125 



LESSON XXVI. 



STRUCTURE OF THE TEETH, THE TONGUE, AND MUCOUS 

 MEMBRANE OF THE MOUTH. 



I. 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 

 dentinal 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 picric acid and stained with logwood or borax- 

 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 haematoxylin, and embedded in paraffin, and the sections mounted 

 by the shellac-creosote process (see Appendix). 



4. Section across the whole tongue of a small mammal ; stain with log- 

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

 the muscular fibres and the structure of the papillae of the mucous mem- 

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

 arrangement of the blood-vessels in the muscular tissue and in the mucous 

 membrane will also be well seen. 



THE TEETH. 



A tooth consists 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 

 crown, and the cement is a layer of bone which invests the root 

 (fig. 150). 



The enamel is formed of elongated hexagonal prisms (fig. 151), 

 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 ap- 

 pearance. Sometimes coloured lines run through the enamel across 

 the direction of its fibres. 



