CONVERSION OF FOOD. 223 



and our mouths wider, and for this change God has 

 provided. 



At this time, the second, or permanent, set of 

 teeth pushes forwards, and gradually displaces, the 

 others. This set, when complete, consists of 

 thirty-two teeth, sixteen in each jaw; but they 

 seldom all appear till we are fourteen or fifteen 

 years of age, as there is not room for them before 

 we have reached nearly our full growth. 



That part of a tooth which stands up above the 

 gum is called the crown, and is covered by a very 

 hard ivory-like substance, termed enamel. The 

 roots of the single teeth, and the fangs of the dou- 

 ble ones are hollow, and contain a nerve, artery, 

 and vein. The nerve, when exposed or inflamed, 

 gives rise to that distressing pain, called tooth- 

 ach. 



The food which we eat to supply the waste in 

 our bodies, is first submitted to the action of the 

 teeth. These, by their hardness and sharpness, 

 break it into small fragments, whilst the glands 

 give out saliva in abundance. 



By these means the food is made into a soft pulp, 

 and in this state it passes into the stomach. When 

 it is received there, the vessels of that part throw 

 out a fluid called the gastric juice. This is a very 

 powerful solvent, and by mixing with the food, 

 brings it into a half fluid state, when it is called 

 chyme, and is of a grayish colour. 



This change being finished, the lower opening 

 of the stomach, which had remained fast closed, 



