TEETH I3Q 



but not our species, others quite outside the genus, but still within the 

 family. 



TEETH OF MAN 



Human teeth are in structure substantially like those of most other 

 mammals, and very like indeed to those of other primates. 



In each tooth three parts are distinguishable, an external enamel- 

 capped crown, a root buried in a bony socket or alveolus, and a neck 

 or constricted region between root and crown. The number of cusps or 

 tubercles on the crown varies in the different teeth. The incisor and 

 canine teeth have a single cusp, the premolars have two, and hence are 

 known as bicuspids, and the molar teeth may have as many as five. The 

 number of roots also varies in the different teeth. Incisors, canines, and 

 premolars have but one, although the roots of the premolars are some- 

 times divided into two. The lower molars have two roots, and the upper 

 molars three. 



The finer structure of a tooth may be best seen in a thin longitudinal 

 section. See Fig. 127. The central portion, the pulp cavity, is filled 

 with connective tissue containing blood capillaries and nerve fibers, which 

 enter the tooth through a minute foramen at the end of the root. The 

 larger mass of the tooth is formed by a bone-like substance, the dentine 

 or ivory. Unlike bone, however, dentine is devoid of cells. In section, 

 the dentine takes on a somewhat fibrous appearance from the presence 

 of parallel tubes, the dental canaliculi, which radiate from the pulp 

 cavity through the dentine. At their peripheral terminations in the 

 dentine, the canaliculi branch profusely. The sensitivity of the dentine 

 to the dentist's drill is probably due to the living protoplasm in these 

 canaliculi, which acts in the manner of nerve fibers. The larger part 

 of the dentine, approximately 75%, consists of inorganic mineral salts 

 such as calcium phosphate and calcium carbonate. The remaining 25% 

 is organic material. At no place on the tooth does the dentine reach the 

 surface, since the crown and neck are covered with enamel, while the 

 root is surrounded by a heavy cement. 



Enamel is the hardest substance in the human body, since it contains 

 only three and a half per cent of organic substance. It is thickest at. the 

 apex of the crown, and thins out towards the neck and root. High 

 magnification shows that the enamel consists of minute parallel hexagonal 

 prisms which rest on the dentine and extend to the outer surface of the 

 crown. Increase in the amount of enamel toward the outside of the 

 crown is effected by means of increase in the number of enamel prisms 

 and not by their enlargement or branching. In this way, the solidity 

 of the enamel is maintained throughout the crown of the tooth. The 

 mineral constituents of enamel are identical with those of dentine. 



