If the thorax of a recently killed frog be opened, the heart can be observed beating in situ, and the different acts in the cycle studied without difficulty.

In mammalians, in order to see the heart in operation, it is necessary to keep up artificial respiration, during which the heart continues to beat regularly, though the thorax be opened. A careful inspection of the beating heart shows that during its cycle of action certain changes take place in the shape and relative position of its cavities. This is owing partly to the change in the amount of their blood contents and partly to the form assumed by the muscular wall when contracting.

During the passive interval the auricles are seen to swell gradually on account of the blood flowing into them from the veins: when the auricular cavities are nearly full, a contraction, commencing in the great venous trunks near the heart, passes with increasing force over the auricles and gives rise to their rapid systolic spasm. The auricles suddenly diminish in size, and appear to become pale. When the blood is being propelled through the auriculo-ventricular openings, the flaccid walls of the ventricles appear to be drawn over the liquid mass by the contraction of the muscular walls of the auricles (just as a stocking is drawn over the foot by the hands), and the base of the ventricles is thus drawn upward. The moment the ventricles have received their full charge of blood from the auricles they contract, becoming shorter by the movement of the base toward the apex, and thicker by their elongated cone becoming rounder. The great arteries are at the same time distended with the blood from the ventricle and elongated, their elastic walls being drawn down over the liquid wedge. The soft elastic tissues are thus in turn made to slide, as it were, over the incompressible fluid that forms the fulcrum, which the muscular walls use as a purchase.

During the systole, when the thorax is open, the ventricles rotate slightly on their long axis, so that the left comes a little forward, and the apex also forward and toward the right. When the systole of the ventricles ceases, they become flaccid and flattened, and the gradual refilling of the cavities begins, as there is nothing to prevent the blood flowing from the veins through the auricles into the ventricles, where the pressure, as in all parts of the thorax, is negative. The semilunar valves being closed, the large arteries grasp firmly the blood, and by their steady resilient pressure force it on toward the distal vessels. During this pause the arteries seem to become shorter and to draw the base of the heart up again by lengthening the flaccid ventricles.

The part of the heart which changes its position most is the line between the auricles and ventricles, while the apex remains fixed in one position, only making a very slight lateral and forward motion, which probably does not take place within the thorax. If a thin needle with a straw attached be made to enter the apex through the wall of the chest, the straw does not move in any definite direction during the systole, but simply shakes.

Cardiac Tambour, which can be strapped on to chest wall, so that the central button lies over the heart beat.

Fig. 120. Cardiac Tambour, which can be strapped on to chest wall, so that the central button lies over the heart beat, and the pressure may be regulated by the screws at the side. To the tube bent at right angles is attached the rubber tube which connects the air cavity with that of the writing tambour shown in Fig. 119.

If, on the other hand, the needle be placed in the base of the ventricles, the straw moves up and down with each systole and diastole.