THE INCISORS. 597 



CHAPTER I. 



THE INCISORS. 



A. Incisors of the First Dentition. 



THESE teeth, also called foetal, milk, deciduous, are twelve in number : six in 

 each jaw, three on each side (Figs. 271 and 272). 



They have received the particular names pincers, intermediates, and corners. 



The pincers are placed almost on the median line, the one to the right, the 

 other to the left ; the intermediates, external to the pincers ; finally, the cor- 

 ners, external to the intermediates. 



Collectively they form a regular parabolic curve. By their free portion they 

 present distinctive characters which depend upon their situation, their width, and 

 their degree of curvature. 



Compared with those of the second dentition, their width is much less, and 

 there exists a well-marked constriction or neck between their free portion or 

 crown, projecting above the gums, and their incased portion, root, embedded in 

 the alveolus. The presence of the neck allows these teeth, in all cases, to be 

 distinguished from the former at a glance (Fig. 273). Finally, their color is of 

 a dull milky-, or yellowish-white ; besides, they are curved in the direction of 

 their length. 



They present for study two faces, two borders, and two extremities. 



Paces. Each of the faces diminishes gradually from the free extremity to 

 the end of the root. 



The anterior, convex in every sense, shows longitudinal striae more or less 

 marked, which consist of small grooves separated from one another by small 

 longitudinal lines in relief. As the animal grows older, this face becomes whiter 

 and more polished under the influence of the friction; a semicircular enlarge- 

 ment of the gums shows its separation from the neck. 



The posterior, concave longitudinally, is slightly convex transversely. It 

 presents nothing of special importance, and the neck is here less apparent. 



Borders. The internal border is thicker than the external. 



Extremities. In the virgin tooth (one that does not show any use) the 

 free extremity (Fig. 274) is flattened from before to behind, and limited by two 

 borders : one anterior, a, the other posterior, b, and, finally, by two sides : one 

 external, the other internal. 



The two borders circumscribe a cavity, the external dental cavity, occupying 

 nearly the whole of the free extremity of the tooth. The anterior, a, is the more 

 elevated and longer from side to side. Sharp and convex transversely, it is the 

 first to pierce the gums ; the posterior, b, penetrates the gums later, and, from the 

 effects of wear, comes on a level with the preceding. 



The external dental cavity, c, which we will examine in detail farther on, 

 diminishes in depth for the same reason, little by little, and finally disappears. 

 The tooth is then levelled. 



As these teeth must be replaced, at a certain epoch of life, by those of the 



