The lymph which enters the lymphatic capillaries passes over into collecting vessels (ducts), which carry it through the lymph-glands (nodes) to the large veins at the base of the neck. The lymph-vessels course in the loose subcutaneous tissues, in the connective tissues between muscles and organs, often accompanying the arteries and veins, sometimes forming networks around them. An idea of their arrangement can be best obtained by glancing at the illustrations of the lymphatics of special regions.
In general they are made up of numerous long, narrow vessels, rarely more than half or three-fourths of a millimeter in diameter, which occasionally communicate with one another, and which radiate toward groups of lymph-glands placed in certain definite regions. In the lymph-glands the afferent lymph-vessels break up into capillaries, which again collect into efferent vessels. Several of these efferents from each lymph-gland may pass to a second lymph-gland, where they undergo a second widening into capillaries. In this way the lymph, passing through one, two, three or more lymph-nodes in succession, eventually reaches the thoracic duct, or one of the short ducts, all of which empty into the large veins at the base of the neck. The thoracic duct, which receives, at its lower end, the lymph from the lower half of the body, is the only lymphatic vessel which attains any considerable size (four to six millimeters in diameter) and is usually the only one large enough to be seen readily without injection.
In structure the lymphatic vessels much resemble the veins. They possess an intima, a media and an adventitia, although the line of demarcation between the different layers is not sharp. In the thoracic duct, the endothelium of the intima is succeeded by a delicate layer of fibers, mainly elastic; outside of this is the media, made up mainly of circular smooth muscle cells, interspersed with elastic and connective-tissue fibers; then follows a layer of coarse elastic and connective-tissue fibers, which is succeeded by the adventitia, containing longitudinal and transverse bundles of smooth muscle-cells, as well as blood-vessels and nerves. The other lymphatic vessels possess the three layers, which, however, toward the capillaries, grow thinner, and eventually reach a stage in which, outside the endothelium, there are found only single muscle cells, or muscle cells in groups of two or three.
The lymphatic vessels are characterized by their great richness in valves, which are present throughout their entire course, from their beginnings in the capillary region to their openings into the veins of the neck. The valves are bi- or tricuspid, and are always arranged so as to prevent the flow of lymph back to the capillaries. They thus aid indirectly in the movement of the lymph, in that any external pressure on the vessels must always force the lymph onward.
Nerves of lymphatic vessels
That the thoracic duct and the smaller lymphatic vessels are provided with nerves has been shown by several observers. According to Kytmanoff (in dogs) the nerves to the lymphatics are mainly non-medullated, and are both motor and sensory. They form four sets of plexuses - adventitial, supramuscular, intermuscular and subendothelial. Sensory nerve-endings are found in adventitia and media, in the form of free-ending threads, and bush-like endings. Motor endings are present in connection with the smooth muscle cells of the media. In the intima there is a plexus of extremely fine varicose threads. The physiological action of the nerves supplying the receptaculum chyli has been tested by Camus and Gley who found in dogs a dilatation of the receptaculum as the result of electrical stimulation of the splanchnic nerve.
Movement of the lymph
It has been estimated (Ludwig) that the amount of lymph which passes through the lymphatic ducts of a dog aggregates, during the twenty-four hours, one-third the body-weight. In the thoracic duct the lymph is under a sufficient pressure to burst the duct behind a ligature. In the absence of any especial propulsive organ, such as the heart for the blood-circulation, what are the forces which move the lymph? There must be recognized primary and accessory forces. As accessory forces there are the movement of the muscles and the general pressure of the organs on the lymph-ducts. Since these are provided with valves, all preventing the lymph from flowing backward, any such pressure causes the lymph to move onward. As accessory agents must also be reckoned the smooth muscle and elastic tissue which is present in the walls of the lymph-vessels and in the lymph-gland. That these forces, however, are not primary is shown by numerous facts. There is an active circulation in the lymphatics of embryos long before valves develop. In many lower animals no valves develop save at the entrance of the lymphatics to the veins. That neither valves nor muscular movements are essential is shown by the fact that, in the tails of frog larvae, where no valves are present and where the muscle movements have been completely paralyzed by an anesthetic, the circulation of lymph continues unchecked.
The primary cause, therefore, for the movement of lymph is to be sought in the capillary region, in the force produced by the passage of lymph through the endothelial wall, whether this process be a filtration and diffusion -in which case the causes would he in the pressure and molecular condition of the tissue fluid outside the lymphatic - or whether it be an active secretion by the endothelium - in which case the driving force would be this secretory power of the endothelium.
From Morris's treatise on anatomy.