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TEXT-BOOK OF PHYSIOLOGY. 



lated tissue, constituting lymph. At the same time products of tissue metab- 

 olism pass through the capillary walls into the blood. The blood then passes 

 into the venules, which, leaving the villus at its base, unite with the veins of 

 the submucous coat to form the intestinal veins. These finally unite with 

 the gastric and splenic veins to form the portal vein, which enters the liver 

 at the transverse fissure (Fig. 93). The excess of lymph within the villus 

 passes into the club-shaped lymph-capillary, to be finally carried by the 

 lymph-vessels of the mesentery into the thoracic duct. During the intervals 

 of digestion and in the absence of food from the intestine there is, of course, 



no absorption of food nor the re- 

 moval from the villus of anything but 

 the excess of lymph and metabolic 

 products. 



Function of the Villi. The villi, 

 and especially the epithelial cells cover- 

 ing them, are the essential agents in 

 the absorption of the products of 

 digestion. It is by the activity of 

 these cells that the new materials are 

 taken out of the alimentary canal and 

 transferred into the lymph-spaces in 

 the interior of the villi, from which 

 they are subsequently removed by 

 the blood-vessels and lymph-vessels. 

 As to the mechanism by which the 

 epithelial cells accomplish this result, 

 nothing definite can be asserted. In- 

 asmuch as the absorption of food does 

 not take place in accordance with the 

 laws of osmosis as at present under- 

 stood, it has been suggested that the 

 cells possess a "selective action" de- 

 pendent on their organization and 

 physiologic activity, an activity which 

 is to a great extent conditioned and 

 limited by the degree of diffusibility of the substances to be absorbed. 



Absorption of Water and Inorganic Salts. Water and inorganic salts 

 after their absorption from the intestine and transference into the lymph- 

 spaces of the villi pass through the walls of the capillary blood-vessels and 

 are carried, by the blood of the portal vein, into and through the liver into 

 the blood of the general circulation. Unless water be present in excessive 

 amounts, there is no appreciable absorption of water by the lymph-vessels. 

 Absorption of Sugar. As previously stated, all the carbohydrates, with 

 the exception possibly of lactose, are transformed by the digestive fluids into 

 either dextrose or levulose, under which forms they are absorbed by the 

 epithelial cells. It is possible, however that soluble dextrin may also be 

 absorbed. Whatever the form under which the carbohydrates are absorbed, 

 they never leave the epithelial cells except as dextrose and levulose. Direct 

 experimentation has shown that the sugars are taken up by the capillary 



FIG. 93'. DIAGRAM OF THE PORTAL 



VEIN (pv) ARISING IN THE ALIMENTARY 



TRACT AND SPLEEN (s), AND CARRYING THE 

 BLOOD FROM THESE ORGANS TO THE LIVER. 

 (Yea's "Text-book of Physiology."} 



