lungs on either side are enclosed in a separate and perfect bag, anil each lung has a distinct pleura. The heart lies under the left lung; and, more perfectly to cut off all injurious connexion or communication of disease between the lungs and the heart, the heart is enclosed in a distinct pleura or bag, termed the
pericardium
. This membrane closely invests the heart, supports it in its situation, prevents too great dilatation when it is gorged with blood, and too violent action when it is sometimes unduly stimulated. Notwithstanding the confinement of the pericardium, the heart, when under circumstances of unusual excitation, beats violently against the ribs, and, were it not thus tied down, would often bruise and injure itself, and cause inflammation in the neighbouring parts.
heart
is composed of four cavities; two above, called
auricles
, from their shape, and two below, termed
ventricles