Anatomists and histologists have more perfectly demonstrated the muscle fibers of the heart and the structure at and around the valves; the physiologic chemists have shown more clearly the action of drugs, metals and organic solutions on the heart; and the physiologists and clinicians with laboratory facilities have demonstrated by various new apparatus the action of the heart and the circulatory power under various conditions. It is not now sufficient to state that the heart is acting irregularly, or that the pulse is irregular; the endeavor should be to determine whit causes the irregularity, and what kind of irregularity is present. CLINICAL INTERPRETATION OF PULSE TRACINGS A moment may be spent on clinical interpretation of pulse tracings. It has recently been shown that the permanently irregular pulse is due to fibrillary contraction, or really auricular fibrillation--in other words, irregular stimuli proceeding from the auricle--and that such an irregular pulse is not due to disturbance at the auriculoventricular node, as believed a short time ago. These little irregular stimuli proceeding from the auricle reach the auriculoventricular node and are transmitted to the ventricle as rapidly as the ventricle is able to react. Such rapid stimuli may soon cause death; or, if for any reason, medicinal or otherwise, the ventricle becomes indifferent to these stimuli, it may not take note of more than a certain portion of the stimuli. It then acts slowly enough to allow prolongation of life, and even considerable activity. If such a heart becomes more rapid from such stimuli, 110 or more, for any length of time, the condition becomes very serious. Digitalis in such a condition is, of course, of supreme value on account of its ability to slow the heart. Such irregularity perhaps most frequently occurs with valvular disease, especially mitral stenosis and in the muscular degenerations of senility, as fibrosis. Atropin has been used to differentiate functional heart block from that produced by a lesion. Hart [Footnote: Hart: Am. Jour. Med. Sc., 1915, cxlix, 62.] has used atropin in three different types of heart block. In the first the heart block is induced by digitalis. This was entirely removed by atropin. In the second type, where there was normal auricular activity, but where the ventricular contractions were decreased, atropin affected an increase in the number of ventricular contractions, but did not completely remove the heart block. He adopted atropin where the heart block was associated with auricular fibrillation. The number of ventricular contractions was increased, but not enough to indicate the complete removal of the heart block.