The Thoracic Duct occupies the middle of the figure. It lies upon the spinal column, at the sides cf which are seen portions of the ribs (1). 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, which has been cut across to show the thoracic duct running behind it; d, lymphatic glands placed in the lumbar regions; h, the superior vena cava formed by the junction cf the right and left innominate veins.

arise (Fig. 5). 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. 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 in one direction only, viz. 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 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

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; A2. arteries to the lower part of the body; H.A. hepatic artery, which supplies the liver with part of its blood; V1. veins of the upper

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.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. 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,

part of the body; V. veins of the lower part of the body; V.P. vena porta; H.V. hepatic vein; VC.I. inferior vena cava; V.C.S. superior vena cava; R.A. rght 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 blocd have dark contours, while those which carry venous blood have light

contours.

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;