MECHANISM OF ABSORPTION 427 



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 membrane of this tube offers an immense 

 surface, multiplied as it is by the valvulae 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 are 

 taken up from the digestive tube, and what paths do they follow on 

 their way to the tissues ? 



Theories of Absorption. Not so very long ago it was supposed by 

 many that the processes of diffusion, osmosis and nitration offered a 

 tolerably complete explanation of physiological absorption. At that 

 time the dominant note of physiology was an eager appeal to chemistry 

 and physics to ' come over and help it ' ; and as new facts were dis- 

 covered in these sciences they were applied, with a confidence that was 

 almost naive, to the problems of the animal organism. The phenomena 

 of the passage of liquids and dissolved solids through animal membranes, 

 upon which the woik of Graham had cast so much light, seemed to find 

 their parallel in the absorptive processes of the alimentary canal. 

 And when digestion was more deeply studied, facts appeared which 

 seemed to show that its whole drift was to increase the solubility and 

 diffusibility of the constituents of the food. But as time went on, and 

 more was learnt of the phenomena of absorption and the powers of 

 cells, these crude physical theories broke down, and discarded ' vital- 

 istic ' hypotheses began once more to arouse attention. Then came 

 the investigations of De Vries, Yan 'T Hoff, and others in the domain 

 of molecular physics, which gave to our notions of osmosis the precision 

 that was wanted before its relation to many physiological processes 

 could be profitably discussed. At the present time it must be admitted 

 that we possess no full explanation of absorption, none which is much 

 more than a confession of ignorance, and does not itself need to be 

 explained. Yet some progress has been made at least in defining the 

 boundary between what is clearly known and what is still dark, and 

 in showing that familiar physical processes are not without influence. 

 Some physiologists, impressed with the vast progress of physics and 

 chemistry, believe that it will eventually become possible to explain 

 on mechanical and chemical principles all the peculiar phenomena which 

 we observe in the passage of substances through the walls of the ali- 

 mentary canal. As an aid to the framing of practical working hypo- 

 theses this attitude has everything in its favour. Others, taking 

 account of the number and nature of these peculiarities, oppressed with 

 the perennial paradox of vital action, incline to the less sanguine view, 



