354 ACTIVITY OF THE WALL OF THE ALIMENTARY CANAL. 



Filtration is the passage of fluid through the coarser intermolecular pores 

 of a membrane dependent upon pressure. The higher the latter and the 

 larger and more numerous the pores, the more rapidly will the nitrate pass through 

 the pores of the membrane. Increase in temperature likewise accelerates nitra- 

 tion. Further, those fluids filter most readily that most rapidly soak into the 

 membrane in question. Therefore, different fluids vary in the readiness with 

 which they pass through different membranes. Further, the greater the con- 

 centration of the solutions, the more slowly, in general, is their passage. The 

 filter has the property of retaining in part matters from the solutions passing 

 through, either substances dissolved in the fluid (particularly colloid substances) , 

 or water (from dilute solutions of potassium nitrate) . In the former case the 

 filtrate is more dilute, in the latter more concentrated, than the fluid was before 

 its passage through the filter. Other substances pass through without material 

 change in concentration. Should the filtrate enter another fluid, the concen- 

 tration of the transudate increases with the pressure under which filtration takes 

 place. Some membranes exhibit a difference according as filtration takes place 

 from their different surfaces; thus the membrana testacea of the egg per- 

 mits of filtration only in the direction from without inward. The mucous mem- 

 brane of the stomach and intestine also exhibits a difference in this respect. 



It was formerly believed that filtration of substances in solution 

 could take place from within the digestive canal into the vessels: i. If 

 the intestine contracted and thereby exerted pressure directly on the 

 contents. This alone, however, could scarcely have any noteworthy 

 influence, even in case the canal were contracted in two places and the 

 intervening musculature, through contraction, compressed the fluid 

 intestinal contents. 2. Filtration under negative pressure may be 

 effected through the villi, which on contracting forcibly evacuate the 

 contents of the blood-vessels and lymphatics in a centripetal direction. 

 The latter particularly will remain empty, as the chyle in the fine 

 lacteals is prevented from passing backward by numerous valves. When 

 the villi are again relaxed, they will by suction be able to fill themselves 

 with the fluids of the digestive tract capable of filtration. On the other 

 hand, the fact must especially be emphasized that, according to Spee 

 and Heidenhain, the muscles of the villus actively dilate the central 

 lymph- vessels. 



ABSORPTIVE ACTIVITY OF THE WALL OF THE ALIMENTARY 



CANAL. 



The process of digestion prepares from the food in part true solutions, 

 in part finely divided emulsions, whose small globules are surrounded by 

 an albuminoid capsule. 



Absorption of Solutions. It cannot be denied that true solutions 

 can pass over into the blood and the lymph of the intestinal canal by 

 endosmosis, but some observations indicate that the cellular elements of 

 the digestive tract also participate in the process of absorption through 

 the functional activity of their protoplasm. It has not as yet been 

 possible to refer the forces effective in this connection to simple physical or 

 chemical processes. When Heidenhain introduced methylene-blue in 

 solution into the intestine, he was convinced that the path of its absorp- 

 tion was in part through, in part between, the epithelial cells. 



The Inorganic Substances: Water, and the dissolved salts necessary 

 for nutrition, are generally easy of absorption, and in large measure 

 by the blood-vessels. In the absorption of salt-solutions by endosmosis, 

 water must naturally pass from the intestinal vessels into the intestine, 

 while the salt-solutions enter the vessels. The amount of water, how- 



