76 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



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

 nuclei, mucus, &c. 



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

 products of decomposition, stercobilin from the bile pigment.^ 



MECONIUM 



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

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

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

 and biliverdin ; it is not stercobilin. 



ABSORPTION 



Pood 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 

 more rapidly in the stomach : the small intestine with its folds and 

 villi to increase its surface is, however, the great place for absorption ; 

 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 proteid being converted re- 

 spectively into the diffusible materials sugar and peptone. 



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 

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

 through which the absorbed substances pass are living, and in virtue 

 of their vital activity not only select materials for absorption, but also 

 change^ those substances while in contact with them. These cells 

 are of two kinds — (1) the columnar epithelium that covers the 

 surface ; and (2) the lymph cells in the lymphoid tissue beneath. It 

 is now generally accepted that of the two the former, the columnar 

 epithelium, is the more importanu. When these cells are removed or 

 rendered inactive by sodium fluoride, absorption practically ceases, 

 though the opportunities for simple filtration or diffusion would be by 

 such means increased. 



' Stercobilin may originate also from the haematin of the food. (MacMunn.) 



