28 VITALISM AND SCHOLASTICISM 



absorption and utilisation by the body. This 

 substance passes through the cells which line 

 a portion of the alimentary tract, and it was 

 formerly supposed that this passage through the 

 cells was of the nature of a filtration, or at 

 least of an osmotic process such as may take 

 place through a dead animal membrane. 



But more recent experiments have shown 

 that the occurrences which take place cannot 

 be explained in any such mechanical way, but 

 that there is another factor or there are other 

 factors at work which do not come into opera- 

 tion in connection with the process of chemical 

 osmosis. Again, the process of secretion by 

 glands, such as those which produce the saliva, 

 is now known not to be one in which the 

 secreting cells " act in a passive inert manner 

 as filtering mechanisms, or as membranes pos- 

 sessing constant permeabilities for different dis- 

 solved substances in the plasma, or as media 

 in which different substances possess different 

 solubilities." This " is proven by many ex- 

 perimental observations. Thus that the rate 

 of secretion is not merely passively dependent 

 upon blood pressure and blood supply (al- 

 though under normal conditions it is subject 

 to variations corresponding to changes in these 

 physical factors) is shown by the observation 

 of Ludwig that the secretion pressure in the 



