ABSORPTION. 311 



atics, and elaborated with the other contents of the lymph in passing 

 through the glands. 



(c.) By Blood- Vessels. In the absorption by the lymphatic or 

 lacteal vessels just described, there appears something like the exercise 

 of choice in the materials admitted into them. But the absorption by 

 blood-vessels presents no such appearance of selection of materials ; 

 rather, it appears, that every substance, whether gaseous, liquid, or a 

 soluble, or minutely divided solid, may be absorbed by the blood-vessels, 

 provided it is capable of permeating their walls, and of mixing with the 

 blood ; and that of all such substances, the mode and measure of ab- 

 sorption are determined almost solely by their physical or chemical 

 properties and conditions, and by those of the blood and the walls of the 

 blood-vessels. 



Method of Absorption. 



(.) Osmosis. The phenomena of absorption of all the materials 

 of the food except the fats are, to a great extent, comparable to that 

 passage of fluids through membrane, which occurs quite 

 independently of vital conditions, and the earliest and best 

 scientific investigation of which was made by Dutrochet. 

 The instrument which he employed in his experiments was 

 named an endosmometer. It may consist of a graduated 

 tube expanded into an open-mouthed bell at one end, over 

 which a portion of membrane is tied (Fig. 226). If now 

 the bell be filled with a solution of a salt say sodium chlo- 

 ride, and be immersed in water, the water will pass into 

 the solution, and part of the salt will pass out into the 

 water ; the water, however, will pass into the solution 

 much more rapidly than the salt will pass out into the 

 water, and the diluted solution will rise in the tube. To 

 this passage of fluids through membrane the term Osmosis 

 is applied. 



The nature of the membrane used as a septum, and its 

 affinity for the fluids subjected to experiment have an im- 

 portant influence, as might be anticipated, on the rapidity FIO. 225.-En- 

 and duration of the osmotic current. Thus, if a piece 

 of ordinary bladder be used as the septum between water and alcohol, 

 the current is almost solely from the water to the alcohol, on account of 

 the much greater affinity of water for this kind of membrane ; while, on 

 the other hand, in the case of a membrane of caoutchouc, the alcohol, 

 from its greater affinity for this substance, would pass freely into the 

 water. 



Absorption by blood-vessels is the consequence of their walls being, 

 like the membranous septum of the endosmometer, porous and capable 



