THE CUTTING OF SECTIONS 95 



In fact, after fixation in corrosive the tissue may be transferred directly 

 to absolute alcohol, the perchloride of mercury l.eing removed after the 

 sections are cut, as will be afterwards described. 



The Cutting of Sections. — 1. By Means of the Freezing 

 Microtome. — Pieces of tissue hardened by any of the above 

 methods must have all the alcohol removed from them by wash- 

 ing in running water for twenty-four hours. They are then 

 placed for from twelve to twenty-four hours (according to their 

 size) in a thick syrupy solution containing two parts of gum 

 arabic and one part of sugar. They are then cut on a freezing 

 microtome and placed for a few hours in a bowl of water so that 

 the gum and syrup may dissolve out. They are then stained, or 

 they may be stored in methylated spirit. 



2. Embedding and Cutting in Solid Paraffin. — This method 

 gives by far the finest results, and 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 

 is the most suitable. The more gradually vthe 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 being of course necessary. The tissues 

 occurring in pathological work have a tendency to become brittle 

 if overheated, and therefore the best results are obtained by 

 using paraffin melting at a somewhat low temperature. 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 — 



1 While the method given is sufficient for ordinary purposes, a more elaborate 

 technique is necessary if it is desired that no changes shall take place in the 

 tissue. Thus after fixation the tissue must be taken up to absolute alcohol 



