38 PRACTICAL HISTOLOGY. 



in a larye volume of the fluid. If chromic acid be used, the bone 

 must be first hardened in this fluid. Place it in .1 per cent, 

 chromic acid for twenty-four hours , renew the fluid, but use .2 per 

 cent., and after a week use .5 per cent. ; shake the vessel from time 

 to time, to bring new fluid into contact with the tissue. 



If a more rapid process is desired, after the bone has been two or 

 three days in dilute chromic acid (.2 per cent.), use the chromic 

 and nitric acid fluid. Decalcification requires about fourteen days. 



To test for the removal of all the salts, push a needle into the 

 bone, or make a section with a blunt razor. Obstruction in either 

 case denotes that the bone has not been sufficiently decalcified. In 

 most cases, the bony tissues should be hardened before they are 

 decalcified. This is specially the case in connection with bone 

 softened in chromic acid. It is better to harden them first in 

 M tiller's fluid, and then to decalcify them in chromic and nitric 

 acid fluid (p. 36). 



Bone decalcified in chromic acid must be thoroughly washed in 

 running water for many hours to remove all the chromic salts, and 

 is then transferred to 70 per cent, spirit and kept in the dark, other- 

 wise there will be a copious deposit. Renew the spirit, and transfer 

 the tissue to strong alcohol, still keeping it in the dark. 



If a bone is to be softened in picric acid, it may be placed at once 

 in this fluid, with the precaution indicated at p. 37. It need not 

 be kept in the dark, but it is better to remove as much as possible 

 of the yellow stain by means of alcohol. It decalcifies somewhat 

 more slowly than chromic acid. 



VIII. METHOD OF PREPARING TISSUES AND 

 ORGANS FOR MICROSCOPICAL EXAMINA- 

 TION. 



As most of the tissues require to be hardened, and it is frequently 

 impossible to obtain human tissues sufficiently fresh, recourse must 

 be had to the fresh tissues of animals. As frequently as possible, 

 however, human tissues should be secured. Most of the tissues 

 may be obtained from a cat, rabbit, or guinea-pig, and for certain 

 special purposes the dog, frog, newt, and salamander are used. 



The cat, rabbit, or guinea-pig or, better, all three are killed 

 by chloroform. The animals are placed in an air-tight box a 

 large saucepan does very well along with a sponge saturated with 

 chloroform. Small animals may be chloroformed under a bell-jar. 

 As soon as the animal is dead, open the thorax by a longitudinal 

 incision through the costal cartilages right and left raise the 



