_The Valves._--The opening from the auricle into the ventricle on either side is guarded on the ventral side by a valve formed of folds of endocardium. The valve on the right side has three flaps or cusps and is called the _tricuspid valve_, while that on the left has two flaps, larger and thicker than those of the tricuspid, and is known as the _bicuspid_ or _mitral valve_. The flaps of either valve are kept from being forced into the auricle in closing by fine tendinous cords, the _chordæ tendineæ_, which are attached to the _columnæ carneæ_, muscular bands or columns projecting from the walls of the ventricle, which contract and hold the chordæ tendineæ taut. The opening into the pulmonary artery is from the posterior part of the right ventricle and is guarded by the _semilunar_ or _pulmonary valve_, while the aortic opening from the left ventricle is guarded by a similar valve, the _aortic valve_, the most important valve in the body. All these valves are planned primarily to prevent regurgitation of the blood during contraction of the heart muscle. Pressure in the ventricle must exceed that in the arteries before the semilunar valves will open and the blood can be driven out, just as the auriculo-ventricular valves remain closed until the pressure in the auricles exceeds that in the ventricles. The _heart beat_ is caused by the twisting of the heart upon its axis during contraction of the muscle. Normally it beats rhythmically and regularly, whatever a person does, at a rate of about seventy-two contractions to the minute in the adult. To the regular _cardiac cycle_, as it is called, there are two periods, the _systole_ and the _diastole_, the former representing the period of contraction of the ventricles, when the blood is sent to the lungs and over the body, and the latter representing the period of rest following the emptying of the ventricles, during which they are refilled. Contraction of the heart occupies one-fifth of the time of one beat, dilatation two-fifths, and the pause two-fifths. There are really two systoles, one of the auricles and one of the ventricles, but they come so close together that they are practically simultaneous so far as sound is concerned, though they can be distinguished by sight. During systole the tricuspid and mitral valves close sharply to prevent regurgitation into the auricles, while the semilunar valves open to let the blood out. The cardiac cycle is, therefore, as follows: [Illustration: FIG. 42.--Diagram of the circulation. (After Kirke.)]