80 PRACTICAL MICROSCOPY. 



which pass in a direction nearly at right angles to the surface of the 

 dentine. They are of extreme density, contain little beside inorganic 

 material and in a vertical section the whole is traversed by parallel 

 striae, not unlike the markings indicating tree-growth the lines of 

 Retzius. 



Crusta Petrosa. The fang portion of the dentine is invested with 

 a thin layer of true bone, arranged in laminae, and containing lacunm 

 and caiialiculi, but no Haversian canals. The crusta is provided with 

 periosteum, which forms the bond of union between the teeth and the 

 process of the maxillae. The lacunar bone corpuscles are in connec- 

 tion, through the canaliculi, with the cells in the interglobular spaces 

 of the dentine. It will be seen that the connective-tissue elements, 

 at least of the pulp, are in eventual histological connection with the 

 bone corpuscles of the crusta. 



PRACTICAL DEMONSTRATION. 



The illustrations common to our text-books have been drawn from 

 dried teeth, ground down to the requisite thinness by means of corun- 

 dum or emery wheels. This is a very tedious process, and is impracti- 

 cable with the student. If such specimens are desired it will be ad- 

 visable to purchase them already mounted. They only give the 

 skeleton of the organ, all the soft tissues being destroyed by the drying 

 and grinding. 



While dry specimens exhibit the plan of a tooth, the soft tissues 

 must be studied in sections made after the inorganic constituents have 

 been removed. Teeth immediately after extraction are to be treated 

 in the same manner as described for bone. A une-sixth per cent of 

 chromic-acid solution, to which five drops of nitric or hydrochloric 

 acid have been added, may be first used. Let the quantity of liquid 

 be liberal, and from time to time, say every three days, add a 

 few drops of the nitric acid. The decalcification should proceed 

 slowly and may be complete in from two to three or four weeks. The 

 earthy matters will first be dissolved from the surface. Watch the 

 action carefully, ascertaining the progress of decalcification by prick- 

 ing a fang with the needle. If the acid be too strong, and the action 

 too rapid, the whole may be destroyed. When the decalcification is 

 complete, a needle may be easily passed through the tooth and sections 

 be made with the razor or knife, with or without a microtome. The 

 form will be preserved except as regards the enamel; this will be en- 

 tirely dissolved. The enamel prisms may be demonstrated by treat- 

 ing broken fragments with dilute acid for a short time only. 



Sections should be stained with carmine and picric acid and 

 mounted in glycerin. For the study of the development of teeth, 

 foetal jaws may be treated as just described; and, when properly de- 

 cnlcified and hardened, should be infiltrated with celloidin, sectioned 

 and stained. I would refer the student to the excellent article on the 

 subject in Dr. C. Heitzmann's "Morphology." 



