ABSORPTION 363 



much of the faeces of a mixed diet has never been physiologically 

 inside the body at all), yet salts and traces of urea are constantly 

 found in the sweat, and salts and mucin in the excretions of the 

 respiratory tract. Further, although the solids and liquids of the food 

 are usually taken in by the alimentary mucous surface, it is possible 

 to cause substances of both kinds to pass in through the skin ; and 

 a certain amount of absorption may also take place through the urinary 

 bladder. So that really it may be considered, from a physiological 

 point of view, as more or less an ac/ident that a man should absorb 

 his food by dipping the villi of his intestine into a digested mass, 

 rather than by dipping his fingers into properly prepared solutions, 

 as a plant dips its roots among the liquids and solids of the soil ; or 

 that he should draw air into organs lying well in the interior of his 

 thorax, instead of letting it play over special thin and highly vascular 

 portions of his skin ; or that the surface by which he excretes urea 

 should be buried in his loins, instead of lying free upon his back. 



It has been already explained that, although digestion is 

 a necessary preliminary to the absorption of most of the 

 solids of the food, we are not to suppose that all the food 

 must be digested before any of it begins to be absorbed. On 

 the contrary, the two processes go on together. As soon as 

 any peptone has been formed from the proteids, or sugar 

 from the starch, they begin to pass out of the alimentary 

 canal ; and by the time digestion is over, absorption is well 

 advanced. 



Even in the mouth it has already begun, and it is con- 

 tinued with far greater rapidity in the stomach. Here 

 peptones, sugar, and diffusible substances like alcohol, and 

 the extractives of meat, which form an important part of 

 most thin soups and of beef-tea, are undoubtedly absorbed. 

 But it is in the small intestine that absorption reaches its 

 height. The mucous membrane of this tube offers an 

 immense surface, multiplied as it is by the valvulae conni- 

 ventes, and studded with innumerable villi. Here the whole 

 of the fat, much sugar, proteose and peptone, certain products 

 of the further action of the unformed and formed ferments of 

 the intestine on the food, and certain constituents of the bile 

 are taken in. In the large intestine, as has been already 

 said, water and soluble salts are chiefly absorbed. 



What now is the mechanism by which these various 

 products are taken up from the digestive tube, and what 

 paths do they follow on their way to the tissues ? 



