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 cave, but pours the blood which it contains directly into the division of the heart into which these venæ cavæ open-that is to say, into the right upper division (Fig. 14 b).

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

L. lung; Tr. trachea. 1, solid cord often present, the remnant of a once open communication between the pulmonary artery and aorta. 2, masses of fat at the bases of the ventricle hiding from view the greater part of the auricles. 3, line of fat marking the division between the two ventricles.

fat covering end of trachea.

D

4, mass of

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 both the heart and the adherent half of the bag. Between the two layers of the pericardium, consequently, there is a completely closed, narrow cavity, lined by an epithelium, and secreting into its interior a small quantity of clear fluid.1

FIG. 9.-TRANSVERSE SECTION OF THE CHEST, WITH THE HEART AND LUNGS IN PLACE. (A little diagrammatic.)

D.V. dorsal vertebra, or joint of the backbone; Ao. Ad. aorta, the top of its arch being cut away in this section; S.C. superior vena cava; P.A. pulmonary artery, divided into a branch for each lung; L.P. R.P. left and right pulmonary veins; Br. bronchi; R.L. L.L. right and left lungs; E. the gullet or oesophagus; p. outer bag of pericardium; pl. the two layers of pleura; v. azygos vein.

The outer layer of the pericardium is firmly connected below with the upper surface of the diaphragm.

But the heart cannot be said to depend altogether upon the diaphragm for support, inasmuch as the great vessels

This fluid, like that contained in the peritoneum, pleura, and other shut sacs of a similar character to the pericardium, used to be called serum; whence the membranes forming the walls of these sacs are frequently termed serous membranes.

which issue from or enter it-and for the most part pass upwards from its base-help to suspend and keep it in place.

Thus the heart is coated, outside, by one layer of the pericardium. Inside, it contains two great cavities or "divisions," as they have been termed above, completely separated by a fixed partition which extends from the base to the apex of the heart; and consequently, having no

FIG. 10.-THE HEART, GREAT VESSELS, AND LUNGS. (FRONT VIEW.) R.V. right ventricle; L.V. left ventricle; R.A. right auricle; L.A. left auricle; Ao. aorta; P.A. pulmonary artery; P.V. pulmonary veins; R.L. right lung; L.L. left lung; V.S. vena cava superior; S.C. subclavian vessels; C. carotids; R.J.V. and L.J.V. right and left jugular veins; V.I. vena cava inferior; T. trachea; B. bronchi.

All the great vessels but those of the lungs are cut.

direct communication with one another. Each of these two great cavities is further subdivided, not longitudinally but transversely, by a moveable partition. The cavity above the transverse partition on each side is called the auricle; the cavity below, the ventricle-right or left as the case may be.

Each of the four cavities has the same capacity, and is capable of containing from 4 to 6 cubic inches of water. The walls of the auricles are much thinner than those of the ventricles. The wall of the left ventricle is much thicker than that of the right ventricle; but no such difference is perceptible between the two auricles (Figs. 11 and 12, 1 and 3).

9. In fact, as we shall see, the ventricles have more work to do than the auricles, and the left ventricle more to do than the right. Hence the ventricles have more muscular substance than the auricles, and the left ventricle than the right; and it is this excess of muscular substance which gives rise to the excess of thickness observed in the left ventricle.

The muscular fibres of the heart are of a peculiar nature, resembling those of the chief muscles of the body in being transversely striped (see Lesson XII.), but differing from them in many other respects.

Almost the whole mass of the heart is made up of these muscular fibres, which have a very remarkable and complex arrangement. There is, however, an internal membranous and epithelial lining, called the endocardium; and at the junction between the auricles and ventricles, the apertures of communication between their cavities, called the auriculo-ventricular apertures, are strengthened by fibrous rings. To these rings the moveable partitions, or valves, between the auricles and ventricles, the arrangement of which must next be considered, are attached.

10. There are three of these partitions attached to the circumference of the right auriculo-ventricular aperture, and two to that of the left (Figs. 11, 12, 13, 14, tv, m v). Each is a broad, thin, but very tough and strong triangular fold of connective tissue (see Lesson XII.) covered by endocardium, attached by its base, which joins on to its fellow, to the auriculo-ventricular fibrous ring, and hanging with its point downwards into the ventricular cavity. On the right side there are, therefore, three of these broad, pointed membranes, whence the whole apparatus is called the tricuspid valve. On the left side, there are but two, which, when detached from all their connexions but the auriculo-ventricular ring, look

R.A. cavity of right auricle; S.V.C. superior vena cava; I.V.C. inferior vena cava; (a style has been passed through each of these ;) a, a style passed from the auricle to the ventricle through the auriculo-ventricular orifice; b, a style passed into the coronary vein. R.V. cavity of right ventricle; tv, tv, two flaps of the tricuspid valve: the third is dimly seen behind them, the style a passing between the three. Between the two flaps, and attached to them by chorda tendineæ, is seen a papillary muscle, pp, cut away from its attachment to that portion of the wall of the ventricle which has been removed. Above, the ventricle terminates somewhat like a funnel in the pulmonary artery, P.A. One of the pockets of the semilunar valve, sv, is seen in its entirety, another partially. 1, the wall of the ventricle cut across; 2, the position of the auriculoventricular ring; 3, the wall of the auricle; 4, masses of fat lodged between the auricle and pulmonary artery.