THE TEETH 137 



larly formed openings, interglobular spaces, which are channeled 

 in the outer part of the dentine. The dentinal terminal fibers are 

 in connection with branched cells which occupy the interglobular 

 spaces. 



The Enamel. The part of the dentine above the neck of the 

 tooth is protected by a covering of enamel. The enamel consists 

 of prisms, 4 p in diameter, united into bundles by a little cement 

 substance, which pass in a direction nearly at a right angle to 

 the surface of the dentine. They are of extreme density, contain 

 little besides 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. 



Cementum. The fang portion of the dentine is invested with a 

 thin layer of true bone, containing lacunce and canaliculi, but no 

 Haversian canals. The cementum is provided with pericementum 

 (periosteum), which forms the bond of union between the teeth 

 and the process of the maxillary bone. The bone- corpuscles are in 

 connection, through the canaliculi, with the cells in the interglo- 

 bular 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 cementum. 



PRACTICAL DEMONSTRATION 



The illustrations given in text-books have been drawn from dried teeth, 

 ground down to the requisite thinness by means of corundum or emery wheels. 

 This is a very tedious process, and is impracticable with the student. If such 

 specimens are desired, it will be advisable 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 one-sixth per cent, chromic-acid solution, to which five 

 drops of nitric or hydrochloric acid have been added, may be 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, ascer- 

 taining the progress of decalcification by pricking a fang with a 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 may be made with the razor or knife, with or without 

 a microtome. The form will be preserved except as regards the enamel; this 



