LYMPH-VASCULAR SYSTEM. - This subdivision of the l vascular system consists partly of small tubes or vessels, the lymph vessels, and partly of collections of lymphoid or adenoid tissue (p. 849), the lymph glands. The lymph vessels or lymphatics are tubes with delicate transparent walls, which convey the fluid called lymph and chyle. They arise in the tissues and terminate by joining the venous system, so that their contained fluid flows towards the heart. They resemble veins in having a course from periphery to centre; in possessing valves, which are generally two in number and semilunar in shape ; in being divided into a superficial and a deep set - the superficial lymphatics being situated, like the superficial veins, in the subcutaneous tissue ; the deep lymphatics accompanying the arteries and deep veins. Lymphatics differ, however, from veins in possessing in their course glandular enlargements, in having thinner coats, in being almost uniform in size, and not uniting into larger vessels as they pass onwards in their course. As a rule they are like fine threads, and their main trunk, the thoracic duct, is not bigger than a crow-quill. The lymph-vessels are divided into lacteal or chyle vessels and lymphatics proper.

The lacteal or chyle vessels, named from the milk-like chyle which they contain, arise in the minute processes called intestinal which project from the free surface of the mucous membrane of the small intestine into the lumen of the boweL The lacteals from adjacent villi form a network in the submucous coat of the intestine, from which larger lacteals arise, which pierce the muscular coat, and then run between the folds of the mesentery to the posterior wall of the abdomen, where, opposite the body of the first lumbar vertebra, they join the deep lymphatic vessels of the abdomen to form the thoracic duct.

The lymphatic vessels proper correspond so closely in their distribution in the extremities and in the head and neck with the veins of those parts, that a special description of their arrangement is not necessary, the more so as a general representation of these vessels is given in Plate XXI. The superficial and deep lymphatics of the lower limbs enter the abdominal cavity, and are joined by the lymphatics of the pelvis. They ascend in front of the bodies of the lumbar vertebrae, join the lacteal vessels to form the thoracic duct, the place of junction being marked by a dilatation of the duct called receptaculum chyli. The Thoracic thoracic duct passes through the opening in the diaphragm duct. which transmits the aorta, ascends in front of the bodies of the dorsal vertebrae, receives in its course the deep lymphatics of the left half of the chest, reaches the root of the neck on the left side, is joined there by the deep and superficial lymphatics of the left upper limb and left side of the head and neck, and opens into the great veins at the angle of junction between the left internal jugular and subclavian. This duet conveys, therefore, the chyle during digestion, and the lymph contained in the lymph-vessels below the diaphragm and in the lymph-vessels situated to the left side of the mesial plane in the parts of the body above the diaphragm. The lymph-vessels on the right side of the supra-diaphragmatic parts of the mesial plane do not join the thoracic duct, but converge to the root of the neck on the right side, where they join to form the right lymphatic duct, which opens into the angle of junction of the right internal jugular and subclavian veins.

The mode of origin of the lymph-vessels has long been a vexed question amongst anatomists. The lacteal vessels were at one time supposed to arise by open mouths on the free surface of the intestinal villi, and this idea has been revived in a modified form by some recent observers, who conceive that the lacteals are continuous with a network formed by the anastomoses of processes proceeding from the deep ends of the goblet cells, the mouths of which cells open on the free surface of the villus. The lymph-vessels proper are in some localities continuous with the serous cavities (p. 848); in others they arise within the textures and organs. The most minute lymph-vessels, called lymph-capillaries, like the blood-capillaries, have walls formed of a single layer of elongated endothelial cells. These capillaries take their rise in the connective tissue of a part or organ, and probably spring from spaces, or juice-canals, between the bundles of the connective tissue, which bundles are invested by an endothelial layer of cells. The juice canals are, therefore, a network of minute canals, situated outside the blood-vessels, which allow the tissues to be permeated by a nutrient juice derived from the blood.

In some localities, as the brain and eyeball, the blood-vessels have been described as enclosed in tubular spaces, called peri-vascular canals, in which cells like the corpuscles of the lymph have been seen, and which are believed to be continuous with the lymphatic system. The researches of Ludwig and some of his pupils into the minute structure of the lachrymal gland, the glands of the skin, and the testis, have shown that lymph-capillaries lie in close relation to the secreting structures of these glands.

The coats of the lymph-vessels resemble in structure those of the veins, but they are thinner and more transparent. The valves arc small and numerous.

The lymphatic glands are small bodies, varying in size from a pea to an almond, situated. in the course of the lymph-vessels in several regions of the body. They are found especially in the groin, armpit, mesentery, back of the abdomen, roots of the lungs, and side of the neck (Plate XXI.) Entering one end of each gland are lymph-vessels, named vasa aferentia, and emerging from the opposite end of the gland are the lymph-vessels named vasa eferentia. Each gland is invested by a capsule of connective tissue, which sends processes into the substance of the gland to divide it into compartments; it consists of adenoid tissue, and the meshes of its retiform connective tissue contain multitudes of lymph corpuscles. Each •gland is permeated by a network of minute canals, which are continuous with both the vasa afferentia and efferentia; the gland, therefore, is traversed by a stream of lymph which washes the lymph corpuscles out of the meshes of the reticulum, and in this manner these corpuscles find their way into the lymph. The lymph glands are, therefore, centres of origin for the lymph corpuscles. The collections of adenoid tissue, forming the solitary and Peyer's glands of the intestine, and found in the tonsils and other localities (p. 849), are also without doubt centres of formation for the lymph corpuscles.