_The middle mediastinum_ is that part within the pericardium or heart sac. It is the largest space of all the mediastinal spaces. It contains the heart, the ascending aorta, the lower half of the superior vena cava, the vena azygos, the bifurcation of the trachea, the pulmonary artery, etc. The middle mediastinum is sometimes called the cardiac cavity, because it contains the heart. =The Pericardium (Heart Sac).=—The pericardium is a serous sac in which is located the heart and the commencement of the great blood vessels. Behind we find the bronchi, esophagus and descending thoracic aorta. To the sides we find the pleura, the phrenic nerve and the accompanying vessels. In front we find the sternum and the remains of the thymus gland. It is attached above to the great blood vessels and below to the diaphragm. =The Heart.=—The heart is a hollow, muscular organ of a conical (cone shaped) form, placed between the lungs and enclosed in the pericardium. The heart is placed obliquely in the chest. The base is directed upward, backward and to the right, and corresponds to the dorsal vertebrae from the fifth to the eighth inclusive. The apex is directed downward, forward and to the left and corresponds to the space between the cartilages between the fifth and sixth ribs. The exact location of the apex of the heart would be ¾-inch to the inner side, and an inch and one-half below the left nipple, or about three and one-half inches from the middle line of the sternum or breast bone. [Illustration: FIG. 19—A cross section of the heart showing valves. (Spalteholz)] The heart is placed behind the sternum, and extends about three inches to the left of the median line, and about one and one-half inches to the right, or in other words, about one-third of the heart lies to the right of the median line, and two-thirds lies to the left of the median line. The heart in the adult measures five inches in length, three and one-half inches in breadth in its broadest part, and two and one-half inches in thickness. The weight of the male heart varies from ten to twelve ounces, and that of the female from eight to ten ounces. The capacity of the ventricles of the heart averages about three and one-half ounces of blood to each ventricle, and the auricle a little less than four ounces, making the total capacity of the heart average about fifteen ounces. [Illustration: FIG. 20—The right auricle and ventricle laid open. (Gray)]