In this progress through the gland the chyle undergoes some important changes. Before entering the gland it was simply a milky fluid, containing very tine particles of matter in suspen sion, and being incapable of coagulation, or clot ting, when heated; but, on leaving the gland, it . ," e and contains numerous white cells, which have been swept away from the gland tissue by the stream. It will now be evident how liable the lymphatic glands are to be irritated and thrown into a condition of inflammation. Suppose the ingoing vessels carry with them irritating material, it filters through the whole gland, irritating in all its course, and swelling and inflammation result. Some times this may save the rest of the body. It may be that the gland not infrequently in tercepts, like a filter, material which, if permitted to go On, and finally to pass into the blood, would affect the whole body, but which, caught by the gland, ex pends its whole force on it, so that while the gland suffers the rest of the body escapes. Having issued from the gland, the chyle flows onward in lacteals in the mesentery till it is poured into the receptacle for the chyle, receptaculum chyli (2, Fig. 126). This recep tacle is a sac-like expansion at the lower end of a duct—the thoracic duct (1, Fig. 126). Not only do all the lacteal vessels pour their contents into the receptacle, but other vessels —lymphatic vessels of the lower limbs, to be described immediately—also join it. It lies on
the front of the spinal column at the level of the upper lumbar vertebrm (p. GO). From the receptaculum chyli the thoracic duct passes upwards along the front of the spine through the cavity of the chest to the root of the neck. Here it curves forwards and joins a large vein from the neck—the internal jugular—just where that • f e;i: joins a large one from the arm (s, Fig. 126). So that the chyle, after passing through the mesenteric glands, is carried up to the root of the neck by the duet, and there poured into the blood. The thoracic duct is from 15 to 18 inches low, and about the size of a small crow-quill.
In the course of the lacteal vessels and thoracic duct there are valves which direct the flow of fluid and prevent it passing backwards, while a valve at the junction of the duct and the veins in the neck permits the contents of the thoracic duct to flow into the vein, but pre vents the blood passing from the vein into the duct.
Fig. 126 shows the course of the thoracic duct. Lymphatic vessels are seen joining it from below, and from the walls of the chest. Glands are shown in the course of the vessels, 4 pointing to a gland of the chest wall. 5 and 6 point to veins.