10 HISTOLOGICAL TECHNIC 



the excess of bichromate, though too prolonged washing seems to be 

 detrimental. A precipitate forms in the alcohols, but this apparently 

 does no harm. 



After mercuric chlorid or Zenker fixation the washing may be 

 done either in water or in alcohol. To avoid precipitates in the tis- 

 sues add a small quantity of an iodin solution (equal parts tincture 

 iodin and lo-per-cent. aqueous solution potassium iodid) to any of 

 the grades of alcohol. As the alcohol becomes clear more of the solu- 

 tion is added until the alcohol remains slightly tinged. 



III. Preserving 



Hardened tissues are usually preserved in 8o-per-cent. alcohol. 

 Formalin in aqueous solutions of i-per-cent. to lo-per-cent. is also 

 used as a preservative. In either case, when it is necessary to pre- 

 serve the specimens for a considerable length of time (several months 

 or longer), the tissues are likely to lose their staining qualities to a 

 certain extent. Preserving the specimens in equal parts of strong 

 alcohol, glycerin, and distilled water is successful as a partial remedy 

 for this. 



IV. Decalcifying 



Tissues containing lime salts, like bones and teeth, must have 

 the lime salts dissolved out before sections can be cut. This process 

 is known as decalcification. 



Tissues to be decalcified must be first fixed and hardened. Fix- 

 ation in Orth's fluid and hardening in graded alcohols give good re- 

 sults. After hardening, the tissue is washed in water and placed in 

 one of the following decalcifying fluids. The quantity of fluid should 

 always be large and the fluid frequently changed. The completion 

 of decalcification can be determined by passing a needle through 

 the specimen or by cutting it with a scalpel. The time required varies 

 with the size and hardness of the specimen and the decalcifying fluid 

 used. When decalcification is complete, the specimen is washed in 

 sufficient changes of water to remove all trace of acid. This may be 

 quickly accomplished by the addition of a little ammonium hydrate 

 to the water. The specimen is then carried through graded alcohols. 



(i) Hydrochloric Acid. — This may be used in aqueous solutions 

 of from 0.5-per-cent. to 5-per-cent. A very satisfactory decalcifying 

 mixture is that known as Ebner's hydrochloric-salt solution. It 

 consists of: 



