APPENDIX 299 



Potassium bichromate. Make a 2 p.c. solution. This is, 

 on the whole, the best reagent for the brain and spinal cord. 

 The cord takes four to six weeks to harden, and the brain two to 

 three months, unless it is cut into slices. 



A 3 p.c. solution is preferred by some observers ; to obtain a 

 sharper nuclear stain acetic acid up to 5 p.c. may be added ; 

 glandular tissues are fixed in this in 1 to 3 days. 



COMPOUND HARDENING AGENTS. 



Chromic acid and alcohol. Mix equal parts of chromic 

 acid -3 p.c. and alcohol 75 p.c. Various other mixtures are 

 used. Make as required and filter. Tissues placed in the 

 mixture should be kept in the dark. After four to six days, 

 wash in running water. 



Chromic and nitric acids (Perenyi's fluid). 



10 p.c. nitric acid 40 c.c. 



95 p.c. alcohol 30 c.c. 



0'5 p.c. chromic acid 30 c.c. 

 The tissue is left in the fluid three to six days. 



Chromic and osmic acids (Flemming's mixture). 



1 p.c. chromic acid 15 c.c. 



2 p.c. osmic acid 4 c.c. 

 Glacial acetic acid *5 to 1 c.c. 



This is a good hardening reagent for small pieces of tissue ; and 

 it is especially used to preserve dividing nuclei. The penetra- 

 tion of the osmic acid is slow. 



The tissue is placed in the fluid for a few hours to a day 

 (cp. b above). If it is desired to see fat globules after hardening 

 in Flemming's fluid, the tissue should be washed with water and 

 cut frozen, or kept in 70 to 80 p.c. alcohol for some time before 

 imbedding. 



The fluid is sometimes modified by taking only -5 to 1 c.c. of 

 osmic acid. 



Chromic acid and platinum chloride (Merkel). 

 1 p.c. chromic acid 25 c.c. 



1 p.c. platinic chloride 25 c.c. 

 Water 150 c.c. 



