ABSORPTION OF SPECIAL MATERIALS. 207 



mentary tract which is especially adapted for taking up the 

 materials elaborated from the food. 



In the upper part of the small intestine the valvuta conni- 

 ventes are most marked, and the villi are long and closely set 

 together. It is here we find the thickest layer of creamy chyme 

 covering the mucous membrane, but seldom any masses of par- 

 tially digested food. All these points, which are directly related 

 to absorption, tend to show that the upper part of the intestine is 

 the part specially adapted for this purpose. The chyme which 

 clings to the mucous membrane contains all the substances which 

 are destined to pass into the economy. Into this mixture the 

 villi dip, so that each villus is completely bathed with chyme. 

 From what has been said of the construction of the villi, it is 

 obvious that such an arrangement is admirably well adapted to 

 the absorption of the nutrient material, which has every facility 

 for making its way into the lacteals and blood vessels. 



The principal ingredients in the chyme may now be examined 

 in detail with reference to their powers of being absorbed. 



Water can be absorbed from the intestinal tract in almost 

 unlimited quantity, but not so with solution of salts. The 

 amount of the solution of any salt capable of absorption seems 

 to depend on its endosmotic equivalent. The lower the en- 

 dosraotic equivalent the more readily the solution passes into 

 the blood vessels. In those cases where the equivalent is very 

 high, such as magnesium sulphate, there is a tendency of 

 the fluid to pass out from the blood vessels into the intestinal 

 cavity ; hence the watery stools caused by this and such like 

 saline purgatives. 



Among the carbohydrates we need only take into account the 

 sugars, for starch unchanged is but little, if at all, absorbed. Only 

 a certain quantity of sugar can be taken up by the intestinal ab- 

 sorbents, since some is found in the faeces when the amount taken 

 with the food exceeds a certain quantity. Some of the sugar in 

 the intestine, moreover, undergoes fermentation, by which it is 

 converted into lactic and butyric acid. How much of the sugar 

 is absorbed as lactic and butyric acid has not been determined, 

 but the amount of sugar found in the portal vessels or lacteals 



