62 



TEETH. 



grinding points. In the upper jaw they have three divergent roots, 

 two of which are external, and one is internal. In the lower jaw 

 they have but two roots, which are anterior and posterior. The third 

 molar or wisdom-tooth is smaller, and less perfectly developed than 

 the other two, and does not appear until manhood. 



Structure. — Each tooth has three textures, ihe ivmy or bony por- 

 tion, the e^iamel^ and the cementum. The ivory forms the largest 

 portion of the body, neck, and root ; it consists of radiating fibres, 

 the particles of which are not absorbed as in bone. It contains no 

 vessels or nerves, and consists of phosphate of lime, gelatin, and 

 water. The enamel is the hardest portion ; it encrusts the body ; it 

 is white, brittle, and semi-transparent, thicker upon the grinding sur- 

 face, and terminates by a thin edge at the neck. Its structure consists 

 of radiating fibres, and it possesses neither vessels nor nerves. The 

 cementum forms a thin coating over the root of the tooth, from the 

 apex to the enamel. It resembles bone in structure. It increases 

 with the advance in age, making the teeth of old persons appear to 

 project. 



A cavity exists in the roots of the teeth, which is filled by a pulp, 

 principally composed of an artery, vein, and nerve, which enter at 

 the small orifice at the point. 



Fig. 50. 



An infant gum contains the rudiments of fifty-two teeth, in sepa- 

 rate cavities ; twenty of them appear between the sixth month and 

 second year of age, and fall out between the sixth and twelfth year ; 

 they are therefore called the deciduous or milk teeth, of which there 

 are two incisors, one cuspid, and two molars on either side of each 

 jaw. The order of the eruption is irregular, but the average is as 

 follows : seventh month, two middle incisors ; ninth month, two late- 

 ral ; twelfth month, first molar ; eighteenth month, canine ; twenty- 

 fourth, two last molars. The teeth of the lower jaw precede those of 

 the upper by a short interval. 



