THE CUTTING OF SECTIONS. 93 



(rf) Methylated Spirit. — Small pieces of tissue may be placed in methy- 

 lated spirit, which is to be changed after the first day. In six to seven days 

 they will be hardened. If the pieces are large, a longer time is necessary. 



The Cutting of Sections. — i. 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 (of which Cathcart's is a good example) 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 

 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. Reichert's, being of cotlrse 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 



