The Cavities.—The heart contains four cavities, two [auricles] above and two [ventricles] below, with a longitudinal septum between the auricle and ventricle on the right and those on the left. The posterior surface is largely made up of the left ventricle and the anterior of the right ventricle. The right auricle, which receives the blood from the general circulation, has a capacity of about two fluid ounces and is larger than the left, which receives the blood returning from the lungs, though its walls are thinner. Of the ventricles the left is the larger and its walls are about three times as thick as those of the right, for it has to send the blood all over the body. All the cavities are lined with smooth, transparent, serous membrane, the endocardium, which is continuous with the intima of the great vessels.
Fig. 41.—Orifices of the heart, seen from above, both the auricles and the great vessels being removed: PA, Pulmonary artery and its semilunar valves; Ao, aorta and its valves; RAV, tricuspid, and LAV, bicuspid valves; mv, segments of mitral valve; lv, segment of tricuspid valve. (Huxley.)
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:
Fig. 42.—Diagram of the circulation.
(After Kirke.)
Circulation.—The blood, after it has given off its oxygen and collected carbon dioxide, returns to the heart through two main channels, the superior and inferior venæ cavæ, the former bringing the blood from the upper part of the body, including the head, neck, and arms, and the latter from the lower part below the diaphragm. The two vessels empty along with the coronary sinus, which is guarded by the coronary valve, into the right auricle. At the same time that they empty into this auricle the four [pulmonary veins], the only veins that carry arterial or oxygenated blood, are emptying the fresh blood from the lungs into the left auricle. When both auricles are full, they contract and send the blood into the ventricles, the auricular systole. As the blood comes through into the ventricles it probably comes around by the walls and closes the auriculo-ventricular valves, though just how the valves close is not certain. When the two ventricles are full they in turn contract, the ventricular systole, and the blood is forced out, that in the right ventricle passing to the lungs for its new supply of oxygen through the [pulmonary artery], the only artery to carry venous blood, and that from the left ventricle entering the aorta for general distribution through the body. Following the systole is a pause, the diastole, while the heart fills again.
Fig. 43.—The fetal circulation.