3 66 A MANUAL OF PHYSIOLOGY 



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

 many that the processes of diffusion, osmosis and filtration 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 na'ive, 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 phenomena 

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

 broke down, and discarded ' vitalistic 5 hypotheses began once more 

 to arouse attention. Then came the recent 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 confession of 

 ignorance, and does not itself need to be explained. Some physiolo- 

 gists, impressed with the vast progress of physics and chemistry, and 

 especially with the strides that have recently been made in the study 

 of osmosis, believe that as our knowledge of these sciences increases, 

 it will become possible to explain on physical 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. And the one must confess what the other pro- 

 claims, 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 pre- 

 decessor, 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, it is at bottom the work of cells with a 

 selective power which we do not understand, and which is probably 

 peculiar to living structures. Thus, dissolved substances pass with 

 equal ease in either direction through an ordinary diffusion membrane,, 

 but they pass in general, more readily out of the intestine than into- 



