"When viewed from the right side of the heart, the bundle can not be seen, because it is covered by the mesial leaflet of the tricuspid valve, whose line of attachment passes obliquely over the membranous septum. Then, if the endocardium is removed from the posterior part of the septum of the auricle up to the membranous septum, the posterior part of the auriculoventricular bundle will be exposed. If, in addition, the membranous septum be removed, the bundle may be traced from the point to which it could be followed when viewed from the left side as it passes posteriorly over the muscular septum. In the region of the auriculoventricular junction it loses its compactness, the fibers divide, and the bundle seems to fork. One branch passes into the superficial part of the valve musculature which descends from the auricles, and the other branch passes directly into the musculature of the auricle. "Briefly, the auriculoventricular bundle runs posteriorly in the septum of the ventricles about 10 mm. below the posterior leaflet of the aortic semilunar valves; with a gentle curve it passes posteriorly just over the upper edge of the muscular septum and sends its fibers into the musculature of the right auricle and of the auricular valves. In the heart of the adult the bundle is 18 mm. long, 2.5 mm. wide, and 1.5 mm. thick." (Erlanger.) All normal impulses have their origin in the sino-auricular node at the junction of the superior vena cava with the right auricle (Fig. 50). From there the impulse travels in the wall of the auricle in the interauricular septum to the node of Tawara or A-V node (Fig. 51), thence through the bundle of His to be distributed to the fibers of the right and left ventricles. This sequence is orderly and perfectly regular. [Illustration: Fig. 50.--Showing alternating periods of sinus rhythm and auriculoventricular rhythm. (After Eyster and Evans.)] [Illustration: Fig. 51.--Period of auriculoventricular or "nodal" rhythm following exercise in sitting posture. (After Eyster and Evans.)] It has also been shown that the independent auricular and ventricular rates vary somewhat, that of the auricle being in general faster than that of the ventricle. A strip of mammalian ventricle placed outside of the body in proper surroundings will begin to beat automatically at the rate of about 40 beats a minute. Experimentally various grades of heart block have been produced in the dog's heart by more or less compression of the bundle at the A-V ring. The block may be partial, when two to nine auricular beats occur to every one of the ventricle, up to absolute complete block when the auricles and ventricles beat independently of one another.