Systems of The action of the heart's mechanism is subject to the absolute control of two sets of nerves. These are the cardiac branches of the two nervous systems, the cerebro-spinal and the sympathetic. They pass to the heart and terminate in its wall as ganglionic cells whose filaments in turn are distributed among the muscular fibres of the heart. The action of each of these two systems is antagonistic to that of the other. The sym pathetic system produce? acceleration of the heart beats and is therefore called the acceler ator nerve; while the influence of the cerebro spinal system is just the opposite and is named the inhibitotry nerve, as it checks the augment ing and accelerating action of its opponent.
Under normal circumstances, there exists an equilibrium in their activity, which results in the rhythmic and steady action of the heart. This consists of the simultaneous contraction of both auricles, termed the auricular systole; followed, after a slight pause, by a simultaneous contraction of both ventricles, called the ven tricular systole. A general relaxation (dias tole) of the whole heart completes the cardiac cycle. The average duration of a cycle is about eight-tenths of a second, which is divided ap proximately as follows: Auricular systole, one tenth; auricular diastole, seven-tenths; ventric ular systole, three-tenths; ventricular dias tole, five-tenths; common pause, four-tenths. The heart rests a little over nine hours out of every 24. During thisperiod of rest, the heart accomplishes for itself what it does for the rest of the body in nearly 14 hours — that is, it feeds itself. This is done by the blood passing through the coronary arteries into the heart, whence the deoxidized blood is returned through the coronary sinus into the right auri cle. The blood that feeds the tissues is called arterial blood. Its color is bright red, and it is carried in vessels known as arteries. (Vessels that carry blood from the heart are known as arteries irrespective of the kind of blood they carry and those bringing the blood to the heart are known as veins). During its travel through the system, the blood gives up its nu tritive elements and becomes dark red in color; and is then transferred, through the intermedi ary of microscopic vessels called capillaries, into another set of vessels, termed veins. The veins, through an extensive ramification of ves sels throughout the body, collect venous blood, and carry it into the right auricle through two large venous trunks called the superior and in ferior vena cava. During the auricular systole,
the venous blood is forced into the right yen icicle. When this chamber is filled, the auricu lo-ventricular bundle of His conveys the im pulse of systolic contraction to the ventricle, and the next phase of the cardiac cycle sets in. During this period the auriculo-ventricular valve (tricuspid on the right side, bicuspid on the left) closes to prevent regurgitation of the blood into the auricle. The vibrations result ing from the ventricular contraction and the forcible closure of the valves are productive of the first sound of the heart. At this time the intra-ventricular pressure is high. The closed auriculo-ventricular valves at this instant are subjected to this pressure, since they then form the roofs of the ventricles. To prevent their diverging upward into the auricles, the valves are provided with non-elastic cords (chordie tendinx), one end of which is attached to the ventricular wall and the other to the under-sur face of the valve. The contraction of the right ventricle forces the blood into the pulmonary artery, the resistance of which causes the clo sure of the semi-lunar valve guarding its ori fice. The closure gives rise to the second sound of the heart. Immediately thereafter, the ventricle relaxes and the auriculo-ventric ular valve opens; and the pressure from be hind forces the bloOd into the lung capillaries, where it becomes oxidized by the absorption of oxygen from the alveoli of the lungs. It is then propelled through the pulmonary veins into the left auricle. This constitutes the pulmonary or lesser circulation.
Governed by identically the same physical laws and accompanied by the same phenomena described in the circulation of the right heart (the same action takes place in the left heart simultaneously with the action in the right heart), the blood which is now arterial is pro pelled from the left auricle into the left ventri cle; from there into the aorta (the largest ar tery in the human body) and traversing the arterial capillary and venous channels, it finally reaches again the right auricle. Thus the greater or systemic circulation is completed.
The systemic circulation controlled by the left heart and the pulmonary circulation con trolled simultaneously by the right, together constitute the entire function of the heart.