FIXING FLUIDS 49 1 



No. 2. Saturate mixture No. i with mercuric bichloride (about 20 

 parts). This is the most rapid and penetrating fixative known, and it 

 affords a very delicate cytological fixation. Immersion for 30 minutes 

 to i hour is sufficient even for the larger pieces. Subsequent treatment 

 as with No. i, except that the crystals of sublimate must be removed from 

 the tissue, either by placing the block in 80 per cent, alcohol and iodine 

 (see Zenker's fluid) ; or after the block has been cut, by treating the sec- 

 tions with iodine (see p. 497). 



Flemming's Fluid. 



Osmic acid, i % aqueous solution 10 



Chromic acid, i % aqueous solution 25 



Glacial acetic acid, i % aq. solution 10 



Distilled water 55 



This solution should be mixed only at the time of using. Only very 

 thin pieces (not over 2 mm. thick) should be used. Fix for 24 hours or 

 longer (sometimes even for weeks). Wash in running water 24 hours. 

 Pass through 50 per cent., 70 per cent. (12 hours in each), to 80 per cent, 

 alcohol. 



Formaldehyde. The gas is soluble in water to the extent of 40 per 

 cent., and solutions of this strength are obtainable under the trade names 

 of formalin, formol, and formalose. 



For fixing tissues, 10 c.c. of the commercial product are added to 90 

 c.c. of water. It penetrates very quickly, but specimens may be left in 

 it for a considerable time without apparent harm. Ordinary blocks are 

 sufficiently fixed in from 12 to 24 hours. Transfer directly to 80 per cent, 

 alcohol. 



Histologically, its chief use is for the preservation of nervous tissue, 

 the fixation of tissue to be cut with the freezing microtome, and the preser- 

 vation of embryos. Small human embryos obtained by practitioners should 

 be put at once into 10 per cent, formalin and forwarded to an embryological 

 laboratory. 



March!' s Fluid. 



Potassium bichromate 2.5 



Sulphate of sodium i . o 



Water 100 . o 



Osmic acid, i% aqueous solution 50.0 



Small pieces are fixed for 5 to 8 days in the dark. Wash 24 hours in 

 running water; 50 per cent, and 70 per cent, alcohol (24 hours each); 

 80 per cent, alcohol. Used for demonstrating degenerated nerve fibers 

 and in making damar mounts of fat and myelin, since the osmium reduced 

 by fat is insoluble in alcohol. Sections must not be treated with xylol, 

 but chloroform should be used instead. 



Orth's Fluid. 



Potassium bichromate 25 



Sodium sulphate 10 



Water. . . . 1000 



