1 82 PHYSIOLOGY. 



called villi (singular, villus). The villi greatly increase 

 the absorbing surface of the small intestine. In each 

 villus is a network of blood capillaries, and the beginning 

 of lymphatic capillaries called lacteals. 



Routes of Different Foods after Absorption. In the 



villi the largest part of the work of absorption is done. 

 The fats are absorbed by the lymph capillaries, or lacteals, 

 and the rest of the foods by the blood capillaries. It 

 should be carefully noted that nearly all of the foods but 

 the fats go at once to the liver, through the portal vein ; 

 but the fats are carried by the main lymph duct (the 

 thoracic duct) to be emptied into the subclavian vein in 

 the neck ; hence do not directly pass through the liver. 



Diffusion, Osmosis, and Dialysis. If a solution of salt and one 

 of sugar are brought into contact, they will gradually mix by diffusion. 

 If these two solutions are separated by parchment, they will still diffuse 

 through the membrane and mingle. This is osmosis. Since substances 

 differ in the readiness with which they pass through a membrane, they 

 may be thus separated. Such separation is dialysis, and the membrane 

 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 peptones, sugars, etc., pass through the mucous 

 membrane into the blood. 



Absorption a Vital Process. "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. 



