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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. 6). The remaining lymphatics are connected by a common canal with the corresponding vein on the right side.
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. 6). 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. 7).
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. 7, V.C.S. V.Č.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. 7, 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. 7, 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. 7, 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 that 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.
The arteries which open into the capillaries of the substance of the heart are called coronary arteries, and arise, like the other arteries, from the aorta, but quite close to its origin, just beyond the semilunar valves. But the coronary vein, which is formed by the union of the small veins which arise from the capillaries of the heart, does not open into either of the venæ cavæ, but pours the blood which it contains directly into the division of the heart into which these cavæ open-that is to say, into the right upper division (Fig. 14 6).
FIG. 7.-DIAGRAM OF THE HEART AND VESSELS, WITH THE COURSE OF THE CIRCULATION, VIEWED FROM BEHIND SO THAT THE PROPER LEFT OF THE OBSERVER CORRESPONDS WITH THE LEFT SIDE OF THE HEART IN THE DIAGRAM.
L.A. left auricle; L. V. left ventricle; Ao. aorta; A1. arteries to the upper part of the body; 42. arteries to the lower part of the body; H.A. hepatic artery, which supplies the liver with part of its blood; VI. veins of the upper
The abdominal viscera referred to above, the veins of which do not take the usual course, are the stomach, the intestines, the spleen, and the pancreas. These veins all combine into a single trunk, which is termed the vena porta (Fig. 7, V.P.), but this trunk does not open into the vena cava inferior. On the contrary, having reached the liver, it enters the substance of that organ, and breaks up into an immense multitude of capillaries, which ramify through the liver, and become connected with those into which the artery of the liver, called the hepatic artery (Fig. 7, H.A.), branches. From this common capillary mesh-work veins arise, and unite, at length, into a single trunk, the hepatic vein (Fig. 7, H.V.), which emerges from the liver, and opens into the inferior vena cava. The portal vein is the only great vein in the body which branches out and becomes continuous with the capillaries of an organ, like an artery.
8. The heart (Figs. 8 and 10), to which all the vessels in the body have now been directly or indirectly traced, is an organ, the size of which is usually roughly estimated as equal to that of the closed fist of the person to whom it belongs, and which has a broad end turned upwards and backwards, and rather to the right side, called its base: and a pointed end which is called its apex, turned downwards and forwards, and to the left side, so as to lie opposite the interval between the fifth and sixth ribs.
It is lodged between the lungs, nearer the front than the back wall of the chest, and is enclosed in a sort of double bag-the pericardium (Fig. 9, p.).. One-half of the double bag is closely adherent to the heart itself, forming a thin coat upon its outer surface. At the base of the heart, this half of the bag passes on to the great vessels which spring from, or open into, that organ; and becomes continuous with the other half, which loosely envelopes the heart and the adherent half of the bag. Between the two
part of the body; V2. veins of the lower part of the body; V.P. vena porta; H. V. hepatic vein; V.C.I. inferior vena cava; V.C.S. superior vena cava; R.A. right auricle; R. V. right ventricle; P.A. pulmonary artery; Lg. lung; P.V. pulmonary vein; Lct. lacteals; Ly. lymphatics; Th.D. thoracic duct; Al. alimentary canal; Lr. liver. The arrows indicate the course of the blood, lymph, and chyle. The vessels which contain arterial blood have dark contours, while those which carry venous blood have light
FIG. 8.-HEART OF SHEEP, AS SEEN AFTER REMOVAL FROM THE BODY, LYING UPON THE TWO LUNGS. THE PERICARDIUM HAS BEEN CUT AWAY, BUT NO OTHER DISSECTION MADE.
R.A Auricular appendage of right auricle; L.A. auricular appendage of left auricle; R. V. right ventricle; L.V. left ventricle; S.V.C. superior vena cava; I.V.C. inferior vena cava; P.A. pulmonary artery; Ao, aorta; A'o', innominate branch from aorta dividing into subclavian and carotid arteries;