98 MICROSCOPIC METHODS 



Finally they are placed in absolute alcohol for twenty-four hours and 

 are then ready to be prepared for cutting. 



If the tissue is very small, as in the case of minute pieces removed 

 for diagnosis, the stages may be all compressed into twenty-four hours. 

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

 to absolute alcohol, the perchloride of mercury being removed after the 

 sections are cut, as will be afterwards described. 



(d) Methylated spirit. Small pieces of tissue may be placed in 

 methylated spirit, which is to be changed after the first day. In from 

 six to seven days they will be hardened. If the pieces are large, a longer 

 time is necessary. 



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 

 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 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 tempera- 

 ture. We have used for some years a mixture of one part of 

 paraffin, melting at 48, and two parts of paraffin melting at 



law, chemists can only sell 8 ounces of pure spirit at a time. Most pathological 

 laboratories are, however, permitted by the Excise to buy "industrial spirit," 

 which contains only one-nineteenth of wood naphtha. 



