100 DIGESTION 



beneath, and the teeth themselves fall out when there is 

 nothing left to hold them in. In this way, all of the tem- 

 porary teeth are displaced one after another by the perma- 

 nent ones, which consist of four sets of eight teeth each. 

 Each set has two incisors, a canine, two bicuspids and 



FIG. 65. The teetn of the right half of the upper jaw In their sockets, viewed 

 from below. 



three molars. The bicuspids are so named because they 

 each have two prominences or cusps on their crowns. 

 They are used for crushing and also, like the molars, for 

 grinding. The third molar of each of the four sets, 

 known as the wisdom tooth, develops late and is often 

 deficient in strength and endurance. 



Development of teeth. The teeth closely resemble 

 bone and are formed as buds within the jawbones, 

 where their roots permanently remain. They are, how- 

 ever, not true bone, as shown by their structure and 

 method of development. They are to be classed rather 

 with the other specialized outgrowths from the skin, such 

 as the nails and hair. 



Roots. By means of roots, the teeth are rigidly held 

 in the bones of the jaws. The incisors and canines have 

 single roots. Some of the bicuspids have single roots, 



