TEETH 193 



ning of human evolution. Along with these, has gone a change in the 

 direction of the incisors, correlated with the appearance of a chin. In 

 the apes, the incisors protrude, in man, they stand upright. Incidentally, 

 the human bite becomes horseshoe-shaped, with the rows of cheek teeth 

 no longer parallel, as in all lower forms, even the apes. In addition, the 

 triangular upper molars of the apes, with three cusps, become in man 

 quadrangular with four, and, correlated with the reduced size of the single 

 teeth, their pulp cavities become relatively still farther reduced, not to 

 sacrifice unduly the thickness of the tooth wall. x\pe teeth are, then, 

 taurodont, so that the X-ray sometimes identifies a fossil human tooth 

 when other tests fail. 



All these differences between apes and men are, however, bridged by 

 various fossil creatures, on the whole human, some of the genus Homo 

 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. 146). 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 prismatic 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% 



