222 THE WONDERFUL HOUSE THAT JACK HAS 



decay. But to understand this, we must first learn 

 some important facts about their structure and history. 



The P art of a tooth outside 

 the gum is called the crown, 



, AU M.U- ' 4.u 



and the part within the gum 



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covering on the outside of 



the crown is called enamel, 



Showing the structure of a tooth ftn( ^ being the hardest Sub- 



stance in the body, is well 



fitted to protect the tooth from injury. Underneath 

 the enamel and forming the principal part of the 

 crown is the dentine, which is a hard substance, but 

 not so hard as enamel. Both the roots and the crown 

 are hollow, the space inside containing the nerve and 

 the blood-vessels. 



Our first or temporary set, twenty in number, are often 

 called milk teeth. Few need to be told that these are 

 followed by a permanent set of thirty-two. The cut 

 on p. 224 shows both milk teeth and the permanent 

 set. The teeth of the lower jaw are similar to those 

 of the upper in both temporary and permanent sets. 



The cut below shows how appropriately the various 

 teeth are named. Incisors mean cutters, and from their 

 shape it can be seen that these four teeth in each jaw 

 are well adapted for cutting. Canine means doglike, 

 and we observe that the two teeth in each jaw bearing 

 that name resemble the teeth of a dog. Next to the 

 canines on each side are the bicuspids. Cusp means 



