DIGESTION OF FOOD IN THE MOUTH 55 



which in animals is the tusk. The next two are larger, 

 with flat surfaces ; they are called bicuspids. The next 

 three, the grinders , or molars, have large, flat surfaces, 

 well adapted to grinding the food. 



In a young child the two bicuspid teeth resemble the molar teeth in 

 the adult, and the three molars are absent. At about the age of six, a 

 whole new set of teeth begins to grow beneath the first teeth, and to 

 press against their roots, cutting off their food supply. The blood 

 takes away the substance of the old teeth as the new ones advance 

 against them, until their projecting parts alone are left attached only by 

 the gum. They finally drop out, while the new ones 

 advance to take their places. The first teeth, like 

 the permanent set, may decay and cause toothache, 

 and should have as good care in filling and cleaning 

 as is given to permanent teeth. 



Sometimes when a baby's gums are being cut 

 through by the growing first teeth, they are tender 



and swollen, making the child fretful. Yet teething 



Section of a tooth, 

 seldom causes sickness in a healthy child. 



a enamel. 



71. Composition of teeth. The teeth b dentine. 



c pulp cavity con- 

 are composed of a very hard kind of bone taming blood 



called dentine, which in some large animals tubes and 



nerves. 



is called ivory. It is nourished by blood d cement, 

 tubes and nerves, which enter at the tip 

 within the jaw and form a pulplike mass in a small cavity 

 in the center of each tooth. The root of the tooth is set 

 into a socket in the jawbone, and a kind of soft bone, 

 called the cement, fixes it in place. The projecting part of 

 each tooth, called the crown, is covered with a hard shell 

 called the enamel. 



72. Care of teeth. When the enamel is too thin, or is worn or 

 broken off, the dentine beneath it may decay. Then the tooth rapidly 

 goes to pieces, often with much pain. Picking the teeth with pins and 

 cracking nuts often break the enamel. Dirt and particles of food be- 

 tween the teeth are great promoters of decay. The saliva deposits a 



