DIGESTION AND ABSORPTION OF NUTRIENTS 89 



tween the crown and the root is the neck of the tooth, which 

 is surrounded by the gums. 



The internal structure of the tooth is well shown in a verti- 

 cal section (Fig. 30). The covering of enamel is thickest 

 over the top of the crown; it becomes thinner down the 

 exposed sides, and disappears in the neck region. The largest 

 part of the tooth is composed of bony dentine. In the central 

 part is the pulp cavity. This region is well supplied with 

 nerves and blood vessels, which enter through a small open- 

 ing at the end of each root. The blood furnishes the teeth 

 with new building material. 



119. Care of the teeth. — Too much stress cannot be 

 laid on the importance of caring for the teeth, since decay- 

 ing teeth are frequently painful, are always unsightly, are 

 usually the cause of an ill-smelling breath, and often lead to 

 indigestion. Immediately after eating, one should remove 

 any bits of food from between the teeth by using a wooden 

 toothpick, dental floss, or thread. Pins, l^nife-blades, or 

 other metallic implements should never be used for this 

 purpose. The teeth should then be brushed thoroughly 

 on all sides, and warm water and a little castile soap or 

 reliable tooth powder should be used. The sides of the teeth 

 should be brushed from the gums toward the crown in order 

 to avoid pushing the gums away from the neck of the tooth. 



Since the enamel that covers the crown of the tooth is 

 composed entirely of mineral matter, it cannot, of course, 

 decay. If, however, food is allowed to decompose on or 

 between the teeth, the acids formed by the action of the 

 bacteria gradually dissolve the enamel until a cavity is 

 formed. When the dentine is reached, the bacteria directly 

 cause this part of the tooth to decay, since it contains living 

 matter. 



