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

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. II and 12, I 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, áre 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 tendinea, 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.

something like a bishop's mitre, and hence bear the name of the mitral valve.

The edges and apices of the valves are not completely free and loose. On the contrary, a number of fine, but strong, tendinous cords, called chorda tendineæ, connect them with some column-like elevations of the fleshy substance of the walls of the ventricle, which are termed papillary muscles (Figs. 11 and 12, pp); similar columnlike elevations of the walls of the ventricles, but having no chorda tendineæ attached to them, are called columnæ

carnea.

It follows, from this arrangement, that the valves oppose no obstacle to the passage of fluid from the auricles to the ventricles; but if any should be forced the other way, it will at once get between the valve and the wall of the heart, and drive the valve backwards and upwards. Partly because they soon meet in the middle and oppose one another's action, and partly because the chorda tendineæ hold their edges and prevent them from going back too far, the valves, thus forced back, give rise to the formation of a complete transverse partition between the ventricle and the auricle, through which no fluid can pass.

Where the aorta opens into the next ventricle and where the pulmonary artery opens into the right ventricle, another valvular apparatus is placed, consisting in each case of three pouch-like valves called the semilunar valves (Fig. 11, S.V.; Figs. 13 and 14, Ao. P.A.), which are similar to those of the veins. Since they are placed on the same level and meet in the middle line, they completely stop the passage when any fluid is forced along the artery towards the heart. On the other hand, these valves flap back and allow any fluid to pass from the heart into the artery, with the utmost readiness:

The action of the auriculo-ventricular valves may be demonstrated with great ease on a sheep's heart, in which the aorta and pulmonary artery have been tied and the greater part of the auricles cut away, by pouring water into the ventricles through the auriculo-ventricular aperture. The tricuspid and mitral valves then usually become closed by the upward pressure of the water which gets behind them. Or, if the ventricles be nearly