THE TEETH 



365 



each tube with Fehling's solution. In which tube or tubes has 

 digestion taken place? 



Take two test tubes, place in each tube an equal amount of starch 

 paste and saliva. Place one tube in warm water, the other in the ice- 

 box. Test each for grape sugar after one hour. In which tube do you 

 find sugar? The above experiments show that the enzymes in the 

 saliva under certain conditions change starches to sugars. You will 

 remember that starch in the growing corn grain was changed to grape 

 sugar by an enzyme called 

 diastase. In saliva a similar 

 action is caused by an 

 enzyme called ptyalin (tl'a- 

 lin), or salivary amylase; but 

 this enzyme acts only in an 

 alkaline medium at about 

 the temperature of the body. 



The teeth. The teeth of 

 man, unlike those of the 

 frog, are used in the me- 

 chanical preparation of the 

 food for digestion. Instead 

 of holding prey, they crush, 

 grind, or tear food so that 

 more of its surface may be 

 exposed to the action of 

 the digestive fluids. The 

 first or " milk " teeth of 

 man are only twenty in 

 number, while the second 

 or " permanent " set contain thirty-two teeth. These teeth are 

 divided, according to their structure, into four groups ; these are 

 the incisors, or cutting teeth; the canines; the premolars; and 

 the flat-top molars, or grinding teeth. 



Each tooth, as the figure shows, is composed chiefly of hard 

 bone or dentine. The crown of the tooth is covered with enamel, 

 the hardest substance in the body. In the interior is a pulp 

 cavity, which during the life of the tooth contains nerves and blood 

 vessels which give the tooth its nourishment. The tooth is held 

 in its bony socket in the jaw by cement. 



When a tooth dies, bacteria often set up an irritation at its base 



Draw this diagram in your workbook and indi- 

 cate the four kinds of teeth in their place in the 

 jaw. 



