APPENDIX TO THE HISTOLOGICAL SECTION. 



PRESERVING, FIXING AND HARDENING FLUIDS. 



General Rules. 1. Fix ax soon after death as possible. 2. The, size 

 of the piece of tissue to be treated is of the first importance and will vary with 

 the density of the tissue and the penetrating power of the reagent used. 3. Not 

 less than 20 volumes of fluid to one of the tissue are to be used, and the object 

 should be suspended by a thread in the upper part of the reagent or laid on 

 a bed of absorbent cotton so that the fluid may have ready access. 



1. Nopmal saline solution. 0.6 P- C - NaCl (6 of dry NaCl 

 are dissolved in 1,000 CC of distilled water). This fluid is used for 

 the examination of fresh tissues, and delays changes in them for a short 

 time ; though not perfectly " indifferent," owing to its convenience is 

 much used. Egg white is a useful substitute. Aqueous humour or 

 iodised serum (blood-serum to which iodine crystals have been added) 

 are sometimes used. 



2. Alcohol. An indispensable reagent in histology. 



(a) Absolute alcohol is chiefly used for final dehydration of tissues. 

 Used alone it fixes and hardens epithelial structures well. 24 hours to 

 several days according to the size and density of the tissue. 



(b) One-third alcohol (Ranvier). "Alcool au tiers." 1 part 90 P- C 

 alcohol and 2 parts water. Ranvier's original receipt is " Alcool a 

 36 de Cartier " (88'5P- C - pure alcohol) 1 part, water 2 parts. Dissociates 

 epithelial structures in 24 to 48 hours. 



(c) After -hardening. With increasing strengths, starting from 7QP- C -, 

 several changes, at intervals of 24 hours, through 8QP- C - to 9QP- C . In 

 the latter, tissues or sections may be kept stored for later use in 

 well-stoppered bottles. Tissues fixed in chromium salts should either 

 be washed in water before the alcohol treatment, or be kept in the dark 

 until all colour has been washed out by changes of alcohol, 7QP- C -, 

 otherwise a green precipitate is apt to form. After corrosive fixing, 

 see precautions indicated later. Prolonged after-hardening in strong 

 alcohol (methylated spirit) for several weeks confers great firmness 

 upon tissues and enables them to resist the strain of embedding 

 more perfectly. 



(d) Methylated spirit does well for nearly all purposes as strong 

 alcohol. If methylated spirit loses its clearness, and turns at all 

 milky on dilution with water, it is unfit for use, being surcharged 

 with resins. 



