e other, so that they are
constantly mixing and mingling. For these and other reasons, they are more
properly regarded as two divisions of a single liquid--one which, by
adapting itself to different purposes,(27) supplies all the conditions of
a nutrient fluid for the cells.

*Movements of the Lymph.*--As compared with the blood, the lymph must be
classed as a quiet liquid. But, as already suggested, it has certain
movements which are necessary to the purposes which it serves. A careful
study shows it to have three well-defined movements as follows:

1. A movement from the capillaries toward the cells.

2. A movement from the cells toward the capillaries.

3. A movement of the entire body of lymph from the lymph spaces into the
lymphatics and along these channels to the ducts through which it enters
the blood.

By the first movement the cells receive their nourishment. By the second
and third movements the lymph, more or less laden with impurities, is
returned to the blood stream. (See Figs. 28 and 30.)

*Causes of the Lymph Movements.*--Let us consider first the movement
through the lymph tubes. No pump, like the heart, is known to be connected
with these tubes and to supply the pressure necessary for moving the
lymph. There are, however, several forces that indirectly aid in its flow.
The most important of these are as follows:

1. _Blood Pressure at the Capillaries._--The plasma which is forced through
the capillary walls by pressure from the heart makes room for itself by
pushing a portion of the lymph out of the lymph spaces. This in turn
presses upon the lymph in the tubes which it enters. In this way pressure
from the heart is transmitted to the lymph, forcing it to move.

2. _Variable Pressure on the Walls of the Lymph Vessels._--Pressure exerted
on the sides of the lymph tubes by contracting muscles tends to close them
up and to push the lymph past the valves, which, by closing, prevent its
return (Fig. 29). Pressure at the surface of the body, provided that it is
variable, also forces the lymph along. The valves in the lymph vessels
serve the same purpose as those in the veins.

[Fig. 29]

Fig. 29--*Diagram* to show how the muscles pump lymph. _A._ Relaxed muscle
beside which is a lymphatic tube. _B._ Same muscle in state of
contraction.

3. _The Inspiratory Force._--When the thoracic cavity is enlarged in
breathing, the unbal