ABSORPTION AND THE ROUTES OF FOOD. 353 



sugar, the salt will tend to flow toward the sugar side and 

 the sugar toward the salt side. These currents will con- 

 tinue until finally the composition of both sides is the same. 

 The ease, however, with which substances pass through 

 membranes varies materially and is probably not the same 

 with any two substances. Some, like the soluble mineral 

 salts and sugars, dialyze easily, while others, such as albu- 

 mins, dialyze with great difficulty. But not only do sub- 

 stances in solution pass through the membrane, but the 

 water itself seeps through. Thus, if a solution of salt be 

 placed on one side of an animal membrane and pure water 

 on the other, not only will the salt seep across, but the 

 water from the pure side will flow toward the salt side, and 

 in this way by the dilution of the salt solution an equilibrium 

 is finally established. 



An example of osmosis can be readily illustrated on an 

 ordinary hen's egg. Every one is familiar with the fact 

 that a hen's egg which has been kept a day or two has at 

 its blunt end an air space. This space is enclosed between 

 the two walls of the shell membrane. If, now, the shell 

 and that part of the membrane adhering to it be removed 

 from above the air space, and the egg then partially immersed 

 in water, osmotic currents will set in. Water will tend to 

 flow into the egg and bits of salt and albumin out into the 

 water. But as albumin is practically non-dialyzable, much 

 more water will flow in than albumin out, and so the contents 

 of the egg will increase in size and the partially collapsed 

 shell membrane again be distended as it was in the fresh egg. 

 If these osmotic currents are allowed to continue, the result 

 will soon be that the shell membrane will be burst by the 

 increased pressure caused by the water which the osmotic 

 currents have carried into it. 



When it was just stated that absorption in the alimentary 

 canal has the vitality of the epithelium cells as an import- 

 ant factor, it was not intended to deny the prominent role 

 played by osmotic currents. Thus, a draught of water 

 readily passes into the blood, 110 doubt for the reason that 

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