TEETH. 



[ '^->^' ] 



TEETH. 



with hot water. Tho 'facing' of the various 

 kinds is mostly distinguishable with a com- 

 mon lens, and when tlie tea is infused forms 

 a sediment, the characters of which may be 

 determined by tho microscope and by che- 

 mical analysis. 



BiBL. liassall, J(w7i^-c.,L'G8; Warington, 

 Tr. Chem. Soc. 1851. 



TEETH.— The teeth of the Mammalia 

 are inserted in sockets or alveolar cavities 

 of the jaws. 



The teeth consist of : — a crown, or that 

 portion which projects beyond the alveolar 

 cavity and the gum ; the things, or the por- 

 tions which are inserted into the bony 

 structures ; and a neck, or narrower inter- 

 mediate portion. The crown of the tooth 

 contains the pulp-cavity, which is closed 

 above, but prolonged laelow through the 

 fangs. 



In regard to their structure, teeth are in 

 part identical with bone, in part closely 

 allied to it ; but in respect to their develop- 

 ment, they must be regarded as formations of 

 the mucous membrane, as modified papillae. 



The substance of human teeth consists of 

 three parts : — the ivory or dentine (fig. 726 d), 



Fig. 726. 



Molar tooth, human : 



longitudinal section, 

 ivory, 



with 



a, enamel ; 6, pulp-cavity ; c, cement ; d, 

 the ivory -tubes. Magnified 5 diameters. 



which constitutes the greater portion of 

 their mass, and to which their form is 



mainly owing ; the cement, or bony portion 

 (tig. 726 c), which forms an external cov(>r- 

 ing, principally of the fangs ; and tho ena- 

 mel (fig. 726 a), which covers the crown. 



The ivon/ or denfitic (figs. 726 d, 727 d) is 

 whitish and of a silky lustre, and, excepting 

 a small portion at the base of the fangs, 

 forms the entire boundary of the cavity of 

 the teeth. It consists of a homogeneous 



Transverse section of the same; the references as above. 

 Magnified 5 diameters. 



matrix enveloping numerous tubes or cana- 

 liculi, called the ivory-tubes (tig. 729 a, b). 

 The tubes are very fine, and pursue an un- 

 dulating course, at first curving, then bifur- 

 cating, throughout gix-ing off" numerous fine 

 lateral communicating branches, which are 

 best seen in a horizontal section (fig. 728), 

 and ultimately ramifying and anastomosing 

 freely. They commence at the surface of 

 the pulp-cavity, in the crown following a 

 somewhat radiating direction from its centre 

 (fig. 726), whilst in the fangs their course 

 is more horizontal. They have distinct 



Fig. 728. 



Transverse section of the ivory-tubes of the fang (a), 

 fig. 729), showing their numerous anastomoses. 

 Magnified 450 diameters. 



walls, about equal in thickness to their 

 calibre, although in transverse sections (fig. 

 730) this thickness is generally exaggerated, 

 on account of their being obliquely divided. 

 They contain air in the dry state, which 



3c 



