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 (lymphatics), and lymphatic glands. Lymph corresponds nearly to the blood without the red corpuscles. It is the familiar liquid seen in a blister, or oozing out where the skin has been grazed without breaking a blood vessel.

Necessity for Lymph and Lymph Spaces.—The body cannot be nourished with the albumin, sugar, oxygen, and other digested food in the blood, until this food passes out of the blood vessels. The food leaves the blood through the thin walls of the capillaries. Many of the cells do not touch the capillaries, and the lymph penetrates the spaces between the cells to reach them (see colored Fig. 3). If there were no lymph spaces, these cells could not get any food. The lymph bathes the cells, and the cells absorb what they want from the nourishing fluid. The red corpuscles bearing the oxygen cannot pass through the capillary walls. Oxygen, being a gas, readily passes through the walls and reaches the cells through the lymph in the lymph spaces. The waste materials must go back into the blood; carbon dioxid passes back through the capillary walls and is taken to the lungs; how the other waste materials formed in the cells pass back will soon be explained.

Need of Lymphatics.—The plasma continually passes into the tissues, but it cannot return directly into the blood. The lymph contains waste material which must be removed, and also much unused food which nature, like an economical housekeeper, will offer to the tissues again. There are vessels called lymphatics that take the lymph back into the blood (see Fig. 64).

The Lymphatic Circulation (Fig. 64).—The blood flow does not begin nor end, but makes a never ending circle. The countless lymphatics begin, with open ends, in the lymph spaces between the cells (colored Fig. 3). The smaller lymphatics unite into larger ones until finally they all unite into two large ones that empty into the large veins