In the past few years an immense amount of work has been done by numerous observers on the changes in the electrical potential of the various portions of the heart during contraction. The very elaborate and delicate electrocardiograph with the string galvanometer devised by Einthoven is used. It has been definitely determined that the impulse to cardiac contraction originates in the sinus node, a collection of differentiated nerve cells situated at the junction of the superior vena cava with the right auricle. From there the impulse travels in certain fibers in the interauricular wall, passes through another node, the auriculoventricular or Tawara node, situated in the auricular wall just above the auriculoventricular ring, thence via the Y-bundle, or bundle of His to the ventricles. This sequence is orderly, regular, and normally invariable. (Fig. 38.) [Illustration: Fig. 38.--Right side of the heart showing diagrammatically the distribution of the two vagus nerves to different parts of the viscus. The impulse to contraction originates at the sino-auricular node and passes over the wall of the auricle to Tawara's node, and thence over His' bundle across the auriculoventricular septum to be distributed throughout the ventricular wall. If the upper, sino-auricular, node is damaged, or if its impulses fail to get across the wall of the auricle, Tawara's node acts in its place to start off the ventricle. If a lesion at the base of the mesial segment of the tricuspid valve damages His' bundle, so that Tawara's node is cut off from the ventricle, then the ventricle may originate its own impulses to contraction. (Hare's Practice of Medicine.)] The sino-auricular (s-a) node is the most irritable portion of the heart, it is endowed with the greatest amount of rhythmicity as well. It is under the control of the vagus nerve. Its inherent rate of rhythmicity is probably more rapid than the usual numbers of impulses per minute, but it is inhibited by the vagus. Paralysis of the vagus endings increases the rate of impulse formation and therefore the rate of the heart. The electrocardiogram is a graphic representation on a photographic film or sensitive bromide paper of the changes of electrical potential during muscular activity. The lines are made by the highly magnified string of the galvanometer as it moves across the slit in the photographic apparatus in response to the induction currents set up in the heart magnified by the special galvanometer. The record is made in three so-called Leads. Lead I The electrodes are attached to right arm and left arm. Lead II The electrodes are attached to right arm and left leg. Lead III The electrodes are attached to left arm and left leg.