SYSTEM.] A N A T M 1 907 of the mucous membrane of the small intestine into the lumen of the bowel The lacteals from adjacent villi form a network in the subrnucous coat of the intestine, from which larger lacteals arise, which pierce the muscular coat, and then run between the folds of the mesentery to the posterior wall of the abdomen, where, opposite the body of the first lumbar vertebra, they join the deep lymphatic vessels of the abdomen to form the thoracic duct. The lymphatic vessels proper correspond so closely in their distribution in the extremities and in the head and neck with the veins of those parts, that a special descrip tion of their arrangement is not necessary, the more so as a general representation of these vessels is given in Plate XXI. The superficial and deep lymphatics of the lower limbs enter the abdominal cavity, and are joined by the lymphatics of the pelvis. They ascend in front of the bodies of the lumbar vertebras, join the lacteal vessels to form the thoracic duct, the place of junction being marked by a dilatation of the duct called receptacidum chyli. The Thoracic thoracic duct passes through the opening in the diaphragm which transmits the aorta, ascends in front of the bodies of the dorsal vertebrae, receives in its course the deep lymphatics of the left half of the chest, reaches the root of the neck on the left side, is joined there by the deep and superficial lymphatics of the left upper limb and left side of the head and neck, and opens into the great veins at the angle of junction between the left internal jugular and sub- clavian. This duct conveys, therefore, the chyle during digestion, and the lymph contained in the lymph-vessels below the diaphragm and in the lymph- vessels situated to the left side of the mesial plane in the parts of the body above the diaphragm. The lymph-vessels on the right side of the supra-diaphragmatic parts of the mesial plane do not join the thoracic duct, but converge to the root of the neck on the right side, where they join to form the right lym phatic duct, which opens into the angle of junction of the right internal jugular and subclavian veins. The mode of origin of the lymph-vessels has long been a vexed question amongst anatomists. The lacteal vessels were at one time supposed to arise by open mouths on the free surface of the intestinal villi, and this idea has been revived in a modified form by some recent observers, who conceive that the lacteals are continuous with a network formed by the anastomoses of processes proceeding from the deep ends of the goblet cells, the mouths of which cells open on the free surface of the villus. The lymph-vessels proper are in some localities continuous with the serous cavities (p. 848) ; in others they arise within the textures and organs. The most minute lymph-vessels, called lymph- capillaries, like the blood-capillaries, have walls formed of a single layer of elongated endothelial cells. These capil laries take their rise in the connective tissue of a part or organ, and probably spring from spaces, or juice-canals, between the bundles of the connective tissue, which bundles are invested by an endothelial layer of cells. The juice canals are, therefore, a network of minute canals, situated outside the blood-vessels, which allow the tissues to be permeated by a nutrient juice derived from the blood. In some localities, as the brain and eyeball, the blood vessels have been described as enclosed in tubular spaces, called peri-vascular canals, in which cells like the corpuscles of the lymph have been seen, and which are believed to be continuous with the lymphatic system. The researches of Ludwig and some of his pupils into the minute structure of the lachrymal gland, the glands of the skin, and the testis, have shown that lymph-capillaries lie in close rela tion to the secreting structures of these glands. The coats of the lymph-vessels resemble in structure those of the veins, but they are thinner and more trans parent. The valves are small and numerous. The lymphatic glands are small bodies, varying in size from a pea to an almond, situated in the course of the lymph-vessels in several regions of the body. They are found especially in the groin, armpit, mesentery, back of the abdomen, roots of the lungs, and side of the neck (Plate XXI.) Entering one end of each gland are lymph- vessels, named vasa afferentia, and emerging from the opposite end of the gland are the lymph-vessels named vasa efferentia. Each gland is invested by a capsule of connective tissue, which sends processes into the substance of the gland to divide it into compartments ; it consists of adenoid tissue, and the meshes of its retiform connective tissue contain multitudes of lymph corpuscles. Each -gland is permeated by a network of minute canals, which are continuous with both the vasa afferentia and efferentia; the gland, therefore, is traversed by a stream of lymph which washes the lymph corpuscles out of the meshes of the reticulum, and in this manner these corpuscles find their way into the lymph. The lymph glands are, therefore, centres of origin for the lymph corpuscles. The collections of adenoid tissue, forming tha solitary and Foyer s glands of the intestine, and found in the tonsils and other localities (p. 849), are also without doubt centres of formation for the lymph corpuscles. BLOOD-VASCULAE GLANDS. Intimately associated with Bloc the vascular system are certain organs to which the names vasc of blood-vascular glands, or glands without ducts, are g applied. These organs are the spleen, the thyroid gland, the thymus gland, the suprarenal capsules, and portions of the pituitary and pineal glands. The Spleen is situated in the cavity of the abdomen between the stomach and the diaphragm. It is invested by peritoneum, and has a fibro- elastic coat in which involuntary muscular fibre-cells are formed. This coat sends multitudes of fine trabeculse-into the interior of the organ, which subdivide it into numbers of minute compartments, in which the red, highly vascular spleen pulp is contained. This pulp consists of collections of small spherical masses of adenoid tissue, forming the Malpighian corpuscles, of the terminal branches of the splenic blood-vessels, and of the lymph-vessels, together with numerous cells, some of which are red blood corpuscles, others lymph corpuscles, others contain pigment granules or fat, others contain in their interior numerous blood cor puscles. The arteries of the spleen in part end in capil laries from which the veins arise, but more frequently they open into lacunae or blood spaces, which give origin to the veins. The Thymus gland, best seen in infancy and child hood, lies in the cavity of the thorax near the base of the heart. It consists of two lobes, each of which is composed of lobules of adenoid tissue, to which numerous lymph- vessels may be traced. In the adult it is converted into a mass of fat. The Thyroid gland is situated in the neck at the front and sides of the windpipe. It consists of multi tudes of minute closed follicles, each of which is lined by a layer of cells. The Suprarenal capsules, two in number, lie in the abdomen one above each kidney. They contain cells, some of which are arranged in columns, others in a reticulated manner, and are well provided with blood vessels, nerves, and lymphatics. Development of the Vascular System. The vascular system is formed in the middle or mesoblast layer of the early embryo. The cells of the mesoblaet lose their origi nal spherical form and become stellate , the processes of adjacent cells unite together and form a network, and the nuclei rapidly increase in numbers. The peripheral part of the protoplasm of the stellate cells differentiates into a wall of nucleated protoplasm, and forms the wall of the blood-vessels, whilst the central part of the protoplasm liquefies, and the nuclei contained in it become the blood- corpuscleg. If the vessel remains as a capillary, its wall
assumes merely the character of a single layer of endo-
