THE CHEMISTRY OF DIGESTION 463 



of the digestive process as it affects the different food-stuffs It 

 will perhaps be helpful to call attention to the comparatively few 

 and simple substances which are finally produced as the result of 

 the numerous reactions that go on in the alimentary tract. All 

 carbohydrates (except the single sugars), the starches, gums, and 

 'double sugars, are hydrolyzed into single sugars during their di- 

 gestion, so that absorption of carbohydrates is altogether in the 

 form of single sugars. All fats are split into fatty acid and glycerin, 

 in which state they are ready to be taken up by the intestinal 

 walls. The proteins, as we saw in Chap. I, are complexes built up 

 of a large number of amino acids. The digestive process splits 

 them into simpler molecules each of which is composed either of a 

 single amino acid, or of two or three of them together. We may 

 say, in general, that proteins are split into their constituent 

 amino acids. 



Tabulating the digestion products we have : 

 from carbohydrates, single sugars; 

 from fats, fatty acid and glycerin; 

 from proteins, amino acids. 



The Saliva. The first digestive fluid that the food meets with 

 is the saliva, which, as found in the mouth, is a mixture of pure 

 saliva, formed in parotid, submaxillary, and sublingual glands, 

 with the mucus secreted by small glands of the buccal mucous 

 membrane. This mixed saliva is a colorless, cloudy, feebly alkaline 

 liquid, " ropy " from the mucin present in it, and usually contain- 

 ing air-bubbles. Pure saliva, as obtained by putting a fine tube in 

 the duct of one of the salivary glands, is more fluid and contains 

 no imprisoned air. 



The uses of the saliva are in part physical and mechanical. It 

 keeps the mouth moist and allows us to speak with comfort; it 

 also dissolves such bodies as salt, and sugar, when they are taken 

 into the mouth in solid form, and enables us to taste them ; undis- 

 solved substances are not tasted, a fact which any one can verify 

 for himself by wiping his tongue dry and placing a fragment of 

 sugar upon it. No sweetness will be felt until a little moisture has 

 exuded and dissolved part of the sugar. 



In addition to such actions the saliva exerts a chemical one on 

 an important food-stuff. It contains an enzym, ptyalin, which 

 has the power of turning starch into a double sugar, maltose. This 



