TOOTH. 63 



a large amount of chromic acid and nitric acid fluid until they are sufficiently decalcified, which 

 may be ascertained by piercing them with a needle. Decalcification takes place in two to three 

 weeks if the fluid be frequently renewed. Wash the pieces frequently in water to remove the 

 surplus acid, and then transfer them to spirit until sections are required. Make a number 

 of vertical sections through the jaw, and the tooth implanted therein. Make a number 

 of sections across the long axis of a tooth and keep them till they are required. In the 

 process of softening, the enamel disappears, owing to the small amount of organic matter 

 present. Stain a vertical section and a transverse section in picrocarmine, and mount each 

 in Farrant's solution. 



Vertical section of a softened tooth. EXAMINATION (L). Observe the absence of enamel, 

 though the general distribution of the parts of the tooth is the same as in the unsoftened 

 tooth. Notice the tooth planted in the alveolus, which is lined by a membrane stained red 

 the pertodontal -membrane which serves as a periosteum for the bony wall of the alveolus. 

 Trace its continuity with the fibrous tissue of the mucous membrane of the gum. In the lower 

 jaw-bone, observe the medullary cavity with branches of a nerve and blood-vessels. They are 

 very apt to fall out, however. The pulp-cavity is seen to contain the pulp. (Indicate the general 

 arrangement of the parts in PI. XII., Fig. 5.) 



(H). Observe the dentinal tubules, which are, however, less distinct than in the dry tooth ; 

 the matrix is stained yellow. Here and there transverse sections of the tubules may be seen ; 

 they appear as fine dots in a homogeneous matrix. Notice the crusta petrosa with its 

 bone-corpuscles. In the pulp-cavity traces of blood-vessels, delicate connective tissue, and, 

 it may be, a layer of columnar cells the odontoblasts may be found lying on the dentine. 

 Place a similar section in one per cent, osmic acid for four hours, mount it in Farrant's 

 solution, and examine. It shows the same details more clearly. 



Transverse section of a softened tooth. EXAMINATION (L and H). Observe the pulp- 

 cavity and the dentinal tubules radiating from it, and many of them cut transversely. 



DEVELOPING TOOTH. 



PREPARATION. Place the head of a new-born rat, kitten, or puppy in a large quantity 

 of Miiller's fluid for two days, and then into a sixth per cent, solution of chromic acid for two 

 days. At the end of another two days, substitute for it the chromic and nitric acid mixture 

 (p. xxxiii) until complete decalcification takes place. Make vertical sections through the jaw, 

 and pass them through a five per cent, solution of sodic bicarbonate, to remove all trace of acid. 

 Stain a section with picrocarmine, another must be left in one per cent, osmic acid for several 

 hours, and mount both in Farrant's solution. 



EXAMINATION (L). Observe the general shape of a tooth-germ still imbedded in the 

 gum. Note the dentine and, it may be, the cap of enamel ; inside the dentine, the large 

 mass of tissue which becomes the pulp. 



(H). Lining the dentine, observe a layer of columnar cells the odontoblasts which give 

 off fibres the fibres of Tomes which pass into the dentinal tubules. 



