DIGESTION AND ABSORPTION IN THE INTESTINES. 807 



may enter the lacteals of the villi, pass into the lymph circulation, 

 and through the thoracic duct of the lymphatic system eventually 

 reach the blood vascular system. The older physiologists assumed 

 that absorption takes place exclusively through the central lacteals 

 of the villi, and hence these vessels were described as the absorb- 

 ents. We now know that the digested and resynthesized fats are 

 absorbed by way of the lacteals, but that the other products of di- 

 gestion are absorbed mainly through the blood-vessels, and therefore 

 enter the portal system and pass through the liver before reaching 

 the general circulation. According to observations made upon a pa- 

 tient with a fistula at the end of the small intestine,* food begins to 

 pass into the large intestine in from two to five and a quarter hours 

 after eating, and it requires nine or more hours before the last of a 

 meal has passed the ileocecal valve ; this estimate includes, of course, 

 the time in the stomach. During this passage absorption of the 

 digested products takes place nearly completely. In the fistula case 

 referred to above it was found that 85 per cent, of the protein had 

 disappeared, and similar facts are known regarding the other food- 

 stuffs. The problems that have excited the greatest interest have 

 been, first, the exact form in which the digested products are ab- 

 sorbed, and, second, the means by which this absorption is effected. 

 With regard to the last question, much work has been done to 

 ascertain whether the known physical laws of diffusion, osmosis, 

 and imbibition are sufficient to account for the movements of the 

 absorbed substances or whether it is necessary to refer them in 

 part to some unknown activities of the living epithelial cells. It 

 would seem that diffusion and osmosis occur in the intestines. 

 Concentrated solutions of neutral salts, sodium chlorid, for instance, 

 if introduced into a Thiry-Vella loop, cause a flow of water into 

 the lumen in accordance with their high osmotic pressure, and, on 

 the other hand, some of the sodium chlorid diffuses into the blood 

 in accordance with the laws of diffusion. It seems equally clear, 

 however, that absorption as it actually takes place is not governed 

 simply by the differences in concentration between the contents 

 of the intestine and the blood or lymph, but depends largely 

 upon the properties of the separating wall of living epithelial cells. 

 Thus, the animal's own serum, t possessing presumably the same 

 concentration and osmotic pressure as the animal's blood, is ab- 

 sorbed completely from an isolated intestinal loop. So also it has 

 been shown that in the absorption of salts from the intestine f the 



* Macfayden, Nencki, and Sieber, " Archiv f. experiment. Pathol. u. Phar- 

 macol.," 28, 311, 1891 



fHeidenhain, "Archiv f. die gesammte Physiologic," 56, 579, 1894. 



t Wallace and Cushny, "Archiv f. die gesammte Physiologic," 77, 202, 

 1899. 



