CHAPTER XIV 

 THE ABSORPTION OF THE FOOD-STUFFS 



IF two unlike solutions are separated by a membrane, 

 such as a sheet of parchment or some artificial substitute, 

 they will usually tend to equalize both in composition and 

 concentration. When, for example, potassium chlorid and 

 sodium chlorid solutions are placed on opposite sides of 

 such a partition, each salt proves its ability to pass through 

 the barrier, and in the course of time there will be uniform 

 mixtures in both compartments. This is said to show that 

 the salts are diffusible and that the membrane is permeable. 

 Different membranes are permeable in very different 

 degrees, and the freedom with which various salts pass 

 through a particular membrane is also far from constant. 

 Some substances may appear freely soluble and may go 

 readily through ordinary filters, but may hardly diffuse at 

 all. This, as a rule, is the case with the proteins. If a 

 mixture of unboiled white of egg and sodium chlorid is on 

 one side of a membrane and the other side is washed with 

 running water, nearly all the salt will escape, leaving the 

 protein practically undiminished. The process by which 

 diffusible salts are encouraged to separate themselves from 

 substances which cannot accompany them through the 

 membrane into the water beyond it is called dialysis. 



The products of digestion are, in general, much more 

 diffusible than the food-stuffs from which they are derived. 

 Starch, even when boiled for a long time, does not make 

 its way through ordinary membranes; the sugars do so 

 with relative ease. Fats are not even soluble in water; 

 soaps and glycerin are diffusible compounds. Peptones 

 arising from the hydrolysis of proteins have some power 

 to penetrate membranes, and the simpler amino-acids pass 



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