroform can be 

 recognised by smell. 



In the case of very small pieces of tissue the time given for each stage 

 may be much shortened, and where haste is desirable Nos. 2 and 4 may 

 be omitted. Otherwise it is better to carry out the process as described. 



6. Cast the tissues in blocks of paraffin as follows : Pairs of L-shaped 

 pieces of metal made for the purpose by instrument makers must be at 

 hand. By laying two of these together on a glass plate, a rectangular 

 trough is formed. This is filled with melted paraffin taken from a stock 

 in a separate dish. In it is immersed the piece of tissue, which is lifted 

 out of its pure paraffin bath with heated forceps. The direction in 

 which it is to be cut must be noted before the paraffin becomes opaque. 

 When the paraffin has begun to set, the glass plate and trough have 

 cold water run over them. AVhen the block is cold, the metal L's are 

 broken off, and, its edges having been pared, it is stored in a pill-box. 



The Gutting of Paraffin Sections. — Sections must be cut as, 

 thin as possible, the Cambridge rocking microtome being, on the 

 whole, most suitable. They should not exceed 8 ft in thickness, 

 and ought, if possible, to be about 4 /a. For their manipulation 



Fis. 37. — Needle with square of paper on end for manipulating 

 paraffin sections. 



it is best to have two needles on handles, two camel's-hair brushes 

 on handles, and a needle with a rectangle of stiff writing-paper 

 fixed on it as in the diagram (Fig. 37). When cut, sections are 

 floated on the surface of a beaker of water kept at a temperature 



through successive dilutions of spirit, not differing from each other by more 

 than 10 per cent. Again, when alcohol has been replaced by chloroform the 

 latter must be saturated with chips of paraffin, tirst at room temperature, then 

 at 37° C, and must be kept at 55° C. as short a time as possible. 



