XXII.] TOOTH. 2$I 



ADDITIONAL EXERCISES. 



Glands of Tongue or Palate. Harden the tongue or soft palate of a rabbit 

 in 3 per cent, nitric acid for 1-2 hours. Wash out all the acid and stain the 

 sections with methylene-blue. Wash out the blue with alcohol until only the 

 glands remain blue. Mount in xylol-balsam. 



Terminations of Nerves in the Lingual Papillae and Glands. These maybe 

 studied by staining small pieces of the tongue of mouse or rat by Golgi's silver 

 nitrate method (p. 78), or by the rapid hardening method (bichromate of potash 

 and osmic acid, Lesson XXVI. 14). The nerve in the papilke contains nerve-cells 

 which are in connection with the nerve-fibres. The nerve-fibres form a plexus 

 of fibres outside the basement membrane of the serous gland acini epilemmal 

 plexus and one within this membrane amongst the secretory cells hypo- 

 leminal plexus (Fusari and Pancud). 1 



LESSON XXII. 



TOOTH (ESOPHAGUS. 



TOOTH. 



THE chief mass of a tooth consists of dentine. It is capped by 

 enamel, and the root or fang is invested by a layer of bone, the 

 crusta petrosa. All three tissues are calcified, and contain calcic 

 phosphate. The enamel, however, is an epithelial structure, and 

 consists of modified and calcified epithelial cells, while the dentine 

 and crusta petrosa belong to the connective tissue group. 



Unsoftened Tooth. This is one of the few preparations which 

 had better be bought. 



1. Longitudinal Section of a Dry Tooth (fig. 241). 



(a.) (L) Observe the crown and fang, and, connecting the two, 

 the neck. 



(/>.) The dentine surrounding the pulp cavity, the enamel 

 covering the dentine of the crown, and the crusta petrosa or 

 cement covering the dentine of the fang. The wavy black lines in 

 the dentine or dentinal tubules are really tubules filled with air, 

 hence they appear black. Note their direction from the pulp cavity 

 towards the outer margin of the dentine. Quite at the apex of the 

 crown of the tooth they run vertically ; in the fang they run nearly 

 horizontally, and in the part of the dentine intermediate between 

 1 Ardiiv. ital. dc Biol. t xiv. p. 240, 1891. 



