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A YEAR IK" SCIENCE 



pieces. The last molar on each side often appears as 

 late as the twentieth year, sometimes later, and is called 

 a wisdom tooth. 



Eustachian Tube 



Esophagus 



-Trachea 



Fig. 150. Median section through the head showing the relation 

 of the mouth and nasal cavities to the esophagus. 



Structure of tooth. The portion of the tooth 

 embedded in the bone is the root of the tooth ; the 

 exposed portion is the crown. Covering the crown is 

 a layer of enamel, which is the hardest tissue in the 

 body. The root has no enamel, but is covered with a 

 layer of cement. The largest part of the tooth is 

 composed of a bone-like substance called dentine. This 

 surrounds a small cavity, the pulp cavity. Through a 



