molecular movements. The absorption of food is, however, not a simple
process, and the passage takes place through an _active_ (living)
membrane. Another difference is that certain foods undergo chemical change
while being absorbed.

*Small Intestine as an Organ of Absorption.*—While absorption may occur to
a greater or less extent along the entire length of the alimentary canal,
most of it takes place at the small intestine. Its great length, its small
diameter, and its numerous blood vessels all adapt the small intestine to
the work of absorption. The transverse folds in the mucous membrane, by
retarding the food in its passage and by increasing the absorbing surface,
also aid in the process. But of greatest importance are the minute
elevations that cover the surface of the mucous membrane, known as

*The Villi.*—Each single elevation, or villus, has a length of about one
fiftieth of an inch and a diameter about half as great (_A_, Fig. 76), and
contains the following essential parts:

1. An outer layer of epithelial cells, resting upon a connective tissue
support.

2. A small lymph tube, called a _lacteal_, 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