THE TEETH. 311 



crowns are somewhat cuboidal. Each has two cusps, an 

 outer towards the cheek, and an inner on the side turned 

 towards the interior of the mouth. The fang is compressed 

 laterally, and has usually a groove partially subdividing it 



FIG. 91. FIG. 92. 



FIG. 90. An incisor tooth. / 



FIG. 91. A canine or eye tooth. 



FIG. 92. A bicuspid tooth seen from its outer side; the inner cusp is, accord- 

 ingly, not visible. 

 FIG. 93. A molar tooth. 



into two. At its tip the separation is often complete. 

 The molar teeth or grinders (Fig. 93) have large crowns 

 with broad surfaces, on which are four or five projecting 

 tubercles, which roughen them and make them better adapt- 

 ed to crush the food. Each has usually several fangs. The 

 milk teeth 'only differ in subsidiary points from those of 

 the same names in the permanent set. 



The Structure of a Tooth. If a tooth be broken open 

 a cavity extending through both crown and fang will be 

 found in it. This is filled during life with a soft vascular 

 pulp, and hence is known as the "pulp cavity" (c, Fig. 

 94). The hard parts of the tooth disposed around the pulp 

 cavity consist of three different tissues. Of these one im- 

 mediately surrounds the cavity and makes up most of the 

 bulk of the tooth; it is dentine (2, Fig. 94); covering the 

 dentine on the crown is the enamel (1, Fig. 94) and, on the 

 fang, the cement (3, Fig. 94). 



The pulp cavity opens below by a narrow aperture at the 

 tip of the fang, or at the tip of each if the tooth has more 

 than one. The pulp consists mainly of connective tissue, 

 but its surface next the dentine is covered by a layer of 



