THE ABSORPTION OF THE FOOD-STUFFS 735 



deeper plexus, but not in the reverse direction. From the muscularis 

 mucosce unstriated muscle fibres pass up through the villus to be attached 

 partly to the outer surface of the central lacteal, partly by expanded 

 extremities to the basement membrane covering the surface of the villus. 

 Contraction of these muscle fibres will tend to empty the central lacteal 

 into the deep plexus of lymphatics and may also cause an expulsion of 

 the contents of the spaces of the retif orm tissue of the villus into the central 

 lacteal. The alimentary canal represents one of the few localities where a 

 formation of lymph is constantly proceeding, even in a condition of complete 

 rest. On placing a cannula in the thoracic duct of a dog an outflow of 

 lymph is obtained which may vary in different animals between 1 c.c. and 

 10 c.c. in the ten minutes. The greater part of this lymph is derived from 

 the alimentary canal, so that any of the intestinal contents which have 

 made their way into the spaces of the villus might be entrained in this 

 lymph current and carried away with it into the thoracic duct and so into the 

 general blood system. 



The other possible channel of absorption is by the capillary blood- 

 vessels of the villus. Each villus is supplied with blood from one or two 

 arterioles which break up into a rich plexus of capillaries lying close under 

 the basement membrane of the villus. The return blood is collected into 

 one or two veins, which join the radicles of the portal vein in the submucosa 

 and in the mesentery. In these capillaries the blood is circulating rapidly, 

 so that a considerable amount of material may pass into them from the 

 spaces of the villus within, say, one hour without altering appreciably the 

 percentage composition of the blood. On the other hand, it must be remem- 

 bered that the blood in these vessels is at a high pressure, probably not less 

 than 30 mm. Hg., so that any absorption into the blood stream must occur 

 against this pressure. It is probable therefore that in explaining any 

 absorption by the blood-vessels we shall have to place out of court any possi- 

 bility of the passage occurring in consequence of hydrostatic differences of 

 pressure, i.e. by a process of filtration. 



When salt solutions are introduced into the small intestine they are 

 rapidly absorbed without the production of any corresponding increase in 

 the rate of lymph flow from the thoracic duct. On the other hand, the 

 absorption of larga amounts of fluid may cause an actual diminution of 

 the solids of the plasma, so that we are justified in regarding the capillary 

 network of blood-vessels at the surface of the villi as solely responsible for 

 the absorption. 



What are the forces which cause this transference of fluid and dissolved 

 substances from one side to the other of the membrane composed of epithelial 

 cells plus capillary endothelium ? Like other cells, those of the intestinal 

 epithelium are probably bounded on their free surface by a ' lipoid ' mem- 

 brane, i.e. one containing some complex of lecithin and cholesterin and per- 

 meable only by such substances as are soluble in lipoids. On the other hand, 

 the cement substance between the cells may be of a different character and 

 possibly permeable to water-soluble substances. The question has been 



