146 HISTOLOGY OF THE TEETH. 



volume, transfer them to a solution of 5 drops acetic acid or to i 

 ounce of strong glycerine, and leave in the fluid for four days. 



To facilitate the Study of the Nerve-Fibrils, the pieces should 

 now be transferred to i oz. of glycerine solution of acetic acid, to 

 which enough strong chromic acid must be added to make an 

 infusion of a pale straw colour. Leave for a week, and then 

 imbed in gum. Cut as many mounts as required. These must 

 now be examined in strong glycerine under (i-inch) low power, 

 and the best sections, mounted in glycerine with two drops of 

 acetic acid to the ounce, will show with higher powers. If you 

 wish to keep any of these, draw off the superfluous fluid with 

 blotting-paper, and the glass cover must be cemented to the slide 

 by painting carefully a thin ring of marine glue or Ward's brown 

 cement round it ; when dry, a second or third coat may be added, 

 and finally finish with white zinc. 



If desired to Study Blood- Vessels of Pulp, chloroform an 

 animal, and just before respiration ceases, open the right auricle 

 and let the vessels empty themselves ; then inject with Prussian 

 blue, warmed to a temperature of 40 deg. C, to render the 

 gelatine fluid and also to prevent any vascular spasm which a cold 

 fluid is very liable to produce. Then place the head in alcohol 

 for twenty-four hours to harden the injection ; when the pulp is 

 removed, immerse in a weak solution of chromic acid, and at the 

 end of ten days sections of it may readily be cut, which are best 

 mounted in glycerine jelly. If the animal is dead, you must wait 

 till rigor mortis has passed off, and inject a ;/^;/-gelatinous Prussian 

 blue, but the first injection is the best. In some animals which 

 have died of strangulation the vessels will be found so gorged 

 with blood as to render any further preparation unnecessary. 



Class IV. — Sections of Tooth-Germs. 



Obtain a very young foetus, 3 to 8 inches in length. xA-void all 

 that have been kept in spirit, especially if it has been weak, or the 

 epithelium will have peeled off, and the jaws will be useless for our 

 purpose. Remove the head, place in a very weak solution of 

 chromic acid (J4^ per cent.), leave for from eight to ten days, when 

 remove to alcohol to absorb water. Imbed in gum, freeze, cut, 



