292 THE ESSENTIALS OF HISTOLOGY. 



The hardening of the brain and spinal cord in Mtiller's fluid takes from 

 3 weeks to as many months. It can be hastened by warmth or by placing 

 small pieces in Marchi's fluid (see below), after they have been a week or 10 

 days in Miiller. 



In no case should the pieces of tissue to be hardened be too thick for the 

 fluid readily to penetrate to every part. They should be taken as soon after 

 death as possible. Each piece should have from 10 to 20 times its bulk of 

 hardening fluid, and this should always be changed after a few hours. 



Embedding of Hardened Tissues, and Preparation of Sections.- 

 Sections are most advantageously made with some form of microtome. It is 

 generally needful to support the hardened tissue whilst it is being cut, and 

 with this object it is embedded in some fatty or other substance which is 

 applied to it in the fluid condition and becomes solid on standing. 1 The em- 

 bedding substance can either simply inclose the tissue, or the tissue may be 

 soaked in it : the latter method is the one most commonly employed. 



The embedding substance chiefly used is paraffin of 110 F. (43 C.) melting 

 point. 



Embedding in paraffin. Before being soaked in melted paraffin, the piece 

 of tissue is stained, dehydrated by absolute alcohol, and is then soaked in 

 turpentine, xylol, or chloroform. From this it is transferred to molten 

 paraffin, which should not be too hot, and it is soaked in this for one or 

 several hours, according to thickness. It is then placed in the desired posi- 

 tion on the microtome and surrounded by melted paraffin. When cold, thin 

 sections can be cut, the paraffin dissolved out by turpentine or xylol, and the 

 sections mounted. 



If it be desired to cut a riband of successive sections, the block of paraffin 

 in which the organ is embedded must be cut with square angles, and some 

 paraffin of low melting point smeared over the opposite sides of the block. 



Preparation of frozen sections. The bichromate solutions are the best 

 fluids to use for preserving tissues which are to be frozen in place of being 

 embedded. The tissue requires to be soaked in gum-water before being 

 placed upon the freezing microtome. 



Embedding in celloidin. The piece to be embedded is dehydrated by 

 alcohol, and is then placed in a solution of celloidin in alcohol and ether. 

 After 24 hours or more it is removed from the celloidin and placed upon 

 a flat wooden or metal holder (which can be fixed in the microtome when 

 the celloidin has been hardened). When the celloidin is set, the holder 

 is plunged in alcohol (80 per cent.), and after a few hours, sections 

 may be cut with a knife wetted with spirit of the same strength. The 

 advantage of this method is that the celloidin, which is quite transparent, 

 need not be got rid of in mounting the sections, and serves to keep the 

 parts of a section together : it is thus very useful for friable tissues or for 

 large sections. The tissue may either be stained in bulk before embedding, 

 or the sections may be stained. The method is especially valuable for the 

 central nervous system. 



1 For rapid work a split piece of alcohol-hardened liver is often used to support the 

 tissue from which sections are to be taken. 



