The heart of man and other warm-blooded animals may be said to be made up of two muscular sacs, the pulmonary and systemic pumps, or, as they are commonly termed, the right and left sides of the heart; between these no communication exists after birth. Each of these sacs may be divided into two chambers - one, acting as an ante-chamber, receives the blood from the veins; it has very thin walls and is called the auricle; the other, the ventricle, is the powerful muscular chamber which pumps the blood into and distends the arteries. (Figs. 113 and 114).

In the empty heart the great mass of the organ, which forms a blunted cone, is made up of the ventricles, while the flaccid auricles are found retracted to an insignificant size at its base. The four cavities have the same capacity, namely, about six ounces or eight cubic inches when distended.

The walls of both the auricles are about the same thickness, while the amount of muscle in the walls of the ventricles differs materially. The wall of the left ventricle, including that part which forms the inter-ventricular septum, is nearly three times as thick as that of the right or pulmonary ventricle.

Interior of Right Auricle and Ventricle exposed by the removal of a part of their walls.

Fig. 113. Interior of Right Auricle and Ventricle exposed by the removal of a part of their walls.

(A/len Thomson).

1. Superior vena cava. 2. Inferior vena cava. 2'. Hepatic veins. 3, 3', 3". Inner wall of right auricle. 4, 4. Cavity of right ventricle. 4'. Papillary muscle. 5, 5', 5". Flaps of tricuspid valve. 6. Pulmonary artery, in the Wall of which a window has been cut. 7. On aorta near the ductus arteriosus. 8,9. Aorta and its branches. 10, 11. Left auricle and ventricle.