xxxii INTRODUCTION. 



4. Ammonium Bichromate Solution. Dissolve 20 grms. in 1,000 c.c. water. This is very 

 useful for hardening the brain and spinal cord, and nervous structures generally. 



5. Ammonium Chromate Solution. Dissolve 50 grms. of ammonium chromate in 1,000 c.c. 

 water, i.e. a five per cent, solution, or it may be made by adding one ounce of the salt to 

 twenty fluid ounces of water. Filter and preserve in a stoppered bottle. It hardens fresh 

 tissue, such as the mesentery, in twenty-four hours. After hardening, the tissue must be washed 

 until no more colour is removed by the water. It may then be stained and mounted in gly- 

 cerine. For other cases, the kidney for instance, small pieces are hardened for forty-eight 

 hours, and after being thoroughly washed are then treated with spirit, as directed above 

 (p. xxxi). This substance is of the utmost value for revealing the rod-like structure in the 

 renal epithelium, and for demonstrating the existence of the intra-cellular and intra-nuclear 

 plexus of fibrils in cells. 



6. Miiller's Fluid. Dissolve 25 grms. potassic bichromate -and 10 grms. sodic sulphate in 

 1,000 c.c. water. This is a most useful reagent, as it penetrates more readily than chromic 

 acid, though it takes longer time to harden tissues usually from five to seven weeks. 



It is frequently advantageous to combine the use of two of the above fluids, thus: place the 

 tissue for a week in Miiller's fluid, and complete the hardening in No. 2. After hardening, 

 the tissues are well washed, and preserved in spirit. 



7. Miiller's Fluid and Spirit. Mix 3 parts of Miiller's fluid and i part of methylated 

 spirit. This does very well for nerve-tissues, muscle, and the retina. It ought to be prepared 

 fresh when required, and the tissue ought not to be placed in it till the mixture cools after the 

 addition of the spirit To prevent the separation of the chromium salts, which is apt to occur, 

 keep the mixture in a dark place. 



8. Methylated Spirit is sometimes, though rarely, used alone. 



9. Absolute Alcohol of S. G. 0-795. This hardens very rapidly in twenty-four hours but 

 it causes considerable shrinking, though it is invaluable for gastric mucous membrane, and for 

 secretory glands generally, e.g. the salivary glands and pancreas. Tissues become stained very 

 readily after hardening in pure alcohol. 



B. THOSE WHICH HARDEN, AND AT THE SAME TIME COLOUR THE TISSUE. 



10. Picric Acid. Make a watery solution saturated in the cold. Keep crystals in the 

 bottle to ensure saturation. Tissues ought to be left in this only for a day or two, else they 

 are rendered too brittle. Its stain is of a bright yellow colour, which is easily removed by 

 prolonged washing in water (Ranvier). Tissues hardened in this fluid become easily stained 

 with picrocarmine. 



11. Kleinenberg's Picric Acid is a modification of the above. To 100 c.c. of a cold saturated 

 watery solution of picric acid add 2 c.c. of strong sulphuric acid, which throws down a yellow 

 precipitate. Filter, and to the filtrate add 300 c.c. of distilled water. This solution is most 

 valuable for fcetal tissues, and especially for early embryos. It produces its effects in from 

 three to ten hours. 



12. Osmic Acid. A one per cent, watery solution is most useful, which can be diluted as 

 required. A \ per cent, solution, prepared with distilled water, is usually employed. It 

 decomposes very rapidly when in contact with organic matter and exposed to light ; and as 

 it is very volatile, it must be kept in a glass-stoppered bottle, covered with paper, to protect it 

 from the light. It hardens in from six to twenty-four hours, and acts specially on fatty 

 matters, which it blackens, and is very useful for tracing the course of medullated nerve-fibres, 



