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Chapter 315
The Lymphatic System
The lymphatic system (Systema lymphaticum) is subsidiary to the venous part of the circulatory system, from which it arises in the embryo. It consists of the lymph vessels and glands.
The lymph vessels (Vasa lymphatica) contain a colorless fluid, the lymph, which contains numerous lymphocytes. They resemble the veins in structure but have thinner walls and are provided with more numerous valves. The vessels are sacculated opposite the segments of the valves and have a characteristic beaded appearance when distended. The collecting lymph vessels do not usually form rich plexuses, as veins often do, their branching is more limited and less tree-like than that of the blood-vessels, and their caliber therefore increases less from the periphery toward their termination. All of the lymph is ultimately carried into the venous system by two trunks, the thoracic duct and the right lymphatic duct. Almost all of the lymph passes through at least one group of lymph glands before entering the blood-vascular system.
The lymph glands or nodes (Lymphoglandulæ) are intercalated in the course of the lymph vessels. They vary widely in size, some being microscopic, others several inches in length. In form they may be globular, ovoid and flattened, elongated, or irregular. In certain situations they are aggregated into groups, and a knowledge of the position of these and the territory drained into them is important. It is convenient, when possible, to indicate their position with regard to arteries on the course of which they are placed. In color they are usually gray or yellowish-brown in the dead subject, pink or reddish-brown during life, but this varies according to their position and functional state. The bronchial glands are often blackened by infiltration with carbon. The mesenteric glands are creamy or white while the chyle is passing through, but pink at other times. Vessels which carry lymph to a gland are called afferent; the efferent vessels which convey it away are larger and fewer. Each gland has a depression, the hilus, at which the blood-vessels enter and the efferent lymph vessels emerge.
Lymph nodules or follicles (Noduli lymphatici) are minute masses of lymphoid tissue which occur in certain mucous membranes. They may be solitary, as in the solitary glands of the intestine, or aggregated into masses or patches, as in the tonsils and the so-called Peyer’s patches.
The lymph nodule or follicle is the unit of structure of the lymph gland. It consists essentially of an artery surrounded by a reticulum of connective tissue, the meshes of which contain numerous lymphocytes. Surrounding this is a rich plexus of lymph vessels, forming the so-called sinus, inclosed in some cases by a fibrous capsule. The gland consists of a mass of follicles, inclosed in a fibrous capsule, from which trabeculæ pass in and unite the follicles. Beneath the capsule is the peripheral sinus, which consists of a very rich plexus of lymph vessels; to this the afferent vessels pass at various points of the surface. In the cortical substance the cells are in rounded masses, the cortical nodules, while in the medullary substance they lie around the arteries, forming the so-called medullary cords. The medulla is redder than the cortex, since it is more vascular; it contains the central lymph sinuses, which have a similar structure to the peripheral sinus.
The hæmolymph glands differ from the lymph glands in color and structure. They are of a deep red color, which is due to the high vascularity of the cortical substance. The peripheral sinuses especially are greatly developed and contain numerous red blood-cells. There is no clear division into cortical and medullary substance, and the trabeculæ contain smooth muscle-cells. Some have afferent and efferent lymph vessels and others do not. They resemble the spleen in some respects, but their significance is not yet clear. They are numerous in the ox and sheep, much fewer in the dog, and apparently are absent in the horse. They occur along the course of the aorta, in the perineal fat, at the portal fissure, and with the gastric and mesenteric lymph glands. In the ox they are also found under the trapezius muscle, under the skin of the upper part of the flank, and in other places less constantly.
The tissue or lymph-spaces are interstices of varying size between cells or in the meshes of connective tissue. They contain a fluid derived from the blood-plasma, which is usually called lymph. They are drained by the veins and lymph vessels. The large serous sacs are often included in this category.
The exact relationship between the lymphatic capillaries and the tissue spaces is still a matter of controversy. It is held by some that the lymph vessels are in direct communication with the tissue spaces, while others maintain that the lymphatics are complete closed tubes. Communication between the spaces and vessels is in general very free. Mall has shown that granules injected into the hepatic artery are returned by the lymphatics as well as by the veins, and intramuscular injections will enter the lymph vessels of the tendon in spite of the absence of lymphatics in muscle.