l_, which occupies the center of
the villus and connects at the base with other lymph tubes, also called
lacteals (_B_, Fig. _76_).

3. A network of capillaries.

The villi are structures especially adapted to the work of absorption, and
they are found only in the small intestine. The mucous membrane in all
parts of the canal, however, is capable of taking up some of the digested
materials.

[Fig. 76]

Fig. 76--*The villi.* _A._ Diagram of a small section of mucous membrane of
small intestine. 1. Villi. 2. Small glands, called _crypts_.

_B._ Diagram showing structure of villi. 1. Small artery. 2. Lacteal. 3.
Villus showing termination of the lacteal. 4. Villus showing capillaries.
5. Villus showing both the lacteal and the capillaries. 6. Small vein. 7.
Layer of epithelial cells.

*Work of Capillaries and Lacteals.*--The capillaries and lacteals act as
receivers of material as it passes through the layer of epithelial cells
covering the mucous membrane. The lacteals take up the digested fats,(66)
and the capillaries receive all the other kinds of nutrients. These
vessels do not, of course, retain the absorbed materials, but pass them
on. Their final destination is the general circulation, which they reach
by two well-defined channels, or routes.

*Routes to the Circulation.*--The two routes from the place of absorption
to the general circulation are as follows:

1. _Route taken by the Fat._--The fat is conveyed by the lacteals from the
villi to the receptacle of the chyle. At this place it mingles with the
lymph from the lower parts of the body, and with it passes through the
thoracic duct to the left subclavian vein. Here it enters the general
circulation. Thus, to reach the general circulation, the fat has to pass
through the villi, the lacteals, the receptacle of the chyle, and the
thoracic duct (Fig. 77). Its passage through these places, like the
movements in all lymph vessels, is slow, and it is only gradually admitted
to the blood stream.

[Fig. 77]

Fig. 77--*Diagram of routes* from food canal to general circulation. See
text.

2. _Route of All the Nutrients except Fat._--Water and salts and the
digested proteids and carbohydrates, in passing into the capillaries, mix
there with the blood. But this blood, instead of flowing directly to the
heart, is passed throug