has two sets of vessels. (1.) The blood-vessels, which, by their
frequent blending, form a complete net-work beneath the external
epithelium; they unite at the base of the villus, forming a minute vein,
which is one of the sources of the portal vein. (2.) In the center of
the villus is another vessel, with thinner and more transparent walls,
which is the commencement of a lacteal.

The _Lacteals_ originate in the walls of the alimentary canal, are very
numerous in the small intestine, and, passing between the laminae of the
mesentery, they terminate in the _receptaculum chyli_, or reservoir for
the chyle. The mesentery consists of a double layer of cellular and
adipose tissue. It incloses the blood-vessels, lacteals, and nerves of
the small intestine, together with its accessory glands. It is joined to
the posterior abdominal wall by a narrow _root_; anteriorly, it is
attached to the whole length of the small intestine. The lacteals are
known as the absorbents of the intestinal walls, and after digestion is
accomplished, are found to contain a white, milky fluid, called _chyle_.
The chyle does not represent the entire product of digestion, but only
the fatty substances suspended in a serous fluid.

Formerly, it was supposed that the lacteals were the only agents
employed in absorption, but more recent investigations have shown that
the blood-vessels participate equally in the process, and are frequently
the more active and important of the two. Experiments upon living
animals have proved that absorption of poisonous substances occurs, even
when all communication by way of the lacteals and lymphatics is
obstructed, the passage by the blood-vessels alone remaining. The
absorbent power which the blood-vessels of the alimentary canal possess,
is not limited to alimentary substances, but through them, soluble
matters of almost every description are received into the circulation.