104 



THE HUMAN BODY. 



Crown.. 



Neck.. 



Root., 



Fig. 38. 



21. A tooth is made of a substance like bone, called 

 dentine. At the end of the root, a very small hole can 

 be seen. This is the entrance to a canal 

 which runs through it lengthwise, and 

 contains the tiny nerve and blood-vessel 

 which supply it. The root is fixed in a 

 socket in the jaw-bone. The crown, or 

 body, projects beyond the gums. The 

 neck is at the junction of crown and root. 

 The crown is covered by enamel, which 

 is the hardest substance in the body. 



22, It is the work of the teeth to bite 

 off mouthfuls of food of the right size, 

 INCISOR TOOTH SAWED an( j then to grind them up fine. This 

 is the first act in digestion. When food 

 is well ground, the juices of the stomach easily mix with 

 it, and act on it. If the teeth are gone, or if food is bolted 

 without being chewed, it enters the stomach in lumps, 

 which the juices can not easily pene- 

 trate. When food is not soon acted on 

 by the digestive juices, it becomes sour, 

 and makes gases, which distend the 

 stomach, and often give pain. 



23. The teeth stop growing after 

 they have taken their places; and, if 

 they are chipped, they do not heal. 

 They may be injured, 



i. By very hot or very cold substances. 

 9. By some medicines. 

 5. By decomposing food between them. 

 They should be brushed every day, and cared for by a 

 dentist if unsound. Good health and good looks both 

 depend much on them. 



Enamel. 



Fig. 39. 



MOLAK TOOTH SAWED IN 

 TWO. 



