The drainage system consists of very delicate vessels known as the _lymphatics_, which come together into larger and larger vessels, just as the veins do, and finally empty into the vena cava just at the point where it enters the heart. There is no back pressure of blood here, so that the movement of fluid through the lymphatics is not hindered. There is no pump for forcing the lymph to move along; the very gradual motion, which is all that is necessary to keep the fluid from accumulating in the tissue spaces, is brought about by pressure upon the lymphatics resulting from the bodily movements. The lymphatic vessels are like the veins in having valves here and there along them. Whenever by any bodily movement either a lymphatic vessel or a vein is squeezed, the valves insure that the liquid shall be passed along in the direction toward the heart and never in the reverse direction. In the case of the veins this action is not absolutely necessary, since the heart itself is able to maintain the circulation, although it does help, particularly in bringing back the blood from the extremities. In the lymphatics this is the only way by which movement of fluid is brought about. The result is that when the body is perfectly quiet, there is very little movement of fluid through the lymphatics; muscular activity, on the other hand, leads to rather active movement through these vessels. The fact is well illustrated in ourselves. One who sits for a long time humped over a desk finds himself feeling very dull; to obtain relief he stirs about, stretches, and yawns. The dullness was merely the result of the stagnation of fluid in his tissues, causing the cells to be more or less poisoned by their own waste products. By making active movements, these stagnating fluids were forced along to be replaced by fresh liquid direct from the blood and the beneficial effect is felt immediately. _Massage_ properly applied has very much the same effect, although it is doubtful whether as good results can ever be obtained thus as by actual vigorous exercise. At various places along the lymphatics are little spongelike lumps of tissue known as _lymph nodes_; the particular spongy substance of which they are composed is called _adenoid tissue_. This adenoid tissue acts as a filter for the fluid passing through it. Any foreign particles, living or nonliving, that get into the stream are caught in the lymph nodes and held there more or less permanently. Most of the nonliving particles that get into our tissue fluids are from the dust that we inhale, which works its way through the mucous membrane of the respiratory passages and so into the body fluids. The lymphatics that drain the lungs carry along these dust particles and they lodge in lymph nodes at the base of the neck. Persons who have lived in dusty regions or have pursued a dusty livelihood, such as coal heaving, will have by the end of their lives lymph nodes which are literally black with dirt.