STRUCTURE OF THE TEETH. 149 



LESSON XXVIII. 



STRUCTURE OF THE TEETH, THE TONGUE, AND MUCOUS 

 MEMBRANE OF THE MOUTH. 



1. STUDY first with the low power and afterwards with the high power a 

 longitudinal section of a human tooth which has been prepared by grinding. 

 It is better to purchase this specimen, for the process of preparation is 

 difficult and tedious without the aid of special apparatus. Examine carefully 

 the enamel, the dentine, and the cement. The dark appearance of the 

 deiitinal tubules is due to their containing air in the dried specimen. 

 Measure the diameter of the enamel prisms and of some of the dentinal 

 tubules. Make sketches from each of the tissues. 



2. Mount in Canada balsam a section of a tooth, in situ, which has been 

 decalcified in chromic or nitric acid and stained with hsematoxylin or 

 carmine. In this section the mode of implantation of a tooth, as well, as the 

 structure of the pulp, can be made out. Make a general sketch under a low 

 power, and under a high power draw a small piece of the pulp showing the 

 processes of the odontoblasts extending into the dentinal tubules. 



3. The development of the teeth and the formation of their tissues are 

 studied in sections made across the snout and lower jaw of foetal animals. 

 The preparation should be stained in bulk with alcoholic magenta, borax- 

 carmine, or hsematoxylin, and embedded in paraffin or celloidin if the 

 former, the sections must be mounted by an adhesive process (see Appendix). 



4. Sections of the tongue vertical to the surface ; stain with hsematoxyliii, 

 and mount in Canada balsam. In these sections the arrangement of the 

 muscular fibres and the structure of the papillae of the mucous membrane 

 may be studied ; and if the organ have been previously injected, the arrange- 

 ment of the blood-vessels in the muscular tissue and in the mucous membrane 

 will also be well seen. 



THE TEETH. 



A tooth consists in man of three calcified tissues : the enamel, which 

 is of epithelial origin, the dentine, and the cement, or crusta petrosa. The 

 dentine forms the main substance of a tooth, the enamel covers the 

 erown, and the cement is a layer of bone which invests the root (fig. 172). 



The enamel is formed of elongated hexagonal prisms (fig. 173), which 

 are set vertically, or with a slight curvature, upon the surface of the 

 dentine. They are marked at tolerably regular intervals with slight 

 transverse shadings producing an indistinct cross-striated appearance. 

 Sometimes coloured lines run through the enamel across the direction 

 of its fibres. 



