=What makes the Lymph Flow?=--The heart does not, for its pressure is not transmitted beyond the blood tubes. _The successive pressures of a working muscle_ move the lymph forward in the lymphatics in the same way that the blood is moved forward in the veins, and the valves keep it from moving back. When riding a trotting horse, or in a jolting vehicle, the lymph is moved beyond the valves at every jolt (Fig. 64). Without exercise the lymph stagnates, and the body becomes poisoned by its own wastes. At every expansion of the lungs lymph is drawn into the chest; and it is forced out of the chest at every contraction. Deep breathing is as great a benefit to the body in moving stagnant lymph as it is in purifying the blood. [Illustration: FIG. 66.--CHIEF LYMPHATIC VESSELS AND GLANDS of trunk. 1, 3, Thoracic duct (emptying at 3); 2, receptacle for chyle (lacteals below it).] =The lymphatic glands= are kernel-like enlargements along the lymphatics, and they contain a great many lymph cells which _purify the lymph as it passes through them_. The lymphatic glands are numerous in the armpits and the groins. The cells in the lymph glands multiply, and some of them are carried by the lymph into the blood _to become those remarkable little bodies, the white corpuscles_. HYGIENE OF THE CIRCULATION =Effects of Work, Fresh Air, and Rest on Corpuscles and Plasma.=--_Work_ uses up the nutritious elements in the blood. A few hours after food is eaten the nutritious materials in the blood are found to be increased. By the breathing of _fresh air_ the carbon dioxid in the plasma is diminished and the oxygen in the colored corpuscles is increased, changing the blood to a brighter red. _Sleep_ gives time for the exhausted cells and depleted blood to be replenished. _Loss of sleep_ means longer hours of activity and greater consumption of nutriment with shorter hours for replacing the nutriment. The pale skin of one who has lost sleep tells of the exhausted condition of the blood. =How the Muscles help the Circulation.=--The imperative need of muscular exercise to keep the body sound exists because of the lack of other means to cause movement in the veins and lymphatics. Good food, pure air, and plenty of exercise are necessary for healthy blood. Many so-called “blood purifiers” are advertised to entrap the ignorant. It is impossible to imagine how “blood purifiers” can aid the blood. _The blood is purified, not by putting anything into the blood, but by something going out of it as it passes through the skin, kidneys, liver, and lungs._ These organs all send out impurities brought to them by the blood.