*The Pericardium.*—Surrounding the heart is a protective covering, called the pericardium. This consists of a closed membranous sac so arranged as to form a double covering around the heart. The heart does not lie inside of the pericardial sac, as seems at first glance to be the case, but its relation to this space is like that of the hand to the inside of an empty sack which is laid around it (Fig. 14). The inner layer of the pericardium is closely attached to the heart muscle, forming for it an outside covering. The outer layer hangs loosely around the heart and is continuous with the inner layer at the top. The outer layer also connects at certain places with the membranes surrounding the lungs and is attached below to the diaphragm. Between the two layers of the pericardium is secreted a liquid which prevents friction from the movements of the heart. [Fig. 14] Fig. 14—*Diagram of section of the pericardial sac*, heart removed. _A._ Place occupied by the heart. _B._ Space inside of pericardial sac. _a._ Inner layer of pericardium and outer lining of heart. _b._ Outer layer of pericardium. _C._ Covering of lung. _D._ Diaphragm. *Cavities of the Heart.*—The heart is a hollow, muscular organ which has its interior divided by partitions into four distinct cavities. The main partition extends from top to bottom and divides the heart into two similar portions, named from their positions the right side and the left side. On each side are two cavities, the one being directly above the other. The upper cavities are called _auricles_ and the lower ones _ventricles_. To distinguish these cavities further, they are named from their positions the right auricle and the left auricle, and the right ventricle and the left ventricle (Fig. 15). The auricles on each side communicate with the ventricles below; but after birth there is no communication between the cavities on the opposite sides of the heart. All the cavities of the heart are lined with a smooth, delicate membrane, called the _endocardium_. [Fig. 15] Fig. 15—*Diagram showing plan of the heart.* 1. Semilunar valves. 2. Tricuspid valve. 3. Mitral valve. 4. Right auricle. 5. Left auricle. 6. Right ventricle. 7. Left ventricle. 8. Chordæ tendineæ. 9. Inferior vena cava. 10. Superior vena cava. 11. Pulmonary artery. 12. Aorta. 13. Pulmonary veins.