THE CUTTING OF SECTIONS 91 



2. Embedding and Cutting in Solid Paraffin. This method 

 gives by far the finest results^nd should always be adopted 

 when practicable. The principle is the impregnation of the 

 tissue with paraffin in the melted state. This paraffin when it 

 solidifies gives support to all the tissue elements. The method 

 involves that, after hardening, the tissue shall be thoroughly 

 dehydrated, and then thoroughly permeated by some solvent 

 of paraffin which will expel the dehydrating fluid and prepare 

 for the entrance of the paraffin. The solvents most in use are 

 chloroform, cedar oil, xylol, and turpentine ; of these chloroform 

 and cedar oil are the best, the former being preferred as it per- 

 meates the tissue more rapidly. The more gradually the tissues 

 are changed from reagent to reagent in the processes to be gone 

 through, the more successful is the result. A necessity of the 

 process is an oven with hot-water jacket, in which the paraffin 

 can be kept at a constant temperature just above its melting- 

 point, a gas regulator, e.g. Eeichert's, being of course necessary. 

 The tissues occurring in pathological work have a tendency to 

 become brittle if overheated, and therefore the best results are 

 not obtained by using paraffin melting about 58 C., such as is 

 employed in most biological laboratories. We have used for 

 some years a mixture of one part of paraffin, melting at 48, and 

 two parts of paraffin melting at 54 C. This mixture has a 

 melting-point between 52 and 53 C., and it serves all ordinary 

 purposes well. An excellent quality of paraffin is that known 

 as the "Cambridge paraffin," but many scientific-instrument 

 makers supply paraffins which, for ordinary purposes, are quite 

 as good, and much cheaper. The successive steps in the process 

 of paraffin embedding are as follows : 



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 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, 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. 



