The _Lymphatics_ are not less important organs in the process of
absorption. Nearly every part of the body is permeated by a second
series of capillaries, closely interlaced with the blood-vessels,
collectively termed the _Lymphatic System_. Their origin is not known,
but they appear to form a _plexus_ in the tissues, from which their
converging trunks arise. They are composed of minute tubes of delicate
membrane, and from their net-work arrangement they successively unite
and finally terminate in two main trunks, called the _great lymphatic
veins_. The lymphatics, instead of commencing on the intestinal walls,
as do the lacteals, are distributed through most of the vascular tissues
as well as the skin. The lymphatic circulation is not unlike that of the
blood; its circulatory apparatus is, however, more delicate, and its
functions are not so well understood.

[Illustration: Fig. 36.
A general view of the Lymphatic System.]

The _lymph_ which circulates through the lymphatic vessels is an
alkaline fluid composed of a plasma and corpuscles. It may be considered
as blood deprived of its red corpuscles and, diluted with water. Nothing
very definite is known respecting the functions of this fluid. A large
proportion of its constituents is derived from the blood, and the exact
connection of these substances to nutrition is not properly understood.
Some excrementitious matters are supposed to be taken from the tissues
by the lymph and discharged into the blood, to be ultimately removed
from the system. The lymph accordingly exerts an important function by
removing a portion of the decayed tissues from the body.

[Illustration: Fig. 37.