Hardening and Preserving Animal Tissues 135 



directly they become opaque throughout. Then wash in 

 repeated changes of 70 per cent, alcohol to which a little 

 tincture of iodine has been added. This process will fix 

 tissues in a few minutes. 



Picric Acid. Make a saturated solution in water. This 

 solution will fix small pieces of tissue in a few minutes ; 

 larger specimens will require from three to six hours' 

 immersion. Then wash out the picric acid with repeated 

 changes of spirit. Water must not be used, as it is 

 hurtful to the tissues that have been prepared by this 

 method. For the same reasons, during all subsequent 

 stages of treatment, water should be avoided, and the 

 staining should be carried out in alcoholic solutions. 



Formaldehyde. This may be used universally if 

 desired. It is sold commercially as * formal ' in a 40 per 

 cent, solution. This must be reduced by the addition of 

 water to a 2 or 4 per cent, solution. It is specially useful 

 for hardening nervous tissues and for eyes ; the latter are 

 completely hardened in 24 hours. 



When in great haste, tissues may instantly be fixed in 

 boiling water. Boil some water in a test-tube, then drop 

 in small pieces of the tissue, and boil again for a few seconds. 

 The specimen may then be placed at once in gum and 

 syrup, and when penetrated, freeze, and make the sections. 

 This method should only be used when a section is urgently 

 wanted. 



General Directions for Hardening Tissues. 



1. Always use fresh tissues. 



2. Cut the organs into small pieces with a sharp knife. 



3. Never wash a specimen in water; when it is necessary 



to remove any matter, allow some normal salt solu- 

 tion to flow over the surface of the tissue, or wash in 

 some of the hardening reagent you are going to use. 



4. All specimens should be hardened in a large quantity of 



the reagent ; too many pieces should not be put into 

 the bottle, and they should be kept in a cool place. 



