87 



For temporary examination, cover in a drop of the potassium 

 acetate solution. For permanent preparations , drain off the potassi- 

 um acetate solution and add a small drop of glycerin or glycerin- 

 jelly. 



Stained preparations. Pour off the potassium acetate solution 

 and add a half saturated solution of alum, letting it remain for 24 

 hours or longer. Tease in water, stain with hematoxylin or carmine, 

 wash away the stain with water, and add a drop of glycerin or gly- 

 cerin-jelh\ Cover and seal (§ no). 



If a large amount is desired, the tissue may be carried through 

 the various steps in a vial. 



§ 16. Ranvier's one-third alcohol. Formula : 95% alco- 

 hol, I part, water 2 parts. This is an excellent dissociating fluid 

 for epithelia. An action of 24 hours is generally sufficient. A 

 weaker solution is often advantageous. 



§ 17. Osmic acid. A ^% solution of osmic acid is a valua- 

 ble dissociator, especially serviceable in the isolation of nerve-fibers, 

 myelinic and amyelinic, and when fat is present, since fat and the 

 myelin of myelinic nerve-fibers are blackened by it. 12-24 hours 

 generally affords sufficient time for it to act. 



FIXATION. 



§ 18. I,iving tissue, when allowed to die and remain undis- 

 turbed, gradually loses the structural features it had in life and 

 undergoes disintegration and decay. K fixer is a fluid (or gas) into 

 whicji the living, or at least very fresh, tissue is placed in order to 

 preserve the structure of its elements as nearly as possible as in life. 



The action of the fixer is a chemical action in which the living 

 substance is coacfulated (fixed) by the reagent, which in some cases 

 forms a chemical union with the organic substance. The chemical 

 action is generally attended with a more or less marked mechanical 

 distortion, as shrinking or swelling, to obviate which chemicals of 

 opposite tendencies are, in the best fixers, combined with each other 

 in proportions intended to neutralize such effects. The chemicals 

 of most service as fixers are (i) osmic acid, (2) platinic chlorid, (3) 

 picric acid, (4) acetic acid, (5) chromic acid, (6) mercuric chlorid, (7) 

 nitric acid, (8) ethyl (or methyl) alcohol ; also, (9) potassium 

 dichromate, (10) sodium sulphate, (11) copper sulphate. The last 



