282. In the accomplishment of this object the heart performs a twofold action; that of contraction and that of dilatation. The auricles contract and thereby diminish their cavities, then dilate and thereby expand them, and the one action alternates with the other. There is the like alternate contraction and dilatation of the ventricles. The first action is termed systole, the second diastole, and both are performed with force. 283. When the heart is laid open to view in a living animal, and its movements are carefully observed, it is apparent that the two auricles contract together; that the two ventricles contract together; that these motions alternate with each other, and that they proceed in regular succession. The interval between these alternate movements is, however, exceedingly short, and can scarcely be perceived when the heart is acting with full vigour; but it is evident when its action is somewhat languid. 284. When the ventricles contract, the apex of the heart is drawn upwards, and at the same time raised or tilted forwards. It is during this systole of the ventricles, and in consequence of this result of their action, that the apex of the heart gives that impulse against the walls of the chest which is felt in the natural state between the fifth and sixth ribs, and which just perceptibly precedes the pulse at the wrist. 285. When the ear is applied to the human chest, over the situation of the heart, a dull and somewhat prolonged sound is heard, which precedes and accompanies the impulse of the heart against the chest. This dull sound is immediately succeeded by a shorter and sharper sound: after this there is a short pause; and then the dull sound and impulse are again renewed. The duller sound and stronger impulse are ascribed to the contraction of the ventricles, and the sharper sound and feebler impulse to that of the auricles. 286. The movement of the heart is effected by the contraction of its muscular fibres. Those fibres rest, as upon a firm support, on the tendinous matter to which they are attached, from which they diverge, and towards which their contraction must necessarily bring all the parts of the heart (267). The result of their contraction is the powerful compression of all the chambers of the heart, and thereby the forcible ejection of their contents through the natural openings.