220 TEXT-BOOK OF PHYSIOLOGY 



asmuch as the absorption of food does not take place in accordance with 

 the laws of osmosis as at present understood, it has been suggested that 

 the cells possess a "selective action" dependent 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 across 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 

 blood-vessels and carried direct to the liver. Analysis of the blood of the 

 portal vein after the ingestion of large quantities of sugar may reveal an 

 increase to 0.25 per cent.; while after the injection of sugar into the intestine 

 the percentage may rise as high as 0.4 per cent. As chemic analysis of 

 lymph obtained from the thoracic duct shows no increase in the percentage 

 of sugar beyond that normally present (o.i per cent.), it is assumed that 

 sugar is not removed from the villi by the lymph- vessels. 



On reaching the liver a portion of the sugar 12 to 20 per cent, passes from 

 the blood stream across the walls of the capillaries into surrounding lymph 

 spaces and comes into direct relation with the liver cells. Then through the 

 agency of an enzyme, the sugar is dehydrated, converted into starch and 

 stored for a variable length of time in the liver cells in the form of hya ine 

 masses which can be readily seen with the aid of the microscope. Under 

 this form the carbohydrate material is retained until the necessity arises 

 for its return to the blood, and this happens, when the percentage of sugar in 

 the blood falls below the normal, viz.: o.io to 0.15 per cent. Under such 

 circumstances the necessary amount of the liver starch is hydrated, con- 

 verted linto sugar, and passed into the blood in quantities sufficient to restore 

 the normal percentage. The apparent necessity for this temporary storage 

 of sugar in the liver is to prevent its too rapid entrance into the arterial 

 blood and hence a rise in the percentage far beyond that which is normal. 

 Should this occur a condition known as hyperglycemia would result and 

 as a consequence an elimination of the excess by the kidneys giving rise to 

 the condition known as glycosuria. 



Absorption of the Products of Protein Digestion. For the reason 

 that the proteins are for the most part transformed through hydration and 

 cleavage by the action of the gastric and pancreatic enzymes into peptones 

 and for the further reason that the peptones are diffusible bodies, it was 

 formerly believed .that they represented the final stages in the digestion of 

 the proteins, and as such were absorbed out of the intestinal contents by 

 the action of the epithelium covering the villi. Inasmuch as chemic analysis 

 failed to detect more than a trace of either peptone or native protein in the 



