CHAPTER XII. 



ABSORPTION. 



In order that the food stuffs, when altered by the various pro- 

 cesses described under digestion, may be of any real use to the 

 economy, the nutritive materials must be distributed through the 

 textures and organs. For this purpose they must pass through 

 the lining membrane of the alimentary canal, and obtain admis- 

 sion to the blood, which is the common mode of intercommuni- 

 cation between the various parts of the body. 



The nutrient part of the food has then to be absorbed out of 

 the alimentary canal by the surrounding tissues, and mixed with 

 the general circulating fluid. 



But the blood is separated from the intestinal contents by a bar- 

 rier, which for it at least is impassable, although it exerts consider- 

 able pressure, and therefore tends to burst out from the vessels. 



The question then arises, How does the elaborated chyme 

 make its way through this barrier, which is sufficient to prevent 

 the flow of blood into the intestinal tract? 



The general answer is easily given, viz. : the blood cannot pass 

 through an animal membrane. But this is not a satisfactory 

 solution of the question, for sometimes, under certain circum- 

 stances, the blood does pass through the wall of the vessels, and 

 normally the plasma escapes from the capillaries into the tissues, 

 in order to nourish them. We must further remember, in con- 

 sidering this point, that the wall of the vessels and the membrane 

 lining of the intestine are both made up of living cells which 

 are endowed with a capability, coincident with their lives, of 

 controlling any passage through or between them. Some of these 

 cell guards, which we might call secreting agents, do allow, or 

 rather cause, a passage of fluid from the blood to the intestinal 

 cavity, and, as we shall presently see, others of them induce a 

 passage of the nutritious materials from the intestinal canal into 

 the surrounding tissues. 



17 193 



