310 HISTOLOGICAL TECHNIQUE. 



Canada balsam lias a somewhat higher refractive index than 

 dammar. It therefore renders the sections a little more transparent 

 and more completely obliterates the structure-picture. When it is 

 desired to retain as much of the structure-picture as possible, 

 which is usually the case, dammar should be chosen for the mount- 

 ing-medium. It dries a little more slowly than balsam, but soon is 

 sufficiently dry at the edges of the coyer-glass to preserve the sec- 

 tion from injury. If the slides are kept in a horizontal position, 

 in a warm place (40° to 50° C. ; 104° to 122° F.), for a couple of 

 days, they will be dry enough for storage, but for several weeks 

 must be handled with care. 



Stained sections may be examined in glycerin, having been 

 mounted by the same manipulations as those used for mounting 

 in balsam, without previous dehydration or clearing. Such mounts 

 are, however, difficult of preservation. The various cements that 

 have been recommended for fastening the edges of the cover-glass 

 to the slide are usually inefficient, as the changes of temperature 

 that are inevitable cause the glycerin to make its way between the 

 glass and cement, loosening the latter. 



A better medium than glycerin for sections that cannot be sub- 

 jected to the action of alcohol for the purpose of dehydration is 

 glycerin-jelly. This is prepared by soaking the best French gelatin 

 in cold water until it has imbibed all it will readily take up, then 

 melting the gelatin, after pouring off the excess of water, and 

 adding an equal bulk of glycerin. A little carbolic acid may be 

 added to the mixture to preserve it. The manipulations for mount- 

 ing are similar to those given above, the sections being transferred 

 from water to the slide. The glycerin-jelly may be melted and a 

 drop placed upon the section, or a little lump of the solid jelly may 

 be placed upon a cover-glass, melted by gentle heat, and the cover- 

 glass then inverted over the section on the slide. After the jelly 

 has dried at the edges of the cover-glass they may be painted with 

 xylol balsam, dammar, or some cement. 



The Rapid Preparation of Sections for Diagnosis. 



The most expeditious means of obtaining sections of fresh tis- 

 sues is to cut them without preliminary treatment with reagents, 

 either free hand with a razor, or with the aid of a freezing mi- 

 crotome. Such sections may be stained with methylene-blue, or they 

 may be examined in neutral salt solution, if they are to be stained, 



