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 unbalanced atmospheric pressure is exerted from all
directions towards the thoracic space. This not only causes the air to
flow into the lungs (Chapter VII), but also causes a movement of the blood
and lymph in such of their tubes as enter this cavity. It will be noted
that both of the large lymph ducts terminate where their contents may be
influenced by the respiratory movements. (See Practical Work.)

*Where the Lymph enters the Blood.*—The fact that the lymph is poured into
the blood at but two places, and these very close to each other, requires
a word of explanation. As a matter of fact, it is impossible for the lymph
to flow into blood vessels at most places on account of the blood
pressure. This would force the blood into the lymph vessels, instead of
allowing the lymph to enter the blood. The lymph can enter only at some
place where the blood pressure is less than the pressure that moves the