726 HISTOLOGIC TECHNIC 



cent, solution of formaldehyd gas, which is to be used in strengths vary- 

 ing from 5 to 20 per cent. Small pieces should be left in the weaker 

 solutions (5 to 10 per cent.) from 'six hours to two days, not longer. 

 In the stronger solutions (10 to 20 per cent.) tissues should remain 

 for only two to six hours. More prolonged immersion in the fixative 

 causes considerable swelling. As a rule, the stronger solutions are pref- 

 erable; this is especially true for the fixation of the cells of lymphoid 

 tissue. The 10 per cent, formalin solution is valuable for the fixation of 

 human embryos. For the hardening of nervous tissues, also, the 10 per 

 cent, solution is very serviceable. 



After fixation., the tissues are transferred directly to 80 per cent, 

 alcohol, with one or several changes. This method gives excellent re- 

 sults with lymphoid and epithelial tissues, but does not bring out the 

 finer details of cytoplasmic structure. 



Potassium Bichromate (Dichromate) . This salt has been used in 

 all sorts of combinations; those which follow may be specially recom- 

 mended. Muller's solution is employed for the fixation of the tissues 

 of the central nervous system, and must be used when fixation is to be 

 followed by any of the Weigert hematoxylin staining methods. Applied 

 to the fixation of other organs, Miiller's fluid is apt to produce some 

 maceration and better results are usually obtained with Miiller-formol or 

 with Tellyesniczky's solution. 



For the special fixation of cytoplasmic granules, and also for after- 

 staining with Mallory's connective tissue stains, Zenker's solution yields 

 better results. 



Miiller's Solution: 



Potassium bichromate 2.5 grm. 



Sodium sulphate 1.0 grm. 



Water 100.0 c.c. 



Pieces of tissue are left in the fluid for one to six weeks; large 

 pieces of the spinal cord or brain require four to six weeks. If left too 

 long the tissues will become brittle. After fixation, the tissue is washed 

 thoroughly in running water for twelve to twenty-four hours, and 

 hardened in graded alcohol. 



A relatively large volume of the fixing fluid should be used, and 

 it should be frequently changed. It should not be allowed to become 

 turbid nor to deposit crystals : this is avoided by frequent changes and 

 by keeping the jars in the dark, or at least in such a position that 

 they are not exposed to a bright light. 



