102 THE HUMAN BODY. 



action. When it gets below the throat, the muscles are 

 still contracting around it and behind it, pushing it on, as 

 we strip water from a soft tube by drawing it between our 

 ringers. But these are involuntary muscles, and we are 

 not conscious of their action. While it remains in the 

 stomach, the muscles are constantly "working" it, very 

 much as a baker works his bread to mix the yeast with 

 it. The same processes continue through the small and 

 the large intestine. 



If the juices are too scanty, or poor in quality, diges- 

 tion does not go on well. If the muscle of the walls of 

 the canal is weak or sluggish, digestion does not go on 

 well. These conditions are called dyspepsia. 



16, AVhat has been said of the changes in the food, that 

 constitute digestion, may be summed up as follows: TJie 

 fats are made into a fine emulsion. The other kinds of 

 food are changed into substances that easily dissolve in 

 the fluids of the canal. 



THE TEETH. 



17, Let us examine more particularly the different 

 parts of the digestive apparatus. Just behind the lips, the 

 outer gates of the alimentary canal, stand the inner gates, 

 the teeth. A child of about five years of age, who has not 

 yet lost any of his first teeth, has twenty in all, ten in each 

 jaw. If we could look deep into his jaw-bones, we should 

 see, beneath these twenty, twenty-eight more, buds of 

 teeth, so to speak, which are his second set (all but four), 

 almost ready to grow out. So that, at that age, a child 

 has really forty-eight teeth, more than at any other time 

 in his life. 



18, The first set are, 



