296 APPENDIX 



As a rule this is done by placing the tissue in a basin of 

 water and allowing a slow stream of water from a tap to fall into 

 the basin. It is left so for three hours to a day or two. This 

 washing in running water should be carried out with tissues 

 hardened in osmic acid, Flemming's fluid, chromic acid. 



It may also be carried out with tissues hardened in chromium 

 salts, mercuric chloride, formol, or Perenyi's fluid, but such 

 tissues may be passed direct to alcohol, which should be renewed 

 two or three times. 



Water should be used as little as possible with portions of 

 the nervous system in which the basophil substance of the cells 

 is to be stained. Prolonged treatment with water deteriorates 

 also the medullary staining of nerve fibres by Weigert's method. 



After washing out, the tissue is left for a day in each strength 

 of alcohol 30 p.c., 50 p.c., 75 p.c. It may be further hardened 

 in 95 p.c. or in absolute alcohol for a week or more. It is kept 

 in 70 or 75 p.c. alcohol. 



Since chromic acid, potassium bichromate and other chromium 

 salts are precipitated by alcohol in the light, tissues hardened in 

 fluids containing these reagents, when transferred to alcohol, 

 should be kept in the dark, until the alcohol on renewal no longer 

 becomes yellow. 



c. Preparation of Solutions. It is generally advisable to 

 make of any given substance a solution of the maximum strength 

 likely to be required, and to dilute this when weaker solutions 

 are needed. If the stronger solution contains x p.c. of the 

 substance and it is required to make a weaker solution of y p.c., 



add 1 c.c. of water to each c.c. of the stronger solution. 



y 



It is rather better to use distilled water in making up the 

 solutions, but in most cases this is not of much importance. 



SIMPLE HARDENING AGENTS. 



Alcohol. Alcohol is especially used for glands and for pre- 

 serving the brain and spinal cord for staining nerve-cells by 

 Nissl's method (cp. p. 117). Strong alcohol (95 p.c. or absolute) is 



