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.