56 APPLIED PHYSIOLOGY 



brown substance called tartar, which may press the gum back from the 

 root of the tooth, until a part of the tooth below the enamel is reached. 

 Then the tooth may decay and break off at the gum, or the gum and 

 bone may be forced back from the crown until the tooth becomes loose 

 and drops out. Thoroughly brushing the teeth twice a day with a 

 tooth brush and water is necessary for preserving the teeth. Particles 



The tongue. 



of food between the teeth should be removed, either by a soft wooden 

 toothpick or else by passing a strong thread between the teeth. Still, 

 with the best of care, some decayed cavities may develop, and these 

 should be filled at once. With this care, almost any set of teeth should 

 last a lifetime. 



The cheeks and lips are thin layers of skin and muscles, 

 which can be moved freely in all directions. 



