There is also a physiological factor of primary importance which modifies the strength of contraction. Briefly it can be stated that, other conditions being equal, the greater the filling of the heart during diastole, the stronger is the following systole. This dependence of strength of contraction is so marked and is of such importance for the whole circulation that Starling named it the "Law of the Heart." It must be remembered, however, that it is not a feature peculiar to the cardiac muscle alone; it belongs to all contractile tissues, whether heart, skeletal muscle or plain muscle, but in the heart it is of a greater immediate vital importance. The practical significance is obvious; it enables the heart to eject the amount of blood which it receives during diastole whether small or large, and thus enables the heart to adapt its beat to considerable variations of the blood flow without chang ing its rate. A dog's heart weighing 5o grams, can put out ioo or 3,00o cc. of blood per minute without changes of the heart rate.
This remarkable adaptation is due to the fact that the larger the output, the more the heart will be filled during each period of relaxation (diastole), and hence its contraction (systole) will be stronger, so that the heart will empty itself of the extra amount of blood. When a heart is in good condition, it performs a given amount of work with a smaller diastolic volume than when the contractility of the heart becomes impaired. The same work may be performed in both cases, but in the second case in order to do the work the heart has to dilate, because at the previous diastolic volume its contractions would be too weak. The dila tion of the heart at constant work is thus the first sign of impair ment of its functions.
The apparatus almost universally used for this purpose is known as the optical manometer. It consists of a small glass or metal tube which can be introduced directly into one of the cavities of the heart. The tube is filled with fluid, and the outside opening is sealed with a thin rubber membrane, which carries an excentrally placed small and light splinter of a mirror. A beam of light re flected from the mirror is made to play on a moving photographic plate or film, and thus the minute movements of the membrane, which are of course proportional to the pressure changes, are greatly magnified without the danger of increasing the inertia of the apparatus.
The pressure changes in the heart (fig. 5) have been described by Starling as follows:— "The cardiac cycle begins with the contraction of the auricles, which may or may not give rise to a slight rise of pressure in the ventricles. As the auricular contraction dies away, the ventricular contraction begins at I. This causes a very rapid rise of pressure. Almost immediately after the beginning of the rise, the auriculo ventricular valves close. The pressure then rises rapidly in the ventricular cavity. During this period, the contraction of the ventricular muscle is isometric. It is raising the pressure within the ventricles without causing any change in its contents, or in the length of the muscle fibres. Directly the pressure exceeds that in the aorta, the aortic valves open at the point marked II., and the aortic pressure thereafter rises with the ventricular pressure. During the whole duration of the ventricular contraction, the aortic pressure remains somewhat below the ventricular pres sure, showing that the blood is flowing continuously from the ventricle into the aorta. The ejection of blood is at first rapid, so that the pressure in the ventricles continues to rise. As the heart gets smaller, the amount of blood ejected into the aorta becomes less than that flowing out in the unit of time through the peripheral branches, so that the pressure begins to fall in the aorta and ventricle, even though the outflow of blood is still going on.