cycle. Taking 75 as the average number of heart-beats per minute,
each cardiac cycle will occupy ·8 seconds. Of this period
| auricular systole occupies | ·1 second |
| auricular diastole occupies | ·7 ,, |
| ventricular systole occupies | ·3 ,, |
| ventricular diastole occupies | ·5 ,, |
In 1861 Chauveau and Marey obtained direct records of the heart of a horse, and determined the sequence and duration of the events happening in the heart, and measured the endo-cardiac pressure by an instrument termed the cardiac sound. The sound—a two-way tube—was pushed down the jugular vein until the orifice of one tube lay in the right ventricle and of the other in the right auricle. The tubes were connected with recording tambours which wrote on a moving drum covered with smoked paper.
Another tambour was used to record the cardiac impulse. The tracings so obtained (fig. 10) teach us the following facts: (1) The auricular contraction is less sudden than the ventricular, and lasts only a very short time, as indicated by the line ab. The ventricle, on the other hand, contracts suddenly and forcibly and remains contracted a considerable time, as shown by the line c′d ′ and by the flat top to the curve which succeeds d ′. (2) The auricular movement precedes the ventricular, and the latter coincides with the impulse of the apex against the wall of the chest. (3) The contraction of the auricle influences the pressure in the ventricle as shown by the small rise a′b′, and that of the ventricle influences the pressure in the auricle somewhat as shown by the waves cd. Much labour has been spent in the contrivance of rapidly acting spring pressure gauges, freed as far as possible from inertia, in order to investigate more exactly the changes of intracardiac pressure, which were first described by Chauveau and Marey.
Fig. 10.—Tracings from the Heart of a Horse, by Chauveau and Marey. The upper tracing is from the right auricle, the middle from the right ventricle, and the lowest from the apex of the heart. The horizontal lines represent time, and the vertical amount of pressure. The vertical dotted lines mark coincident points in the three movements. The breadth of one of the small squares represents one-tenth of a second.
As the intraventricular pressure may rise 150 mm. of mercury in one-tenth of a second, it is no easy matter to contrive an instrument which will respond as rapidly and yet yield an accurate result without overshooting the mark. The final result of a most careful inquiry is the confirmation in almost every point of Chauveau and Marey’s pressure curves. Karl Hürthles differential manometer has proved to be an instrument of great value and precision. A double-bored tube cannula is introduced so that one tube reaches the right auricle and the other the right ventricle. In observations on the left side of the heart, one tube is placed in the left ventricle and the other in the aorta, and each of these tubes is brought into connexion with a tambour. The two tambours are placed one on either side of the fulcrum of a lever. This lever works against a light spring, which in its turn sets in motion a writing-style. The style records the pressure changes on a drum covered with smoked paper. By this, means there can be recorded the exact moment at which the auricular pressure exceeds that in the ventricle, that is to say, the moment when the auriculo-ventricular valves open; likewise the moment when the ventricular pressure becomes greater than that in the auricles, and the auriculo-ventricular valves shut. Similarly, there can be recorded the moment when the intraventricular pressure exceeds that in the aorta and the semilunar valves open, and the moment at which the diastole of the ventricle begins, when the aortic pressure becomes the greater, and the semilunar valves shut. The smoothness with which the heart works is shown by the fact that neither the opening nor the closing of the valves is marked by any peak or point on the pressure curves.
The absence of a mechanism for preventing regurgitation of blood from the auricles-of birds and mammals is remarkable, for in fishes, amphibia and reptiles this is effected by valves guarding the sino-auricular junction. In the warm-blooded vertebrata with the appearance of the diaphragm the sinus becomes merged into the right auricle, and the venous cistern formed by the superior and inferior venae cavae, the innominate, iliac, hepatic and renal veins takes the place of the sinus. Six pairs of valves prevent regurgitation from this cistern, viz. those placed in the common femoral, the sub-clavian and jugular veins. The cistern when filled holds some 400 c.c. of blood; in the liver there is some 500 c.c. of blood, and this can be expressed into the cistern by abdominal pressure; in the portal venous system, when distended, another 500 c.c. may be held, which can be expressed through the liver into the cistern. A large volume of blood is thus at the disposal of the heart for it to draw on during diastole. Respiration by the aspirating action of the thorax sucks this blood into the heart, while the inspiratory descent of the diaphragm squeezes the abdominal contents and forces blood from the liver and cistern into the heart. These forces take the place of the sinus and are far more efficient. The intra-abdominal pressure may be raised on bending or straining till it becomes equivalent to the pressure of a column of mercury 80-100 mm. high (Keith). Under such conditions the pericardium prevents the right side of the heart being over-distended with venous blood.
A. Keith, in Journal of Anatomy and Physiology.
Fig. 11.—Diagram of the Venous Cistern from which the Heart is filled. The abdominal or infra diaphragmatic part of the cistern is indicated in black; the thoracic or supra-diaphragmatic is stippled.
With these facts in view, we can now describe the complete course of a cardiac cycle. We will start at the moment when the blood is pouring from the venae cavae and pulmonary veins into the two auricles. The auricles are relaxed and their cavities open into the ventricles by the funnel shaped apertures formed by the dependent segments of the tricuspid and mitral valves. The blood passes freely through these apertures into the ventricles.
From Diseases of the Heart, by James Mackenzie, M.D., by permission.
Fig. 12.—Tracings of the Jugular Pulse Apex Beat, Carotid and Radial Pulses. The perpendicular lines represent the time of the following events.:—1, the beginning of the auricular systole; 2, the beginning of ventricular systole; 3, the appearance of the pulse in the carotid; 4, the appearance of the pulse in the radial; 5, the closing of the semilunar valves; 6, the opening of the tricuspid valves.
The small positive pressure which is always present in the venous cistern (aided by the respiratory forces) is at this time filling the right heart, while the positive pressure in the pulmonary veins is filling the left heart. The auricular systole now takes place. The circular muscle bands compress the blood out of the auricles into the ventricles, while the longitudinal bands aid in this and pull up the base of the ventricles to meet the load of blood. As the contraction starts from the mouths of the venae cavae, and sweeps towards the ventricles, there can
