64 STRUCTURE OF TEETH. 



nine, and eight molars. The temporary molars have four tubercles, 

 and are succeeded by the permanent bicuspides, which have only two 

 tubercles. 



Each tooth is divisible into a croum, which is the part apparent 

 above the gum ; a constricted portion around the base of the crown, 

 the neck; and a root or fang, which is contained within the alveolus. 

 The root is invested by periosteum, which lines the alveolus, and is 

 then reflected upon the root of the tooth as far as its neck. 



The incisor teeth (cutting teeth) are named from presenting a sharp 

 and cutting edge, formed at the expense of the posterior surface. The 

 crown is flattened from before backwards, being somewhat convex in 

 front and concave behind ; the neck is considerably constricted, and 

 the root compressed from side to side ; at its apex is a small opening 

 for the passage of the nerve and artery of the tooth. 



The canine teeth (cuspidati) follow the incisors in order from before 

 backwards; two are situated in the upper jaw, one on each side, and 

 two in the lower. The crown is larger than that of the incisors, con- 

 vex before and concave behind, and tapering to a blunted point. 

 The root is longer than that of all the other teeth, compressed at each 

 side, and marked by a slight groove. 



The bicuspid teeth (bicuspidati, small molars), two on each side in 

 each jaw, follow the canine, and are intermediate in size between 

 them and the molars. The crown is compressed from before back- 

 wards, and surmounted by two tubercles, one internal, the other ex- 

 ternal ; the neck is oval ; the root compressed, marked on each side 

 by a deep groove, and bifid near its apex. The teeth of the upper 

 jaw have a greater tendency to the division of their roots than those 

 of the lower, and the posterior than the anterior pair. 



The molar teeth (multicuspidati, grinders), three on each side in 

 each jaw, are the largest of the pennanent set. The crown is quadri- 

 lateral, and surmounted by four tubercles, the neck large and round, 

 and the root divided into several fangs. In the upper jaw the first 

 and second molar teeth have three roots, sometimes four, which are 

 more or less widely separated from each other, two of the roots being 

 external, the other internal. In the lower there are but two roots, 

 which are anterior and posterior ; they are flattened from behind for- 

 wards, and grooved so as to mark a tendency to division. The third 

 molars, or dentes sapientiae, are smaller than the other two ; they pre- 

 sent three tubercles on the surface of the crown ; and the root is single 

 and grooved, appearing to be made up of four or five fangs compressed 

 together, or partially divided. In the lower jaw the fangs are fre- 

 quently separated to some distance from each other, and much curved, 

 so as to offer considerable resistance in the operation of extraction.* 



Structure. The base of the crown of each tooth is hollowed in its 

 interior into a small cavity which is continuous with a canal passing 

 through the middle of each fang. The cavity and canal, or canals, 



* See an excellent practical work, " On the Structure. Economy, and Pa- 

 thology of the Teeth," by Mr. Lintott. 



