(From lympha, and duco, to carry). Vasa lymphatica. Lymphatic vessels. The lymphatics arise from, the cells of the membrana cellu-laris, the cavities of the intestines, of the urine and gall bladders, and of every other viscus, carrying a pellucid liquor towards the receptaculum chyli and thoracic duct, in which they almost exclusively terminate. The coats of these vessels are thin and transparent, much crowded with valves, so as, like the lactcals, to resemble, when injected with quicksilver, strings of beads. The lymphatics frequently anastomose, and in their way pass through the lymphatic glands, ramifying before they enter a gland, and uniting in their passage from it. See Lactea vasa, Ductus thoracicus.

The course of the lymph and of the chyle is from the extreme parts of the body towards the centre; and the lymphatics cornmonly lie close to the large blood vessels of the extremities. All the lacteals, and most of the lymphatics, open into the thoracic duct, which lies upon the spine, and runs up towards the neck, where it commonly opens into the angle between the jugular and subclavian veins of the left side; and thus both the chyle and the lymph are gradually mixed with the blood.

The coats of these vessels are thinner and more pellucid than those of the blood vessels, but stronger; for they can support the weight of quicksilver, which will rupture the coats of even the arteries. The internal coat is smooth, dense, and highly polished, projectingby little duplicatures into the cavity of the vessel forming the valves. The second coat consists chiefly of muscular fibres, running in every possible direction; but usually in a circular one, surrounding the internal membrane. The outward coat is similar to the pleura, or peritonaeum.

The coats of the lacteal and lymphatic vessels have, in common with all other parts of the body, arteries and veins for their nourishment, and nerves for their animation: from the blood vessels running through them they, are subject to inflammation, and, from their numerous nerves, they are more irritable than any other vessels in the human body. Their valves are two in number, of a semilunar shape; and are so frequently interposed, that three or four pair may be sometimes found in the space of one inch. They are occasionally fewer, and in some parts wholly wanting.

The lymphatics, like the lacteals, open into the cavities, and draw in the various fluids which these contain by a capillary attraction. It is probable, as we have said, that they convey fluids only, or solid substances very minutely divided. That they carry the bony matter we know, from the fact recorded by Mr. Ches-ton, where, in a case of mollities ossium, the thoracic duct was filled with an osseous matter. Bones are not, however, absorbed so rapidly as the frequently quoted experiment, with madder, would lead us to believe; for it is now found, that, though the colouring part of madder has a considerable affinity to the phosphat of lime, of which the bones consist, it has greater affinity to the serum of the blood. In such experiments the colouring matter is, therefore, only deposited, and again washed away, without any other corresponding change on the earthy salt. The fluids, when once absorbed, are carried, by the action of the vessel, or by the pressure of the adjoining muscles, beyond the first pair of valves; and, by the frequent recurrence of these valves, every action assists the progress of the fluid, since regurgitation is prevented. It is by no means certain that all the lymphatics pass into the thoracic duct. Some trunks have been discovered which escape it, and there is rather a probability that lymphatics occasionally terminate in veins farther distant from the heart.

Lymphatics, as well as lacteals, are not always excited to action: in other words, their extremities are not erected so as to become capillary tubes, a circumstance depending on a variety of causes, of which we can perceive with distinctness only general debdity, or a sufficient supply already in the system. We have had occasion also to suggest, that an elective affinity seems to influence the admission of some fluids, and the rejection of others. Perhaps sedatives may, for a time, paralyze the sensible orifices of the lacteals; stimulants excite them too violently, or astringents contract them too much. This may be deemed conjectural; but some facts might be adduced in favour of each supposition. The lymphatic vessels of the lower extremities are the superficial, or those more deeply seated. The former lie between the skin and the muscles, and are connected with the surface, and the cellular membrane, which lies immediately under it, absorbing fluids from each; one branch of the superficial lymphatics runs upon the top of the foot, another generally under the inner ankle. The branch on the foot runs up on the outside of the tendon of the tibialis anticus, until it rises above the ankle; and running over the shin bone, it divides and forms a plexus, still ascending in the cellular membrane to the inside of the knee, from whence it advances up the inside of the thigh under the skin, and, arriving at the groin enters the lymphatic glands. These glands are seven or eight in number, some of which lie in the angle between the thigh and the abdomen, and others a little below on the fore part of the thigh. Into these upper glands only lymphatic vessels of the genitals enter, so that the venereal bubo, which arises in consequence of an absorption of matter from these organs, is always seated in these; and the lower glands are never affected, except from their vicinity to the glands first diseased. As the upper glands are affected from the genitals, so the lower are usually inflamed from the absorption of acrid matter in the parts below them. The lymphatic vessels of the genitals having joined those of the thigh, a net work is formed, which enters the abdomen under the edge of the tendon of the external oblique muscle, called Poupart's ligament: some branches of this plexus embrace the iliac artery. As no considerable branches can be distinguished on the outside of the leg or thigh, it is probable that all the lymphatic vessels of those parts bend towards the inside. Upon these superficial vessels, from the foot to the groin, there is scarcely in any instance one gland. Besides the superficial lymphatic vessels which lie above all the muscles, or in the cellular membrane under the skin, there is some seated amongst the muscles, accompanying the crural artery. Of these the principal trunk can be discovered by cutting down to the posterior tibial artery, near the inner ankle. From this part the vessel passes up with the posterior tibial artery, and is hid amongst the muscles on the back part of the tibia. About the middle of the leg it enters a small gland met with in most subjects, and from hence runs up to the back part of the ham, still lying close to the artery, and in the ham it usually passes through three glands. After it has passed these glands, this single vessel commonly divides into two or three branches, which still accompany the crural artery, and pass with it through the perforation in the triceps. Having passed the muscle, they go up with the artery and enter a gland deeper seated than those which appear on the groin, from which they pass in to the superficial gland.