72 PHYSIOLOGY AND TEMPERANCE. 



organs. Iron, also, is a constituent of the blood, but it can 

 only become so through the digestive organs. 



Any one of these kinds of food is not alone sufficient to 

 supply the wants of the body. Nitrogenous foods have all 

 the elements necessary for nutrition, but not all of them in 

 sufficient quantities for ec r .iomical living, while their exclu- 

 sive use would, in time, overtax the digestive system. The 

 same lack of economy and overwork of the digestive system 

 would be evident if we confined ourselves to fats, starch, or 

 sugar as a diet. Too much of one kind will not make up for 

 too little of another. 



21. The Digestive System a Complete Workshop. 



The process of digestion is carried on in a most perfect and 

 fully equipped workshop. It commences the moment food 

 enters the mouth. While being made ready to swallow, the 

 change is started by the saliva, an alkaline fluid, containing 

 a ferment (diastase) which converts the starchy parts of food 

 into a kind of sugar. Starch is insoluble, while sugar is freely 

 dissolved. Eating is, therefore, not a mere grinding of the 

 food until, with a mouthful of tea or other drink, it can be 

 swallowed. It is the first step in digestion, and in order that 

 the saliva may do its work properly, the food should be well 

 ground and thoroughly moistened and softened by the saliva 

 only. The necessity for this is more plain when we know the 

 saliva can only act upon the starch in a mixture which is 

 slightly alkaline, and that as soon as swallowed the food 

 meets with a secretion in the stomach which is acid. 



22. Stomach Digestion. The work begun by the saliva 

 is not completed until the food reaches the intestine. The 

 gastric juice has little effect on the starchy matters. In the 

 stomach the albumens, such as are contained in meat, eggs, 

 cheese, bread, etc., are acted upon. Fats are not affected by 

 the gastric juice, although the cells are dissolved and the oil 

 set free. The free acid in the gastric juice keeps the food 



