158 HOW FOOD IS PREPARED FOR BODY USES 



lor vena 



Internal Structure of a Villus. — The internal structure of a 

 villus is best seen in a longitudinal section. We find the outer wall 

 made up of a thin layer of cells, the epithe'lial layer. It is the duty 

 of these cells to absorb the fluid food from within the intestine. 

 Underneath these cells lies a network of very tiny blood vessels 

 and in the core of the villi are spaces which, because of their white 

 appearance after the absorption of fats, have been called lac'teals. 

 Absorption of Foods. — While diffusion and osmosis are important 

 factors in the passage of food and water through the walls of the 



intestine, most physiologists 

 agree that the living matter 

 in the cells lining the villi 

 exerts a selective action on 

 the substances that pass into 

 the blood and lacteals. The 

 cells act as tiny chemical 

 laboratories, actually allow- 

 ing some food substances to 

 pass through them and with- 

 holding the passage of others. 

 Fats, for example, are built 

 up again in these cells from 

 fatty acids and glycerin, and 

 in the form of fats are passed 

 into the central part of the 

 villus, eventually reaching 

 the blood by v/ay of the 

 lacteals and the thoracic duct 

 without passing through the 

 liver. (See diagram.) On 

 the other hand, simple sugars 

 and amino-acids pass directly into the blood and reach the portal 

 circulation. These pass through the liver, where, as we have seen, 

 sugar is taken from the blood and stored as glycogen. From the 

 liver, the food within the blood is carried to the heart, pumped to 

 the lungs, returned to the heart, and is pumped to the tissues of the 

 body. A large amount of water and some salts are also absorbed 

 through the walls of the stomach and intestine. 



Mesenhric y€[i 

 Mesentery 



Lymph 

 node 



Diagram showing how nutrients reach the 

 blood. 



