NUTRITION 



157 



2. Absorption 



The purpose of digestion is, first, to make the food soluble so 

 that it may enter the body proper: pass through the epithelium 

 lining the digestive tract wall ; and, second, to put it into a chemical 

 form that can be used by the cells. The passage into the body, or 

 absorption, occurs chiefly in the 

 small intestine, the walls of which 

 are lined with millions of minute 

 projections, or villi, that bring 

 tiny vessels into intimate contact 

 with the absorptive membrane 

 while greatly increasing the effec- 

 tive absorptive surface. Appar- 

 ently absorption is not merely the 

 result of simple physical processes 

 such as diffusion and osmosis, but 

 is largely the function of the 

 actual living cells forming the 

 epithelium of the villi. (Figs. 114, 

 144.) 



3. Distribution 



Lacteal- 



Locte 



Fig. 114. — Diagram 



'Artery 



of a sec- 



The transportation system of 

 vessels in the villi consists of blood 

 vessels which take up the absorbed 

 products of protein and carbohy- tion through a villus. (From Pea- 

 drate digestion, and lymph vessels, ° y an un •> 

 here called lacteals, which receive the derivatives of the fats. 

 Both also take absorbed water and salts. 



Accordingly the path through which the proteins and carbohy- 

 drates are transported differs from that of the fats. The blood 

 vessels returning blood from the digestive organs finally merge 

 to form a large vessel, the portal vein, which proceeds to the 

 liver to allow that organ to regulate certain of the blood constitu- 

 ents — in particular to store up sugar, in the form of glycogen, 

 after a meal and later dole it out to the blood as conditions de- 

 mand. On the other hand, the lacteals merge into larger and larger 

 lymph vessels and finally into the thoracic duct which empties 

 into the blood vascular system — the fats being switched, as it 



