DIGESTION AND ABSORPTION 1 63 



pounds in order to meet the conditions of a system saturated 

 with water ; second, to convert the miscellaneous food material 

 into certain uniform compounds to meet the conditions of 

 further metabolic change. 



So far as we know the inorganic compounds which serve as 

 food do not require digestive action, so that the process of 

 digestion has to do entirely with the organic constituents. 

 In the following discussion we shall take up each one of the 

 three organic food constituents by itself and consider first the 

 digestion of carbohydrates, next the digestion of proteins and 

 then the digestion of fats. Of course, with a mixed food such 

 as almost all food substances are, separate digestion does not 

 take place, the entire food mass being digested more or less at 

 the same time. However, as we shall find, each constituent 

 is acted upon by certain digestive enzymes which act upon it 

 alone, so that in order to avoid seeming confusion we shall 

 treat them as separate actions. 



Digestive Region. — The digestive region in animals begins 

 with the mouth and continues through the stomach and the 

 small intestine. In man practically no digestive action takes 

 place in the large intestine. The entire digestive tract, or ali- 

 mentary canal, as it is termed, consists of {a) mouth, (b) esoph- 

 agus, (c) stomach, {d) small intestine, (e) large intestine, with 

 several contributory organs which connect with it, e.g. pan- 

 creas and gall bladder. By the time the food mass reaches the 

 large intestine practically all of the food capable of digestion 

 has been digested and absorbed into the circulation through the 

 cell membranes of the digestive tract. 



DIGESTION OF CARBOHYDRATES 



The carbohydrate food materials belong to the three classes 

 of carbohydrates, viz. : 



Monosaccharoses, typified by glucose and fructose sugars; 

 Disaccharoses, typified by cane sugar, milk sugar and malt sugar ; 

 Polysaccharoses not sugars, typified by starch and cellulose. 



