CHAPTER 10 



Mounting 



A mounting medium is a substance in which tissues are immersed 

 for examination under the microscope. The same name should 

 not be appHed to adhesives that merely stick sections to slides and 

 do not enclose the tissues at the moment of examination. Mount- 

 ing is necessarily the last process to which tissues are subjected in 

 the making of a microscopical preparation. 



Solid, unshrinkable objects may be mounted dry. If so, air is 

 to be regarded as the mounting medium. In electron-microscopy 

 sections are examined dry, in a vacuum; if the embedding medium 

 has been removed, the tissue constituents are exposed directly to 

 the electron beam. With these exceptions, tissues are always 

 mounted in media that are fluid at the outset, though many 

 sohdify later. The liquid may be a single pure substance, a mix- 

 ture of liquids, or a solution of one or more solids in a liquid; it 

 must be optically homogeneous and stable. Volatility is allowable, 

 because the edge of the mount can be sealed (p. 133). The mount- 

 ing medium must fill even the minutest spaces formed within cells 

 by coagulant fixatives, for otherwise different parts of the speci- 

 men would be permeated by fluids of diff'erent refractive indices, 

 and the image would therefore be confused (see fig. 4, p. 26). The 

 mounting medium must be transparent and nearly or quite colour- 

 less. It must be capable of wetting the various tissue-constituents, 

 especially proteins (that is to say, it must be able to lie against 

 them without leaving any intervening spaces). It must not dissolve 

 or corrode the tissue-constituents that are to be studied, though 

 solvents of particular constituents (such as lipids) are permissible 

 if the intention is to study the insoluble remainder of the tissue. It 

 must not support the growth of moulds or bacteria. 



123 



