d
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.
1. A representation of a lymphatic
vessel highly magnified. 2. Lymphatic
valves. 3. A lymphatic gland and its vessels.]

In all animals which possess a lacteal system there is also a lymphatic
system, the one being the complement of the other. The fact that lymph
and chyle are both conveyed into the general current of circulation,
leads to the inference that the lymph, as well as the chyle, aids in the
process of nutrition. The body is continually undergoing change, and
vital action implies waste of tissues, as well as their growth. Those
organs which are the instruments of motion, as the muscles, cannot be
employed without wear and waste of their component parts. Renovated
tissues must replace those which are worn out, and it is a part of the
function of the absorbents to convey nutritive material into the general
circulation. Researches in microscopical anatomy have shown that the
skin contains multitudes of lymph