300 



STRUCTURE OF THE TEETH. 



mastication is regulated by the sensibility of the teeth, and the muscular 

 sensibility of the muscles of mastication, as well as by the general 

 sensibility of the mucous membrane of the mouth and lips. At the 

 same time, the mass is mixed with saliva, the divided particles cohere, 

 and are formed into a mass or bolus of a long, oval shape by the 

 muscles of the tongue. The bolus then rests on the back of the tongue, 

 ready to be swallowed. 



Nerves Of Mastication. The muscles of mastication and the buccinator 

 receive their motor nerves from the third branch of the trigeminus; the mylo- 

 hyoid and the anterior belly of the digastric being supplied from the same source. 

 The genio-, omo-, and sterno-hyoid, sterno-thyroid, and thyro-hyoid are supplied 

 by the hypoglossal, while the facial supplies the posterior belly of the digastric, 

 the stylo-hyoid, the platysma, and the muscles of the lips. The general centre for 

 the muscles of mastication lies in the medulla oblongata. 



When the mouth is closed, the jaws are kept in contact by the pressure of the 

 air, as the cavity of the mouth is rendered free from air, and the entrance of air is 

 prevented anteriorly by the lips, and posteriorly by the soft palate. The pressure 

 exerted by the air is from 2-4 mm. Hg. (Metzger and Bonders). 



154. Structure and Development of the Teeth. 



A tooth is just a papilla of the mucous membrane of the gum, which has under- 

 gone a characteristic development. In its simplest form, as in the teeth of the 

 lamprey, the connective-tissue basis of the papilla is covered with many layers of 

 corneous epithelium. In human teeth, part of the papilla is transformed into 

 a layer of calcified dentine, while the epithelium of the papilla produces the 

 enamel, the fang of the tooth being covered by a thin accessory layer of bone, the 

 crusta petrosa or cement. 



The dentine or ivory which surrounds the pulp cavity and the canal of the 

 fang (Fig. 122) is very firm, elastic, and brittle. Like the matrix of bone, dentine, 



when treated in a certain 

 way, presents a fibrillar 

 structure (v. Ebner). It 

 is permeated by innumer- 

 able long, tortuous, wavy 

 tubes the dentinaltuluks 

 (Leeuwenhoek, 1678) 

 each of which communi- 

 cates with the pulp cavity 

 by means of a fine opening, 

 and passes more or less 

 horizontally outwards as 

 far as the outer layers of 

 the dentine. The tubules 

 are bounded by an ex- 

 tremely resistant, thin, 



Fig. 123. 



Transverse section of dentine 

 The light rings are the walls 

 of the dentinal tubules ; the 

 dark centres with the light 

 points are the fibres of 

 Tomes lying in the tubules. 



soft fibres, the "fibres of Tomes" (1840), which are merely greatly elongated and 

 branched processes of the odontoblasts of the pulp (Waldeyer, 1865). 



Fig. 122. 



Vertical section of a 



tooth p, pulp 



cavity; d, dentine; 



c, cement; s, enamel. 



cuticular membrane, 

 which strongly resists 

 the action of chemical 

 reagents. These tubules 

 are filled completely by 



