THE CUTTING OF SECTIONS. 101 



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. 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 they must be put inside. 



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, other- 

 wise sufficient chloroform will be present to vitiate the final result and 

 not give the perfectly hard block obtained with pure paraffin. With 

 experience, 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. When 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 Cutting of Paraffin Sections. Sections must be 

 cut as thin as possible, the Cambridge rocking microtome 



FIG. 39. Needle with square of paper on end for 

 manipulating paraffin sections. 



being, on the whole, most suitable. They should not 

 exceed 8 //, in thickness, and ought, if possible, to be about 

 4 //,. For their manipulation it is best to have two needles 

 on handles, two camel's-hair brushes on handles, and a 



