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.' 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 which issue from or enter it-and for the most part pass

D.V. dorsal vertebra, or joint of the backbone; Ao. Ao. 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.

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.

4, mass of

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

D

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 direct communication with one another. Each of these two great cavities is further subdivided, not longitudinally

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; VS. 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.

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 I capable of containing from 4 to 6 cubic inches of water.

FIG. 11.-RIGHT SIDE OF THE HEART OF A SHEEP.

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.. 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.

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 not smooth, nucleated bands, like those of the vessels, but are bundles of transversely-striped fibres, and resemble those of the chief muscles of the body, except that they have no sheath, or sarcolemma, such as we shall find to exist in the latter.

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, t v, m v). Each is a broad, thin, but very tough and strong triangular fold of the 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 something like a bishop's mitre, and hence bear the name of the mitral valve.

FIG. 12.-LEFT SIDE OF THE HEART OF A SHEEP (LAID OPEN). P.V. pulmonary veins opening into the left auricle by four openings, as shown by the styles: a, a style passed from auricle into ventricle through the auriculo-ventricular orifice; b, a style passed into the coronary vein, which, though it has no connection with the left auricle, is, from its position, necessarily cut across in thus laying open the auricle.

M.V. the two flaps of the mitral valve (drawn somewhat diagrammatically): Pp, papillary muscles, belonging as before to the part of the ventricle cut away; c, a style passed from ventricle in Ao. aorta; Ao1. branch of aorta (see Fig. 8, A'o'); P.A. pulmonary artery; S.V.C. superior vena cava. 1, wall of ventricle cut across; 2, wall of auricle cut away around auriculoventricular orifice; 3, other portions of auricular wall cut across; 4, mass of fat around base of ventricle (see Fig. 8, 2).