200 MANUAL OF PHYSIOLOGY. 



many of which surround the bloodvessels, aid in the passage of 

 substances into the blood-currents, is not known ; but most likely 

 they have some such effect, for the experiments showing direct 

 absorption by the bloodvessels leave the local lymph-channels in 

 operation, while at the same time the normal flow of lymph 

 towards the thoracic duct is more or less prevented. 



Any part which has only the protection of a single layer of 

 epithelial cells well supplied with superficial bloodvessels has also 

 a supply of absorbent vessels, and therefore is capable of absorb- 

 ing materials in solution which are in contact with the surface, 

 and large quantities of fluids and solutions of various materials 

 are absorbed from the stomach and large intestine partly, no 

 doubt, by means of the lacteals or lymphatics, and partly by the 

 minute bloodvessels themselves. 



However, the small intestine seems to be the part of the ali- 

 mentary tract which is especially adapted for taking up the ma- 

 terials elaborated from the food. 



In the upper part of the small intestine the valvulse 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 raucous 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 ob- 

 vious 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 bloodvessels. 



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 un- 

 limited quantity, but not so with solutions of salts. The amount 

 of the solution of any salt capable of absorption seems to depend 

 on its endosmotic equivalent. The lower the endosmotic equiva- 



