TARTARIC ACID. 



[ 630 ] 



TEETH. 



TARTARIC ACID. The crystals of this 

 substance, which belong to the oblique pris- 

 matic system, exhibit beautiful colours under 

 the polariscope. 



TAXUS, L. Taxus baccata is the Yew- 

 tree, belonging to the Coniferae. Its wood 

 (PI. 39. fig. 4), as also that of T. canadensis, 

 shows the remarkable combination of spiral 

 fibres with the coniferous pits. Its embryo- 

 logy is also interesting. See CONIFERS 

 and OVULE. 



TAYLORIA, Hook. A genus of Splach- 

 naceae (Acrocarpous operculate Mosses), 

 containing some of Splachna of authors. 



Tayloria serrata, Br. and Sch. y tennis = 

 Splachnum tenue, Dicks. 



TEETH. The teeth of the Mammalia are 

 inserted in sockets or alveolar cavities of the 

 jaws. 



The teeth consist of a crown, or that por- 

 tion which projects beyond the alveolar 

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

 tions which are inserted into the bony struc- 

 tures ; and a neck, or narrower intermediate 

 portion. The crown of the tooth contains 

 the pulp-cavity, which is closed below, but 

 prolonged above through the fangs. 



Fig. 731. 



Molar tooth, human ; longitudinal section, 

 a, enamel ; b, pulp-cavity ; c, cement ; d, ivory, with 

 the ivory-tubes. Magnified 5 diameters. 



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 pa- 

 pillae. 



The substance of human teeth consists of 

 three parts : the ivory or dentine (fig. 731 d}, 

 which constitutes the greater portion of their 

 mass, and to which their form is mainly 

 owing; the cement, or bony portion (fig. 

 731 c), which forms an external covering, 

 principally of the fangs; and the enamel 

 (fig. 731 a), which covers the crown. 



The ivory or dentine (fig. 731 d, 732 d) is 

 whitish and of a silky lustre, and, excepting 



Fig. 732. 



Transverse section of the same, the references as above. 

 Magnified 5 diameters. 



a small portion at the base of the fangs, 

 forms the entire boundary of the cavity of 

 the teeth. It consists of a homogeneous 

 basis enveloping numerous tubes or canali- 

 culi, called the ' ivory-tubes ' (fig. 734 a, b). 

 These are very fine, and pursue an undu- 

 lating course, at first curving, then bifur- 

 cating, throughout giving off numerous fine 

 lateral communicating branches, which are 

 best seen in a horizontal section (fig. 733), and 

 ultimatelyramifyingand anastomosing freely. 

 They commence at the surface of the pulp- 

 cavity, in the crown following a somewhat 

 radiating direction from its centre (fig. 731), 



Fig. 733. 



Transverse section of the ivory- tubes of the fang (a, fig. 

 734), showing their numerous anastomoses. 



Magnified 450 diameters. 



whilst in the fangs their course is more ho- 



