It is only by means of these valvular arrangements that the heart is enabled to perform its function of pumping the blood in a constant direction onward to empty the veins and fill the arteries.

This pumping is carried on by the successive contractions and relaxations of the muscular walls of the various cavities.

The blood, flowing from the systemic and pulmonary veins, passes unopposed into the right and left auricles respectively. As 23 soon as the auricles are full their walls suddenly contract and press the blood into the right and left ventricles, upon which the ventricles immediately contract, and force it into the great arteries.

The contraction of each pair of cavities is followed by their relaxation.

The blood cannot pass back into the veins from the auricles when they contract, because the auricular contraction commences in the bundles of muscular fibre which surround the orifices of the great venous trunks; and it cannot flow back to the auricles, because, as has been seen, the force of the blood current on its entry into the ventricles closes the valves; while a backward flow from the large arteries is at once prevented by the current distending the semilunar pockets, and thus firmly closing the valves.

The Orifices of the Heart seen from above, both the auricles and the great vessels being removed.

Fig. 118. The Orifices of the Heart seen from above, both the auricles and the great vessels being removed. {Huxley).

PA. Pulmonary artery and its semilunar valves. Ao. Aorta and its valves. RA V. Tricuspid, and LA V. Bicuspid valves.

When viewed for the first time, the beat of the heart appears to be a single act, so rapidly does the ventricular follow the auricular beat. More careful examination shows that this single action is composed of different phases of activity and repose, which together make up the cycle of the heart beat. The contraction of the cavities of the heart is called their systole, the period of rest is called their diastole.