THE DIGESTIVE JUICES 95 



the action of the digestive juices. On a moderate diet unaltered 

 protein is never found. 



3. Indigestible constituents of the food : cellulose, keratin, mucin, 

 chlorophyll, gums, resins, cholesterin. 



4. Constituents digestible with difficulty : uncooked starch, ten- 

 dons, elastin, various phosphates, and other salts of the alkaline earths. 



5. Products of decomposition of the food : indole, skatole, phenol, 

 acids such as fatty acids, lactic acid, &c. ; haematin from haemoglobin ; 

 insoluble soaps like those of calcium and magnesium. 



6. Bacteria of all sorts and debris from the intestinal wall ; cells, 

 nuclei, mucus, &c. This forms a very large contribution. 



7. Bile residues : mucus, cholesterin, traces of bile acids and their 

 products of decomposition, stercobilin from the bile pigment. 1 



MECONIUM 



Meconium is the name given to the greenish-black contents of the 

 intestine of new-born children. It is chiefly concentrated bile, with 

 debris from the intestinal wall. The pigment is a mixture of bilirubin 

 and biliverdin ; it is not stercobilin. 



ABSORPTION 



Food is digested in order that it may be absorbed. It is absorbed 

 in order that it may be assimilated that is, become an integral part 

 of the living material of the body. 



Having now considered the action of digestive juices, we can study 

 the absorption which follows. In the mouth and oesophagus the 

 thickness of the epithelium and the quick passage of the food through 

 these parts reduce absorption to a minimum. Absorption takes place 

 to a small extent in the stomach ; the small intestine, with its folds 

 and villi to increase its surface, is, however, the great place for absorp- 

 tion ; and, although the villi are absent from the large intestine, 

 absorption occurs there also, but to a less extent. 



Foods such as water and soluble salts like sodium chloride are 

 absorbed unchanged. The organic foods, however, are considerably 

 changed, colloid materials like starch and protein being converted re- 

 spectively into the diffusible materials sugar and amino-acids. 



There are two channels of absorption, the blood vessels (portal 

 capillaries) and the lymphatic vessels or lacteals. 



Absorption, however, is no mere physical process of diffusion and 

 nitration. We must also take into account the fact that the cells 



1 Stercobilin may originate also from the heematin of the food. (MacMunn.) 



