In those vessels which have their origin in the wall of the small intestines, the contained fluid has, especially during digestion, a more or less milky appearance, owing to the lymphocytes being loaded with particles of fat which they have taken up from the intestinal contents. On this account, these vessels are usually spoken of as lacteals, although it must be recognized that they are merely portions of the general lymphatic system. In certain respects the vessels of the system strongly resemble the veins. They arise from a capillary network, their walls have a structure closely resembling that of the veins, they are abundantly supplied with valves, and it may be said that the fluid which they contain flows from the tissues towards the subclavian veins. With these similarities there are combined marked differences. One of the most important of these consists in the fact that the capillaries of the lymphatics are closed and do not communicate with any other set of vessels as the venous capillaries do with the arterial; and another important difference is to be found in the frequent occurrence upon the lymphatic vessels of characteristic enlargements, the so-called lymphatic nodes or glands, quite different from anything occurring in connection with the veins. Throughout the body spaces of varying size are found, containing a clear, more or less watery fluid, which are called lymph spaces. These spaces do not communicate with the capillaries of the lymphatics, but are in such close relationship with them that the fluid easily finds its way into the lymph capillaries by osmosis, absorption, lymphocytes going out into these spaces and returning filled with the lymph fluid. The lymphatic capillaries, which are arranged in the form of networks of very different degrees of fineness and complexity, closely resemble in structure the blood capillaries, their walls consisting of a single layer of endothelial cells. They differ from those of the blood vascular system not only in their ultimate branches being closed, but also in their general appearance. They are of greater caliber.