490 HISTOLOGY 



found at the moment of its action; render structures insoluble, and harden 

 them so that they will not be altered by the various after-steps; and give 

 a certain degree of optical differentiation. 



No single compound has yet been found which successfully fulfills all 

 of these conditions, nor are any of the recommended fixatives adequate 

 in all cases or for all special studies. Only the fluids commonly employed, 

 which have proven most useful, are here given. 



Small pieces of tissue, preferably less than i cm. in thickness, should 

 be dropped into a considerable amount of fluid. The tissue should be 

 handled as little as possible, in order that delicate structures may not be 

 destroyed. For example, contact between the fingers and the peritoneum 

 is sufficient to destroy the thin epithelium. 



In order to insure uniform action of the fixing fluid, it is ,of ten advisable 

 to place a little absorbent cotton in the bottom of the vessel. Frequent 

 gentle mechanical agitation will serve the same end. Tubular organs 

 should be washed out, or cut open and their contents and any adherent 

 blood washed away, with salt solution. Membranes may be kept flat 

 and smooth by tying them across the end of a short tube or detached 

 bottle neck. 



Alcohol. Small or thin pieces of tissue are supported on a little ab- 

 sorbent cotton in absolute alcohol, for 12 to 24 hours, changing after 3 

 or 4 hours. Large pieces are fixed by successive immersion in 70 per cent., 

 80 per cent., and 95 per cent, alcohol for 24 hours each. 



Alcohol is a valuable dehydrating and hardening agent, but its fixing 

 qualities are inferior, so that it is rarely used alone as a fixative. Small 

 embryos or blocks of tissue obtained in an emergency should be preserved 

 in 10 per cent, formalin, rather than in alcohol. 



Bouin's Fluid. 



Picric acid, saturated aqueous solution 75 



Formalin 20 



Glacial acetic acid 5 



This fluid is particularly recommended for the fixation of embryos, 

 for which it is unexcelled. Small embryos are fixed in 4 to 6 hours. Larger 

 objects may be fixed 24 to 48 hours or longer. For washing out the fixing 

 fluid, alcohol, first 70 per cent., then 80 per cent., should be employed. 

 Renew the alcohol as often as discolored. 



Carney's Mixtures. 



No. i Absolute alcohol ". 6 



Chloroform 3 



Glacial acetic acid i 



This is a very rapid fixative, even large pieces being fixed in \ to i 

 hour. Wash in absolute alcohol until the odor of acetic acid is lost, chang- 

 ing every 12 hours, and imbed; or grade through 95 per cent, to 80 per 

 cent, alcohol. 



