DIGESTION AND ABSORPTION IN THE INTESTINES. 733 



Absorption in the Small Intestine. Absorption takes place 

 very readily in the small intestine. The general correctness of this 

 statement may be shown by the use of isolated loops of the intestine. 

 Salt solutions of varying strengths or even blood-serum nearly 

 identical in composition with the animals' own blood may be ab- 

 sorbed completely from these loops. Examination of the contents 

 of the intestine in the duodenum and at the ileocecal valve shows 

 that the products formed in digestion have largely disappeared in 

 traversing this distance. All the information that we possess in- 

 dicates, in fact, that the mucous membrane of the small intestine 

 absorbs readily, and it is one of the problems of this part of physiology 

 to explain the means by which this absorption is effected. Anatomi- 

 cally two paths are open to the products absorbed. They may enter 

 the blood directly by passing into the capillaries of the villi, or they 

 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 absorbents. 

 We now know that the digested and resynthesized fats are absorbed 

 by way of the lacteals, but that the other products of digestion 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 patient 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 from nine to twenty-three 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 



* Macfadyen, Nencki, and Sieber, " Archiv f. experiment. Pathol. u. 

 Pharmakol.,"28, 311, 1891. 



