SECTION CUTTING. 187 



must be described briefly. They are allowed to remain 

 in the solution for twenty-four hours and no longer, and 

 are then washed for twenty-four hours in running water 

 to remove the perchloride of mercury. They are then 

 passed through the various strengths of spirit (as will be 

 described subsequently), a little tincture of iodine being 

 added to each to remove any mercury which may still 

 remain. The other steps are the same as those which 

 are used if other methods of fixation have been 

 adopted. 



2. Formalin. This should be used in a 5 per cent, 

 solution in water. It yields very good results, and is 

 perhaps the fluid which can be most warmly recom- 

 mended to a practitioner who is going to send his 

 material to a public laboratory/" The fluid has very 

 great powers of penetration, and the slices may be much 

 thicker than we have recommended. The one objection 

 to the fluid is that it interferes somewhat with the way 

 in which the sections stain. 



3. Alcohol is not a very good fixing fluid, as it tends to 

 cause a good deal of shrinkage. When it is used the 

 blocks should be cut small and placed at once in undi- 

 luted methylated spirit. 



SECTION CUTTING BY THE FREEZING 

 METHOD. 



Sections which are prepared by the freezing method 

 are rarely as thin as those prepared by one or other of 



* Formalin should not be used for tissues which are to be searched 

 for the tubercle bacillus, as it prevents the decolorising action of the 

 acid. 



