The lymphatic system is composed of lymph vessels, lymph nodes, and organs. The functions of this system include the absorbtion of excess fluid and its return to the blood stream, absorption of fat (in the villi of the small intestine) and the immune system function.

Lymph vessels are closely associated with the circulatory system vessels. Larger lymph vessels are similar to veins. Lymph capillaries are scatted throughout the body. Contraction of skeletal muscle causes movement of the lymph fluid through valves.

Lymph organs include the bone marrow, lymph nodes, spleen, and thymus. Bone marrow contains tissue that produces lymphocytes. B-lymphocytes (B-cells) mature in the bone marrow. T-lymphocytes (T-cells) mature in the thymus gland. Other blood cells such as monocytes and leukocytes are produced in the bone marrow. Lymph nodes are areas of concentrated lymphocytes and macrophages along the lymphatic veins. The spleen is similar to the lymph node except that it is larger and filled with blood. The spleen serves as a reservoir for blood, and filters or purifies the blood and lymph fluid that flows through it. If the spleen is damaged or removed, the individual is more susceptible to infections. The thymus secretes a hormone, thymosin, that causes pre-T-cells to mature (in the thymus) into T-cells.

THE LYMPHATIC SYSTEM consists (1) of complex capillary networks which collect the lymph in the various organs and tissues; (2) of an elaborate system of collecting vessels which conduct the lymph from the capillaries to the large veins of the neck at the junction of the internal jugular and subclavian veins, where the lymph is poured into the blood stream; and (3) lymph glands or nodes which are interspaced in the pathways of the collecting vessels filtering the lymph as it passes through them and contributing lymphocytes to it. The lymphatic capillaries and collecting vessels are lined throughout by a continuous layer of endothelial cells, forming thus a closed system. The lymphatic vessels of the small intestine receive the special designation of lacteals or chyliferous vessels; they differ in no respect from the lymphatic vessels generally excepting that during the process of digestion they contain a milk-white fluid, the chyle.

The Development of the Lymphatic Vessels.—The lymphatic system begins as a series of sacs 108 at the points of junction of certain of the embryonic veins. These lymph-sacs are developed by the confluence of numerous venous capillaries, which at first lose their connections with the venous system, but subsequently, on the formation of the sacs, regain them. The lymphatic system is therefore developmentally an offshoot of the venous system, and the lining walls of its vessels are always endothelial.

    In the human embryo the lymph sacs from which the lymphatic vessels are derived are six in number; two paired, the jugular and the posterior lymph-sacs; and two unpaired, the retroperitoneal and the cisterna chyli. In lower mammals an additional pair, subclavian, is present, but in the human embryo these are merely extensions of the jugular sacs.

    The position of the sacs is as follows: (1) jugular sac, the first to appear, at the junction of the subclavian vein with the primitive jugular; (2) posterior sac, at the junction of the iliac vein with the cardinal; (3) retroperitoneal, in the root of the mesentery near the suprarenal glands; (4) cisterna chyli, opposite the third and fourth lumbar vertebr? . From the lymph-sacs the lymphatic vessels bud out along fixed lines corresponding more or less closely to the course of the embryonic bloodvessels. Both in the body-wall and in the wall of the intestine, the deeper plexuses are the first to be developed; by continued growth of these the vessels in the superficial layers are gradually formed. The thoracic duct is probably formed from anastomosing outgrowths from the jugular sac and cisterna chyli. At its connection with the cisterna chyli it is at first double, but the two vessels soon join.

    All the lymph-sacs except the cisterna chyli are, at a later stage, divided up by slender connective tissue bridges and transformed into groups of lymph glands. The lower portion of the cisterna chyli is similarly converted, but its upper portion remains as the adult cisterna.

Lymphatic Capillaries — The complex capillary plexuses which consist of a single layer of thin flat endothelial cells lie in the connective-tissue spaces in the various regions of the body to which they are distributed and are bathed by the intercellular tissue fluids. Two views are at present held as to the mode in which the lymph is formed: one being by the physical processes of filtration, diffusion, and osmosis, and the other, that in addition to these physical processes the endothelial cells have an active secretory function. The colorless liquid lymph has about the same composition as the blood plasma. It contains many lymphocytes and frequently red blood corpuscles. Granules and bacteria are also taken up by the lymph from the connective-tissue spaces, partly by the action of lymphocytes which pass into the lymph between the endothelial cells and partly by the direct passage of the granules through the endothelial cells.

    The lymphatic capillary plexuses vary greatly in form; the anastomoses are usually numerous; blind ends or cul-de-sacs are especially common in the intestinal villi, the dermal papill? and the filiform papill? of the tongue. The plexuses are often in two layers: a superficial and a deep, the superficial being of smaller caliber than the deep. The caliber, however, varies greatly in a given plexus from a few micromillimeters to one millimeter. The capillaries are without valves.

Distribution — The Skin.—Lymphatic capillaries are abundant in the dermis where they form superficial and deep plexuses, the former sending blind ends into the dermal papill?. The plexuses are especially rich over the palmar surface of the hands and fingers and over the plantar surface of the feet and toes. The epidermis is without capillaries. The conjunctiva has an especially rich plexus.

    The subcutaneous tissue is without capillaries.

    The tendons of striated muscle and muscle sheaths are richly supplied. In muscle, however, their existence is still disputed.

    The periosteum of bone is richly supplied and they have been described in the Haversian canals. They are absent in cartilage and probably in bone marrow.

    The joint capsules are richly supplied with lymphatic capillaries, they do not, however, open into the joint cavities.

    Beneath the mesothelium lining of the pleural, peritoneal and pericardial cavities are rich plexuses; they do not open into these cavities.

    The alimentary canal is supplied with rich plexuses beneath the epithelium, often as a superficial plexus in the mucosa and a deeper submucosal plexus. Cul-de-sacs extend into the filiform papill? of the tongue and the villi of the small intestine. Those portions of the alimentary canal covered by peritoneum, have in addition a subserous lymphatic capillary plexus beneath the mesothelium.

Lymph Glands — The lymph glands are small oval or bean-shaped bodies, situated in the course of lymphatic and lacteal vessels so that the lymph and chyle pass through them on their way to the blood. Each generally presents on one side a slight depression—the hilus—through which the bloodvessels enter and leave the interior. The efferent lymphatic vessel also emerges from the gland at this spot, while the afferent vessels enter the organ at different parts of the periphery. On section a lymph gland displays two different structures: an external, of lighter color—the cortical; and an internal, darker—the medullary. The cortical structure does not form a complete investment, but is deficient at the hilus, where the medullary portion reaches the surface of the gland; so that the efferent vessel is derived directly from the medullary structures, while the afferent vessels empty themselves into the cortical substance.

Lymph — Lymph, found only in the closed lymphatic vessels, is a transparent, colorless, or slightly yellow, watery fluid of specific gravity about 1.015; it closely resembles the blood plasma, but is more dilute. When it is examined under the microscope, leucocytes of the lymphocyte class are found floating in the transparent fluid; they are always increased in number after the passage of the lymph through lymphoid tissue, as in lymph glands. Lymph should be distinguished from "tissue fluid" 109 which is found outside the lymphatic vessels in the tissue spaces.