ORGANS AND PROCESSES OF DIGESTION 167 



bring about a diseased condition of the liver. It may be 

 noted, however, that the bad effects of alcoholic beverages 

 upon the stomach, the liver, and the body in general are 

 less pronounced when these are taken as a part of the 

 regular meals. 



Effects of Tea and Coffee. In addition to the stimulating 

 agent caffeine, tea and coffee contain a bitter, astringent 

 substance, known as tannin. On account of the tannin 

 these beverages tend to retard digestion and to irritate the 

 lining of the stomach effects that may be largely obviated 

 by methods of preparing tea and coffee which dissolve 

 little of the tannin. (They should be made without con- 

 tinued boiling or steeping.) The caffein may do harm* 

 through its stimulating effect upon the nervous system 

 (page 56) and through the introduction of a special waste 

 into the body. In chemical composition caffeine closely 

 resembles a waste, called uric acid, and in the body is con- 

 verted into this substance. If one is in a weakened condi- 

 tion, the uric acid may fail to be oxidized to urea, as occurs 

 normally, or to be thrown off as uric acid. In this case it 

 accumulates *in the body, causing rheumatism and related 

 diseases. It thus happens that while some people may 

 use tea and coffee without detriment, others are injured 

 by them. 



Summary. The main structure in the digestive system 

 is the alimentary canal. This provides cavities where 

 important dissolving processes take place, and tubes for 

 joining these cavities, while glands connecting with the 

 canal supply the necessary liquids for changing and dissolv- 

 ing the foods. The general plan of digestion is that of pass- 

 ing the food through the canal, beginning with the mouth, and 

 of acting on it at various places, with the final result of re- 

 ducing most of it to the liquid state. The digestive fluids 



