|Henry Gray (18251861). Anatomy of the Human Body. 1918.|
| The kidney is supplied with a coarse subserous plexus and a deeper plexus of finer capillaries in the capsule. Lymphatics have been described within the substance of the kidney surrounding the tubules.|
| The urinary bladder has a rich plexus of lymphatic capillaries just beneath the epithelial lining, also a subserous set which anastomoses with the former through the muscle layer. The submucous plexus is continuous with the submucous plexus of the urethra.|
| The prostate has a rich lymphatic plexus surrounding the gland and a wide-meshed subcapsular plexus.|
| The testis has a rich superficial plexus beneath the tunica albuginea. The presence of deep lymphatics is disputed.|
| The uterus is provided with a subserous plexus, the deeper lymphatics are uncertain. Subepithelial plexuses are found in the vagina.|
| The ovary has a rich superficial plexus and a deep interstitial plexus.|
| The heart has a rich subserous plexus beneath the epicardium. Lymphatic capillaries have also been described beneath the endocardium and throughout the muscle.|
| Lymphatic capillaries are probably absent in the central nervous system, the meninges, the eyeball (except the conjunctiva), the orbit, the internal ear, within striated muscle, the liver lobule, the spleen pulp and kidney parenchyma. They are entirely absent in cartilage. In many places further investigation is needed.|
Lymphatic Vessels.The lymphatic vessels are exceedingly delicate, and their coats are so transparent that the fluid they contain is readily seen through them. They are interrupted at intervals by constrictions, which give them a knotted or beaded appearance; these constrictions correspond to the situations of valves in their interior. Lymphatic vessels have been found in nearly every texture and organ of the body which contains bloodvessels. Such non-vascular structures as cartilage, the nails, cuticle, and hair have none, but with these exceptions it is probable that eventually all parts will be found to be permeated by these vessels.
Structure of Lymphatic Vessels.The larger lymphatic vessels are each composed of three coats. The internal coat is thin, transparent, slightly elastic, and consists of a layer of elongated endothelial cells with wavy margins by which the contiguous cells are dovetailed into one another; the cells are supported on an elastic membrane. The middle coat is composed of smooth muscular and fine elastic fibers, disposed in a transverse direction. The external coat consists of connective tissue, intermixed with smooth muscular fibers longitudinally or obliquely disposed; it forms a protective covering to the other coats, and serves to connect the vessel with the neighboring structures. In the smaller vessels there are no muscular or elastic fibers, and the wall consists only of a connective-tissue coat, lined by endothelium. The thoracic duct has a more complex structure than the other lymphatic vessels; it presents a distinct subendothelial layer of branched corpuscles, similar to that found in the arteries; in the middle coat there is, in addition to the muscular and elastic fibers, a layer of connective tissue with its fibers arranged longitudinally. The lymphatic vessels are supplied by nutrient vessels, which are distributed to their outer and middle coats; and here also have been traced many non-medullated nerves in the form of a fine plexus of fibrils.
| The valves of the lymphatic vessels are formed of thin layers of fibrous tissue covered on both surfaces by endothelium which presents the same arrangement as on the valves of veins (p. 501). In form the valves are semilunar; they are attached by their convex edges to the wall of the vessel, the concave edges being free and directed along the course of the contained current. Usually two such valves, of equal size, are found opposite one another; but occasionally exceptions occur, especially at or near the anastomoses of lymphatic vessels. Thus, one valve may be of small size and the other increased in proportion.|
| In the lymphatic vessels the valves are placed at much shorter intervals than in the veins. They are most numerous near the lymph glands, and are found more frequently in the lymphatic vessels of the neck and upper extremity than in those of the lower extremity. The wall of the lymphatic vessel immediately above the point of attachment of each segment of a valve is expanded into a pouch or sinus which gives to these vessels, when distended, the knotted or beaded appearance already referred to. Valves are wanting in the vessels composing the plexiform net-work in which the lymphatic vessels usually originate on the surface of the body.|