404 A MANUAL OF PHYSIOLOGY 



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 discovered 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 work 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 pheno- 

 mena of absorption and the powers of cells, these crude physical 

 theories broke down, and discarded ' vitalistic ' hypotheses began 

 once more to arouse attention. Then came the investigations 

 of De Vries, Van '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 explanation of absorption which is more than a con- 

 fession of ignorance, and does not itself need to be explained. 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 alimentary canal. 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, that after all 

 physical explanations have been exhausted, the real secret of the 

 cell will still lurk in some ultimate ' vital ' property of structure or 

 of function, and still elude our search. Both the optimist and the 

 pessimist, the adherent of the physical and the adherent of the 

 vitalistic hypothesis, admit that the phenomena of absorption are 

 essentially connected with the cells that line the alimentary canal. 

 But the one must confess what the other proclaims, that while the 

 processes carried on in these cells are definite, well ordered, and 

 evidently guided by laws, these laws have as yet denied themselves 

 to the modern physiologist, with chemistry in one hand and physics 

 in the other, as they denied themselves to his predecessor, equipped 

 only with his scalpel, his sharp eyes, and his mother-wit. So that 

 in the present state of our knowledge all we can really say is that, 

 while absorption is certainly aided by physical processes, like osmosis 

 and diffusion, possibly by physical processes like imbibition, it is at 

 bottom the work of cells with a selective power which we do not 

 understand. Thus, dissolved substances pass with equal ease in 

 either direction through an ordinary diffusion membrane, but in 

 general they pass, with the water in which they are dissolved, more 

 readily out of the intestine than into it. This normal direction of 

 the stream is still maintained for a considerable time after stoppage 

 of the circulation, provided that the intestine is kept in good con- 

 dition ' by being suspended in well-oxygenated blood. Water or 



