ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 257 



but the physiological membrane with which we are dealing 

 here need not, and in fact usually does not, coincide with the 

 morphological cell-wall. This is shown particularly well in 

 vegetable cells in which the physiological membrane lies 

 entirely within the morphological membrane. Certain cells 

 do not have any morphological membrane at all, yet a physio- 

 logical membrane permeable to certain substances and im- 

 permeable to others no doubt exists in these cases. This is 

 so, for example, in the red blood-corpuscles, in amoebae, and 

 in the intestinal epithelium. Finally, when we deal with 

 tissues, whole cells arranged in layers may constitute a mem- 

 brane through which diffusion occurs from one medium into 

 another. Under these circumstances the protoplasm of the 

 cell, as a whole, constitutes the membrane. 



What has been said of the cells themselves holds true also 

 fox groupings of these cells as they exist in the various mem- 

 branes of the body such as the absorptive mucous mem- 

 branes of the alimentary and urinary tracts, the synovial 

 membranes, etc. As the individual cells differ from each 

 other in permeability, so do also the various tissues built up 

 of these cells. In dealing with tissues we have yet to take 

 into consideration the permeability of the intercellular sub- 

 stance. This may be entirely different from the permeability 

 of the cells themselves. 



4. The Forces Active in Absorption. If an odorous gas 

 is liberated in one corner of a room, it soon spreads through- 

 out the whole room, so that it may be detected anywhere in 

 it. We say that .the gas diffuses through the room, and, 

 according to the kinetic theory, this diffusion takes place 

 because the molecules of the gas are in constant motion and 

 move in all directions. The odorous gas continues to diffuse, 

 if nothing prevents it, until the concentration of this substance 

 is the same in all portions of the room. 



If a substance (such a copper sulphate) is put into a 

 vessel and a solvent (such as distilled water) is carefully 

 poured upon it, we find that after a while the soluble sub- 



