MKTIIODS OF FIXATION. 403 



line or an entirely different color. The coloring-matters, when so 

 employed, not only bring out the structure of the tissue by creating 

 a color-picture, but they also serve as valuable reagents in revealing 

 the nature of the substances to which they impart a color. Again, 

 it is often necessary that a certain method of fixation or other pre- 

 liminary treatment should be used before the particular dye selected 

 can display its greatest selective power for a particular substance. 

 These facts explain the great number of formulae for stains and the 

 preparation of specimens that are found in the technical text-books 

 and journals. The subject has become so expanded within recent 

 years that it has almost created a distinct branch of learning ; but 

 it will only be necessary for the student of medicine to acquire a 

 knowledge of a few methods that will serve to reveal the general 

 structure of cells and the characters of the intercellular substances. 

 The general outline of the procedures in common use for this pur- 

 pose are as follows : 1, fixation ; 2, hardening ; 3, impregnation ; 

 4, embedding ; 5, cutting ; 6, staining ; 7, dehydration ; 8, clearing ; 

 9, mounting. 



Some methods of preparation combine one or more of these steps 

 in a single manipulation, thus considerably reducing the time requi- 

 site for the completion of the process. Other methods necessitate 

 the intercalation of still other manipulations, or the subdivision 

 of those already enumerated. 



Methods of Fixation. 



1. Mliller's Fluid. This classic fixing- and hardening-solution con- 

 sists of potassium bichromate, 2.5 per cent., and sodium sulphate, 

 1 per cent., dissolved in water (preferably distilled water). It is 

 slow in action, requiring from six to eight weeks for the preservation 

 of an average specimen, but with proper care can be made to yield 

 excellent results when the finer details of structure are not to be 

 studied. It is important to use large quantities of the fluid, at 

 least ten times the volume of the tissues immersed in it, and to 

 renew the fluid so frequently that its strength shall be constantly 

 maintained. When fresh tissues are placed in Miiller's fluid they 

 speedily render it cloudy. This is a sign that the fluid should be 

 renewed, even if only an hour has elapsed since the tissues were 

 placed in it. When cloudiness no longer appears the fluid should 

 be renewed once a day for the first two weeks : after that, two or 

 three times a week till the process is completed. 



