All contractile tissues that exhibit an inherent rhythmicity in their performance present the problem of where and how the muscular movements are initiated and what physiologic device facilitates the orderly sequence of contractile changes. This well regulated progression of performance is exemplified in the action of the heart, in which the beat of one part is followed under precisely defined conditions by the contraction of another chamber. Even slight disturbance of this rhythmic program is indicative of a pathologic condition. Modern study has directed attention to the so-called pacemaker of the heart. The cardiac organs of the lower forms exhibit a series of different chambers, beginning with the sinus venosus, into which the great veins open and from which the excitation wave spreads. In the human and other mammalian hearts, a remnant of the sinus venosus is found in the right auricle at the junction of the superior vena cava.