THE DIGESTIVE AND RESPIRATORY SYSTEM 273 



teeth are inserted by what is termed a thecodont articulation 

 into deep pits in the bone called thccce or alveoli (Fig. 75). 



That portion of the tooth which fits into an alveolus is 

 termed the root and is covered by a sort of bone called cement, 

 while that which appears above the gum is called the crown, 

 and is the only part overlaid by enamel. 



All teeth are hollow and contain a central pulp cavity, which 

 encloses a nutrient corium papilla; in cases where the cusp is a 

 part of the bone the pulp cavity is open along one side to 

 admit the passage of nerves and blood-vessels. In thecodont 

 teeth, however, the latter come up through the bottom of the 

 theca and enter the pulp cavity at the inner end of the root. 



a b c d 



FIG. 75. Types of teeth. 



(a), pleurodont; (b), acrodont; (c) thecodont with open root; (d) thecodont 

 with closed root. 



In this respect there are two kinds of roots, open "and closed, 

 in the first of which the root is widely opened and not dis- 

 tinct in structure from the crown, while in the latter the 

 root is nearly solid and its lumen is restricted to a fine canal 

 through which the vessels may reach the pulp. In both cases 

 layers of new dentine are constantly, though slowly, added to 

 the rest through the agency of a layer of cells called odonto- 

 blasts, which cover the pulp and are firmly applied to the inner 

 walls of the pulp cavity. In the first type, in which the root 

 is widely open and the entire tooth fits like a cap over the 

 pulp, the tooth is gradually pushed upward by the addition 

 of new layers underneath, and is thus continually elongating, 

 while in the second or closed type the addition of new layers 



