METHODS OF FIXATION. 405 



maldehyde (40 per cent.) is to be added to every 100 cc. of the 

 Miiller's fluid. 



Orth's fluid fixes in three or four days. The pieces of tissue 

 should not be more than 1 cm. thick. The time for fixation can 

 be shortened if smaller pieces are used and the process is carried 

 on at a slightly elevated temperature ; e. g., in an incubator kept at 

 37 C. (98.6 F.). After fixation the specimens should be washed 

 in running water, as in the previous methods. 



4. Mercuric Chloride Solution. A saturated solution of corrosive 

 sublimate in 0.5 per cent, salt solution is prepared by heating an 

 excess of sublimate crystals in the salt solution and allowing the 

 mixture to cool. The clear fluid is decanted from the crystals when 

 desired for use. The penetration and action of the solution are 

 favored by the addition of 5 per cent, of glacial acetic acid at the 

 time of using. The thickness of the pieces of tissue should not 

 exceed 5 mm., and much thinner pieces are better. Fixation takes 

 place within six hours, after which the tissues may be washed in 

 running water, or placed at once in 70 per cent, alcohol. If acetic 

 acid has been used, it is best to wash in water before immersing in 

 alcohol. Tincture of iodine should be added to the alcohol for the 

 reasons given in the description of Zenker's fluid. 



5. Formaldehyde. This gas is capable of being absorbed by 

 water to form a 40 per cent, solution, but its volatility renders such 

 a solution liable to deterioration. The strength employed for fixa- 

 tion is usually 4 per cent., and may be prepared by adding 10 cc. 

 of 40 per cent, formaldehyde to 90 cc. of distilled water. A 0.75 

 per cent, solution of common salt may be substituted for the distilled 

 water with possible advantage. 



Formaldehyde penetrates deeply and quickly into the tissues, 

 which may be 1 cm. in thickness, and accomplishes fixation within 

 twenty-four hours, but the preservation of structural detail is not 

 very perfect. The solution is useful where the general characters 

 of the tissues are to be determined and the details of the cells are 

 of comparatively little consequence. After fixation the tissues may 

 be washed in water, or placed directly in 70 per cent, alcohol ; or 

 frozen sections may be at once prepared. Satisfactory sections may 

 be obtained from small pieces of tissue if they are put in the for- 

 maldehyde solution for an hour or two and then cut with the 

 freezing-microtome. After they have been washed for a short time 

 in water they may be stained by any of the more usual methods. 



