ABSORPTION 141 



purpose appears to be mainly to change the surface of 

 absorption. Though propulsion of the food is involved, 

 this is dependent principally upon the more rarely occur- 

 ring rush-peristalsis, which, when fully developed, may 

 sweep along the whole length of the intestine in about a 

 minute. Peristalsis is more active in the upper than in 

 the lower part of the intestine. At the approach of a 

 wave to the lower end of the intestine the ileocsecal valve 

 opens. 



The vagus, while not causing the intestinal movements, 

 nevertheless influences them in the direction of increased 

 activity after initial inhibition. The sympathetic, on the 

 other hand, inhibits all movement and tonus, and at the 

 same time causes vaso-constriction, but it closes the 

 ileocaecal valve. 



Absorption from the Small Intestine 



The small intestine is peculiarly adapted anatomically 

 and physiologically for absorption ; anatomically by its 

 great length, by the folding of its internal surface into 

 the valvulse conniventes and by the projection from its 

 mucous membrane of the innumerable viUi ; physiologically 

 by the complex movements which it undergoes. 



The food as it reaches the ileocsecal valve, though as 

 fluid as it was when it entered the duodenum, is greatly 

 diminished in volume and altered in composition, practically 

 all the carbohydrates and the greater part o^ the fat and 

 protein having been absorbed, together with most of the 

 water. 



The Nature of Absorption 



How far are physical processes, such as osmosis, respon- 

 sible for the passage of water and substances in solution? 

 We may say at once that osmosis alone cannot account 

 for the process, since not only water but sahne solutions 

 isotonic with blood and even the animal's own serum are 



