344 LIFE : OUTLINES OF GENERAL BIOLOGY 



(bilirubin and biliverdin), which are formed in the liver and else^ 

 where by the breakdown of the red pigment ha?moglobin of the 

 blood. These pigments are probably only waste-products to be cast 

 out. That is also, most probably, the fate of the cholesterol in the 

 bile, for bile is the only fluid of the body in which cholesterol is at 

 all soluble — and even here it tends to form hard, solid masses 

 (gall-stones). Thirdly, the bile contains salts of two complex 

 nitrogenous acids (taurocholic and glycocholic acid). The bile 

 contains no digestive enzymes, but it acts as a natural laxative and 

 disinfectant ; and it aids in a most remarkable degree the action of 

 the pancreatic enzymes, especially of the fat-splitting steapsin. 

 The salts of the bile combine with the fatty acids set free, and thus 

 form compounds which, unlike the fatty acids themselves, can be 

 absorbed by the cells of the intestine and taken into the body. Thus 

 the bile salts are not lost to the body, but go through a regular cycle, 

 passing into the gut only for a time. 



In the intestine the digested food (chyme) receives no further 

 active juices, except small amounts of the enzyme erepsin, which is 

 also present in duodenal secretion, and indeed in almost all cells. 

 This continues the resolution of proteins into amino-acids, and 

 small amounts of enzymes of lesser importance. By this time the 

 carbohydrates are all converted into glucose, the fats into glycerol 

 and fatty acids, and the proteins into amino-acids (with the excep>- 

 tion of such members of these groups as may be indigestible) ; and 

 it is in the small intestine that the absorption of these simplified 

 products takes place. However, the food-materials may yet be 

 further and ver}' considerably altered by the action of the bacteria 

 which are present in enormous and increasing numbers from the 

 duodenum onwards, and above all in the large intestine. The great 

 majority of these are fortunately harmless, and may even do a 

 certain amount of service in digesting such materials as the complex 

 carbohydrate cellulose, which resists all vertebrate digestive juices; 

 but they tend to form large amounts of gases (carbon dioxide, 

 methano, sulphuretted hydrogen), besides evil-smelling (scatol, 

 indol) or even dangerously poisonous (ptomaine) compounds. If 

 their growth is encouraged by irregular habits or unsuitable diet, 

 disease-causing bacteria (of colitis, dysentery, paratyphoid, and 

 so on) may profit by the favourable conditions. The number of 

 these organisms normally present is inconceivably great; five 

 million million may be eliminated daily. 



Relatively little is known in regard to the absorption of substances 

 from the intestine into its lining cells, and eventually into the 

 blood. It is one of the most difficult aspects of the difTicult problem 

 of cell-permeability which we have discussed elsewhere. It will be 

 enough to say that the amino-acids and simple sugars enter the 

 cells and thence pass into the capillaries and are carried to the 



