THE TEETH. 205 



in teeth which have been softened by immersion in 

 an acid. 



The acid generally used when the structure of the 

 (decalcified) dentinal substance only is to be investi- 

 gated is the hydrochloric. A 10 per cent, solution 

 may be employed, and the tooth is steeped in this 

 until entirely soft, after which it may be preserved 

 in spirit. Sections are to be made in planes both 

 parallel with and across the direction of the dentinal 

 tubes. 



Preparation 3. Dentinal sheaths. To show 

 the sheaths which line the dentinal tubules, a piece 

 of such softened tooth is transferred to strong hydro- 

 chloric acid (contained in a watch-glass, which is 

 covered by another, inverted). In this it may be 

 left for about an hour, after which time all that 

 will be found is a tenacious' soft mass occupying its 

 place. If some of this be removed with a small 

 pointed piece of wood, placed on a slide, covered, and 

 examined, it will be found to consist wholly of fine 

 tubular threads the dentinal sheaths. These, being 

 composed of a substance which resists the solvent 

 action of strong hydrochloric acid longer than the 

 other animal tissues, remain for a time visible after 

 the rest of the dentinal substance has disappeared. 



Preparation 4. Soft tissues of the teeth. 

 But to study the soft tissues that is to say, the 

 pulp and odontoblasts with the processes which 

 these send into the dentinal tubules we must, as 

 in the parallel case of bone, employ a reagent which, 

 whilst softening the hard parts, at the same time 

 preserves and hardens the soft parts. Of these, picric 

 acid is the best to employ. The freshly- extracted 

 tooth is placed in a saturated solution of the acid, 

 and crystals of the latter are from time to time 

 added as required ; the solution being stirred as fre- 

 quently as possible with a glass rod. It is well to 

 break the tooth first, so as to expose the pulp cavity, 

 if this can be done without disarranging the con- 

 tents too much. When softened throughout this 

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