The blood flowing through the body in the delicate capillary vessels yields to the various tissues a kind of irrigation stream of plasma, which leaving the capillaries permeates every tissue and saturates them with nutrient fluid. The surplus of this irrigation stream is collected and carried back to the blood current by a special set of fine flattened vessels with slender walls, called the lymph vascular system, which acts as the drainage of the tissues, and pours its contents into the veins.

Tendon of Mouse's Tail treated with nitrate of silver, showing clefts or cell spaces around the bundles of fibrils as white patches.

Fig. 81. Tendon of Mouse's Tail treated with nitrate of silver, showing clefts or cell spaces around the bundles of fibrils as white patches. These interstices may be called the smallest lymph channels or spaces. (Sc/zafer).

When the nutrient fluid escapes from the capillaries, it lies in the interstices between the tissue elements, and here bathes the cells which commonly occupy these lymph spaces. (Figs. 81 and 86).

Communicating freely with the interstices of the tissues are irregular anastomosing flattened channels, which convey the lymph or any fluid forced between the tissues into vessels with definite boundaries. These vessels, which are lined with characteristic endothelium, form a more or less dense network of lymphatic capillaries, from which spring the tributaries of the lymph vessels. (Figs. 82 and 83).

The lymphatic vessels are throughout slender, thin-walled channels with frequent anastomoses and close-set valves, usually in pairs. They lie imbedded in the connective tissue, and when empty are difficult to see, owing to the extreme thinness of their coats. They converge toward a central vessel called the thoracic duct, which, passing from the abdominal cavity through the thorax, reaches the left side of the neck, and opens into the angle of junction of the two great veins from the head and upper extremity. (Fig. 80.) On the right side a smaller trunk, conveying the lymph from the right arm and that side of the head, chest and neck, opens into the corresponding venous trunks.

Lymph Channels from the thoracic side of the central tendon of the diaphragm of the rabbit, treated with silver nitrate.

Fig. 82. Lymph Channels from the thoracic side of the central tendon of the diaphragm of the rabbit, treated with silver nitrate. The fine lines indicate the boundaries of the endothelium cells lining the lymph channels. The dark part shows the islets between the lymphatic network. {Klein).

The thoracic duct is much larger than any of the numerous tributaries which enter it at close intervals from all directions.

Its lower extremity or point of origin is an irregular dilatation called the receptaculum chyli, because the lymphatic vessels from the stomach and intestines, or lacteals as they are called, pour their contents into it. The chyle from the intestines thus flows into the same main channel as the lymph which is derived from the drainage of the tissues and organs of the lower extremity, the trunk and left side of the head, neck and arm, and the two fluids are mixed in the receptaculum chyli, and other parts of the thoracic duct.

Diagram of a Lymphatic Gland, showing (a I) afferent and (el) efferent lymphatic vessels.

Fig. 83. Diagram of a Lymphatic Gland, showing (a I) afferent and (el) efferent lymphatic vessels; (c) Cortical substance; (m) Medullary substance; (c) Fibrous coat sending trabecular (t r) into the substance of the gland, where they branch, and in the medullary part form a reticulum; the trabeculae are surrounded by the lymph path or sinus (Is), which separates them from the adenoid tissue (lh). (Sharpey).