Hitherto we have spoken of the lymphatic system as a labyrinth of communicating spaces containing stagnant fluid, which is kept in a fitting state by egress and ingress out of and into blood. Such a mental picture is substantially correct. But the system is complicated by the presence of lymphatic vessels. Cells of the connective-tissue sponge-work arrange themselves side by side. They flatten into endothelial scales. The borders of the scales close up. They form lymphatic channels, wider than blood-capillaries, but strictly comparable in every other respect. The lymph capillaries unite into larger vessels. The larger vessels are connected by cross-branches; they form plexuses. Their walls are strengthened with fibrous tissue. Like the veins, they are abundantly provided with valves, which check any tendency to a backward flow on the part of the fluid which they contain. Lymphatic plexuses surround and accompany the larger bloodvessels. They are disposed on the surface of muscles and glandular tissues. They are abundant beneath the skin. Nearly three centuries ago the lymphatic vessels of the mesentery, which collect products of digestion, especially fat, from the walls of the alimentary canal, were recognized owing to the milkiness of their contents after a meal. They were, on this account, termed “lacteals.” Other lymphatic vessels, owing to their transparent walls and colourless contents, are not easily seen; but they are readily injected with mercury or other fluids which render them conspicuous. In the upper part of the thigh, in the armpit, or in the neck, they are about large enough to admit a crow-quill. Those from the lower limbs, from the viscera, and from the walls of the abdomen converge to a receptacle which lies in front of the spinal column. The receptaculum chyli is continued upwards as the thoracic duct, which pours the lymph into the great veins of the left side of the neck and of the left arm just where they join together.