CHAPTER X. 



ABSORPTION. 



ABSORPTION is generally considered to consist of two processes; the 

 first, having for its object the introduction into the blood of fresh mate- 

 rial, and which is called absorption from without, takes place chiefly 

 from the alimentary canal, and to a less extent from the skin and lungs; 

 the second, having for its object the gradual removal of parts of the 

 body itself when they need removal, is called absorption from within, 

 and takes place everywhere within the tissues of the body. 



The conditions of absorption from the alimentary canal which may 

 be taken as an example of the first of these processes are the following: 

 on one side is a fluid containing matters which have been so acted upon 

 by the digestive juices as to be in a fit condition to be absorbed. On 

 the other side are blood-capillaries and capillaries of the lymphatic sys- 

 tem, and separating the two are epithelium and connective tissue, as 

 well as the endothelium of the vessels themselves. The problem which 

 has to be considered is, how does the fluid on the one side of the 

 organic membrane reach the blood or lymphatic vessel ? Until within 

 recent date it was assumed that the passage of the fluid from one side 

 of this membrane to the other came about solely by definite physical 

 laws, and these were practically independent of the vital condition of 

 the tissues. In the first place, it was taught, came in osmosis, the pas- 

 sage of fluids through an animal membrane, which occurs independent 

 of vital conditions, and in the next place came infiltration, the passage 

 of fluids through the pores of a membrane under pressure. It is now 

 believed, however, that there is another factor concerned in absorption, 

 viz., the vital and selective action of the epithelium, and possibly of the 

 tissue which separates the fluid to be absorbed, from the blood and 

 lymph stream. About this vital action of the epithelium very little 

 definite is known, but the mere fact that fats are principally absorbed 

 in one part of the intestine, and as we shall see pass through the cells 

 of the intestinal villi, is some evidence in its favor. It will be as well 

 to consider briefly the two physical processes of osmosis and filtration. 



Methods of Absorption. 



Osmosis. The phenomenon of the passage of fluids through animal 

 membrane, which occurs quite independently of vital conditions, was 

 first demonstrated by Dutrochet. The instrument which he employed 



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