THE MOVEMENTS OF THE HEART consist of an alternate contraction and expansion. The former is called the _sys'-to-le_, and the latter the _di-as'-to-le_. During the diastole, the blood flows into the heart, to be expelled by the systole. The alternation of these movements constitutes the beating of the heart which we hear so distinctly between the fifth and sixth ribs. [Footnote: Two sounds are heard if we put our ear over the heart,--the first and longer as the blood is leading the organ, the second as it falls into the pockets of the two arteries, and the valves then striking together cause it. The first sound is mainly the noise made by the muscular tissue. During the first, the two ventricles contract; during the second the two auricles do so. The hand may feel the heart striking the ribs as it contracts,--a feeling called the impulse, or, if quicker and stronger than usual, palpitation. This is not always a sign of disease, but in hypochondriacs is often an effect of the mind on the nerves of the heart.--MAPOTHER] FIG. 38. [Illustration: _Chambers of the Heart_ A, _right ventricle;_ B, _left ventricle,_ C, _right auricle,_ D, _left auricle,_ E, _tricuspid valve,_ F, _bicuspid valve;_ G, _semilunar valves,_ H, _valve of the aorta;_ I, _inferior vena cava,_ K, _superior vena cava,_ L, L, _pulmonary veins._] THE AURICLES AND VENTRICLES--The heart is divided into four chambers. In an adult, each holds about a wineglassful. The upper ones, from appendages on the outside resembling the ears of a dog, are called _auricles (aures_, ears). are termed _ventricles_. The auricle and ventricle on each side communicate with each other, but the right and left halves of the heart are entirely distinct, and perform different offices. The left side propels the red blood; and the right, the dark. The auricles are merely reservoirs to receive the blood (the left auricle, as it filters in bright and pure from the lungs; the right, as it returns dark and foul from the tour of the body), and to furnish it to the ventricles as they need. Their work being so light, their walls are comparatively thin and weak. On the other hand, the ventricles force the blood (the left, to all parts of the body; the right, to the lungs), and are, therefore, made very strong. As the left ventricle drives the blood so much farther than the right, it is correspondingly thicker and stronger.