ABSORPTION 403 



yet in diarrhoea, whether natural or caused by purgatives, the intes- 

 tines themselves may, instead of absorbing, contribute largely to the 

 excretion of water. Again, although the solids of the excreta are 

 normally given off in far the greatest quantity in the urine and faeces 

 (only part of the latter is truly an excretion, since 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 accident 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, or, at 

 least, any amino-acids, have been formed from the proteins, or 

 any dextrose 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, although the amount 

 of absorption here is quite insignificant, and it is continued with 

 greater rapidity in the stomach. Here a not inconsiderable 

 part of the proteins at least, in the easily digested form of 

 animal food a certain amount of the sugar representing the 

 carbo-hydrates 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. Water is 

 very sparingly taken up by the stomach. It is in the small 

 intestine that absorption reaches its height. The mucous mem- 

 brane of this tube offers an immense surface, multiplied as it is 

 by the valvulse conniventes, and studded with innumerable 

 villi. Here the whole of the fat, much sugar, proteose and 

 peptone, or rather the products of the further action of the 

 ferments of the intestine on these derivatives of the native 

 proteins, 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 



26 2 



