DIGESTION. 



61 



ing portion of the jaws. The crown of the molars is large, 

 with a broad, uneven surface, intended for grinding the food. 

 The first three kinds of teeth have a single root or fang, 

 but the bicuspids, being marked by a groove on each side, 

 are partially divided into two, hence their name. The molars 

 have two, three, and sometimes four fangs. 



5. Structure of a Tooth. The tooth consists of a hard 

 outer portion, the ivory, and the pulp within. The bulk of 

 the outer portion, situated next the pulp, is called the dentine. 



FIG. 21. Diagram showing how the teeth' fit into each other. 



Over this, on the crown, is the enamel. Covering the dentine 

 of the root is the cement. Chemically, the dentine is like 

 bone tissue, but the tooth is harder than bone. The pulp is 

 composed of connective tissue, with blood-vessels and nerves. 

 These enter the tooth through the extremity of the fang. 



The life of the tooth does not correspond with the life of 

 the body. The permanent set begins to appear, as we. have 

 said, when the child is about six and a half to seven years 

 of age. Prior to this the crown has been formed, and the 



