638 THE MECHANISM OF ABSORPTION 



a normal condition. But Cohnheim's experiments do not prove, 

 and we do not know, that this property is peculiar to the living 

 epithelium, and that poisonous solutions would not produce just 

 the same loss of it in a dead epithelium. 



It is necessary to discuss briefly the explanation and signifi- 

 cance of this remarkable one-sided permeability to NaCl. Ham- 

 burger and Hober consider that it has a physical basis, and 

 Cohnheim that it depends upon a vital activity of the cells. 

 Considering our complete ignorance of what the permeability of 

 any membrane to a solution means, it is obvious that it is no 

 more an explanation to say that it is due to a vital activity of 

 the cell than it would be to explain it as a physical peculiarity 

 of the cell. With regard to the significance of the phenomenon, 

 it is unfortunate that the evidence should be conflicting as to 

 whether it is confined to the living or is equally shared by dead 

 epithelium ; for, if it were confined to the living epithelium, we 

 should ascribe it to vital activity. But it must be remem- 

 bered that even if it is confined to the living epithelium, 

 it is not a final proof that it is due to an active thrusting back 

 of NaCl in one direction. Nor does the fact that various ions 

 and drugs can annul the property prove that they do so by 

 inhibiting a vital activity and not by causing a physical altera- 

 tion in the cell. To say, as Cohnheim did, that only living 

 organised material can be poisoned, is in the present state of 

 our knowledge untrue ; for we have seen that the properties of 

 dead materials can be altered by chemical agents. The fact 

 that the decomposition of hydrogen peroxide by platinum black, 

 and the activity of digestive ferments can be "inhibited and 

 stimulated by various ions, would, according to Cohnheim's line 

 of argument, indicate that platinum black and these ferments 

 must possess vital activity. It is true that we know of no 

 physical membrane with such a one-sided permeability, but 

 that does not prove that a cell could not have such a physical 

 constitution as to allow of such a property being exhibited. 

 Instances of other tissues with similar one-sided permeabilities 

 are known. Reid has shown that the permeability to fluid of 

 the living frog's skin is greater from without ^inwards than in 

 the reverse direction ; and that when the skin is dead the per- 

 meability does not become equal in the two directions, but 

 is now greater from w^iurj outwards. Hill and Ridewood have 



