bones* -also contain another set of what are termed lymphatic capillaries, mixed up with those of the blood-vascular system, but not directly communicating with them, and, in addition, differing from the blood capillaries in being connected with larger vessels of only one kind. That is to say, they open only into trunks which carry fluid away from them, there being no large vessels which bring anything to them. These trunks further resemble the small veins in being abundantly provided with valves which freely allow of the passage of liquid from the lymphatic capillaries, but obstruct the flow of anything the other way. But the lymphatic trunks differ from the veins, in that they do not rapidly unite into larger and larger trunks, which present a continually increasing calibre and allow of a flow without interruption to the heart. On the contrary, remaining nearly of the same size, they, at intervals, enter and ramify in rounded bodies called lymphatic glands, whence new lymphatic trunks arise (Fig. 8). In these glands the lymphatic capillaries and passages are closely interlaced with blood capillaries. Sooner or later, however, the great majority of the smaller lymphatic trunks pour their contents into a tube, which is about as large as a crow-quill, lies in front of the backbone, and is called the thoracic duct. This opens at the root of the neck into the conjoined trunks of the great veins which bring back the blood from the left side of the head and the left arm (Fig. 9). The remaining lymphatics are connected by a common canal with the corresponding vein on the right side. * It is probable that these exceptions are apparent rather than real, but the question is not yet satisfactorily decided. The Thoracic Duct occupies the middle of the figure. It lies upon the spinal column, at the sides of which are seen portions of the ribs. At the bottom of the figure the psoas muscles appear. a, The receptacle of the chyle; b, the trunk of the thoracic duct, opening at c into the junction of the left jugular (f) and subclavian (g) veins as they unite into the left innominate vein; e, the right innominate vein formed by the union of the left jugular and subclavian veins; d, lymphatic glands placed in the lumbar and intercostal regions; hh, the cut oesophagus. Two veins are seen running alongside the lower part of the thoracic duct, and just above its middle, one (the left) crosses under the duct and joins the other. These are the azygos veins. Where the principal trunks of the lymphatic system open into the veins, valves are placed, which allow of the passage of fluid only from the lymphatic to the vein. Thus the lymphatic vessels are, as it were, a part of the venous system, though, by reason of these valves, the fluid which is contained in the veins cannot get into the lymphatics. On the other hand, every facility is afforded for the passage into the veins of the fluid contained in the lymphatics. Indeed, in consequence of the numerous valves in the lymphatics, every pressure on, and contraction of, their walls, not being able to send the fluid backward, must drive it more or less forward, towards the veins. 6. The lower part of the thoracic duct is dilated, and is termed the receptacle, or cistern, of the chyle (a, Fig. 9). In fact, it receives the lymphatics of the intestines, which, though they differ in no essential respect from other lymphatics, are called lacteals, because, after a meal containing much fatty matter, they are filled with a milky fluid, which is termed the chyle. The lacteals, or lymphatics of the small intestine, not only form networks in its walls, but send blind prolongations into the little velvety processes termed villi, with which the mucous membrane of that intestine is beset (see Lesson VI.). The trunks which open into the network lie in the mesentery (or membrane which suspends the small intestine to the back wall of the abdomen), and the glands through which these trunks lead are hence termed the mesenteric glands. 7. It will now be desirable to take a general view of the arrangement of all these different vessels, and of their relations to the great central organ of the vascular system-the heart (Fig. 10). All the veins of every part of the body, except the lungs, the heart itself, and certain viscera of the abdomen, join together into larger veins, which, sooner or later, open into one of two great trunks (Fig. 10, V.C.S. V.C.I.) termed the superior and the inferior vena cava, which debouch into the upper, or broad end of the right half of the heart. All the arteries of every part of the body, except the lungs, are more or less remote branches of one great trunk—the aorta (Fig. 10, Ao.), which springs from the lower division of the left half of the heart. The arteries of the lungs are branches of a great trunk (Fig. 10, P.A.) springing from the lower division of the right side of the heart. The veins of the lungs, on the contrary, open by four trunks into the upper part of the left side of the heart (Fig. 10, P.V.). Thus the venous trunks open into the upper division of each half of the heart-those of the body in general into that of the right half; those of the lungs into the upper division of the left half; while the arterial trunks spring from the lower moieties of each half of the heart-that for the body in general from the left side, and that for the lungs from the right side. Hence it follows that the great artery of the body, and the great veins of the body, are connected with opposite sides of the heart; and the great artery of the lungs and the great veins of the lungs also with opposite sides of that organ. On the other hand, the veins of the body open into the same side of the heart as the artery of the lungs, and the veins of the lungs open into the same side of the heart as the artery of the body. |