This system contains and conveys a liquid called the lymph. It consists of lymph spaces, lymph tubes, (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 under the collar bones, near the neck. The one that empties under the left collar bone (3, Fig. 66) is called the _thoracic duct_ because it goes up through the thorax just in front of the spinal column (1, Fig. 66). The other at the right side of the neck is called the _right lymphatic duct_ (see Figs. 64, 65). [Illustration: FIG. 64.--SURFACE LYMPHATICS OF HAND.] In persons with the _dropsy_, the lymph accumulates in the lymph spaces and is not drained away by the lymph flow. Dropsy usually shows itself first by swelling of the feet and the leg below the knee. (Why? See Exp. 2.) There is a set of lymphatics called _lacteals_, situated in the abdomen, which have the function of absorbing digested fats from the intestine (Figs. 66, 100, and colored figure 2). [Illustration: FIG. 65.--DIAGRAM TO SHOW THE TWO PARTS OF THE BODY DRAINED BY THE TWO LYMPH DUCTS.]