ABSORPTION OF WATER AND SOLUBLE SALTS. 395 



different membranes with varying rapidity. The nitration is usually slower, the 

 greater the concentration of the fluid. The filter has the property of retaining 

 some of the substances from the solution passing through it, e.cj. , colloid sub- 

 stances or water (in dilute solutions of nitre). In the former case, the filtrate is 

 more dilute, in the latter, more concentrated than before filtration. Other sub- 

 stances filter without undergoing any change of concentration. Many membranes 

 behave differently, according to which surface is placed next the fluid; thus the 

 shell-membrane of an egg permits nitration only from without inwards ; [and the 

 same is true to a much less extent with an ordinary filter paper the smooth side 

 of the filter paper ought always to be placed next the fluid to be filtered]. There 

 is a similar difference with the gastric and intestinal mucous membrane. 



Filtration of the soluble substance may take place from the canal of 

 the digestive tract when: (1) The intestine contracts and thus exerts 

 pressure upon its contents. This is possible when the tube is narrowed 

 at two points, and the musculature between these two points contracts 

 upon the fluid contents. (2) Filtration, under negative pressure, may be 

 caused by the mill (Briicke). When the villi contract energetically, 

 they empty their contents towards the blood- and lymph-vessels. The 

 lymph-vessels remain empty, as the chyle is prevented from passing 

 backwards into the origin of the lacteal within the villi, owing to the 

 presence of numerous valves in the lymphatics. When the villi pass 

 again into the relaxed condition, they again become filled with the 

 fluids of the intestinal contents. 



192. Absorptive Activity of the Wall of the 



Intestine. 



The process of digestion produces from the food, partly solutions and 

 partly finely divided emulsions, whose fine particles are surrounded by 

 an albuminous envelope, the haptogen membrane [of Ascherson], where- 

 by these particles become more stable. Unchanged colloid substances 

 may also be present in the intestinal tract. 



I. Absorption of Solutions. True solutions undoubtedly pass by 

 endosmosis into the blood-vessels and lymphatics of the intestinal walls, 

 but numerous facts indicate, that the protoplasm of the cells of the tube 

 take an active part in the process of absorption. The forces concerned 

 have not as yet been referred simply to physical and chemical 

 processes. 



(1.) The Inorganic Substances. Water and the soluble salts neces- 

 sary for nutrition are easily absorbed. When saline solutions pass by 

 endosmosis into the vessels, water must pass from the intestinal vessels 

 into the intestine. The amount of water, however, is small, owing to 

 the small endosmotic equivalent of the salts to be absorbed. More 

 salts are absorbed from concentrated than from dilute solutions (Funke). 



