828 PHYSIOLOGY 



circulation. In the centre of the villus is the central lacteal, a club- 

 shaped vessel bounded by a complete layer of delicate endothelial 

 cells. This leads into a plexus of lymphatics placed superficially 

 to the muscularis mucosse. From the superficial plexus communi- 

 cating branches pass vertically to a corresponding plexus lying in 

 the submucosa. The central lacteal and the superficial plexus are 

 free from valves, which, however, are present in abundance in the 

 deeper plexus, so that fluid can pass easily from the lacteal to the 

 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 

 retiform 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 possibility of the passage occurring in con- 

 sequence 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 



