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 are some large serous cavities, such as the abdomen, that may be considered as extended lymph spaces.

8.—Diagram of a lymphatic gland, showing afferent (a. l.) and efferent (e. l.) lymphatic vessels; cortical substance (C); medullary substance (M); fibrous coat (c); sending trabeculæ (tr) into the substance of the gland, where they branch, and in the medullary part form a reticulum; the trabeculæ are surrounded by the lymph path or sinus (l. s.), which separates them from the adenoid tissue (l. h.). (Sharpey.)

The lymphatic vessels have delicate, transparent walls, with three coats like the arteries, though much thinner, and anastomose even more freely than the veins. They have a beaded appearance due to the presence of numerous valves, which form constrictions on their surface. The right lymphatic duct, which is only about an inch long, drains all the lymphatics of the right half of the upper part of the trunk, the head, and the neck approximately, while the thoracic duct drains those of the rest of the body. The latter, which is the largest vessel of the system, begins opposite the second lumbar ver