ABSORPTION. 221 



to be emptied into the subclavian vein in the neck, hence do 

 not directly pass through the liver. 



If a solution of salt and one of sugar are brought into con- 

 tact, they will gradually mix ; and this is called Diffusion. If, 

 now, a piece of parchment be tightly tied over the end of a 

 tube, and one of the above solutions be placed inside, and the 

 other around the tube, they will still diffuse through the mem- 

 brane, and mingle. This is called Osmosis, or Dialysis ; and the 

 parchment is called a dialyzing membrane. In the digestive 

 tube the mucous membrane represents the dialyzing membrane, 

 with blood or lymph on one side, and the contents of the 

 digestive tube on the other. Soluble materials, such as pep- 

 tones, sugars, etc., pass through the mucous membrane into 

 the blood. 



" The process of osmosis, and to a lesser extent of filtration 

 and imbibition, as they are known to occur outside the body, 

 were supposed to account for the absorption of all the soluble 

 products. This belief has now given way, in large part, to 

 newer views, according to which the living epithelial cells 

 take an active part in absorption, acting under laws peculiar 

 to them as living substances, and different from the laws of 

 diffusion, filtration, etc., established for dead membranes. 



"Unlike sugars and peptones, fats are absorbed chiefly in 

 a solid form that is, in an emulsified condition. There can 

 be no question, in this case, of osmosis. It has been shown 

 by .nearly all recent work that the immediate agents in the 

 absorption of fats are again the epithelial cells of the villi of 

 the small intestine. The fat-droplets are taken up by these 

 cells, arid can be seen microscopically after digestion in the 

 act of passing, or rather of being passed, through the cell- 

 substance. The epithelial cells, in other words, ingest the 

 fat-particles lying against their free ends, and then pass 

 them slowly through their cytoplasm into the substance of 

 the villus." HOWELL. 



