414 
DIGESTIVE ORGANS AND FOOD 
naked eye, these folds appear as a covering of minute hairs, 
called villi (vil'li). Their structure is shown in Figure 374. 
The process of osmosis, which has been so frequently 
referred to in Part I, is the chief factor in the passing of 
the food into the blood vessels. This process is assisted by 
the action of the living cells in a manner not well understood. 
The digested proteins and sugars pass directly into blood 
vessels which lead to the liver. In the liver, these blood 
vessels unite to form the portal 
(por'tal) vein, which is divided 
into minute branches that dis¬ 
tribute the blood to the cells 
of the liver. As the blood thus 
passes among the liver cells, 
the larger part of the sugar is 
changed into glycogen (gll'ko- 
jen), an animal starch, and 
stored temporarily in the liver 
cells. This stored-up starch, 
glycogen, does not yield energy 
to the body. It is given out 
gradually from the liver and 
changed back into sugar in 
which form it does yield energy 
to the body and results in 
keeping a uniform amount of 
sugar in the blood. 
The fats pass into certain distinct vessels, lacteals (lak'- 
te-als), which in turn open into larger ones. Eventually 
these vessels unite to form a large duct — the thoracic — 
which empties into one of the veins near the heart. The 
food is now in the blood stream and is carried to the individ¬ 
ual cells of the body. Each cell takes the kind of food which 
it needs and by a series of changes, as yet only partly known, 
makes the food into living protoplasm. 
Figure 374. — Diagram of a Villus 
from the Inner Wall of the 
Intestine. 
