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

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.

upwards from its base-help to suspend and keep it in place.

76

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 CT CL.J.V.

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

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.