Atrioventricular node and bundle of His in newborns and infants.   A. The landmarks for sampling the AV node in newborns are more subtle that in the adult, except for the limbus of the fossa ovalis. This photograph shows the triangle of Koch drawn in. The anatomic landmarks or this triangle are: 1) the coronary sinus and the Thebesian valve (CS) posteriorly; 2) the line of insertion of the septal leaflet of the tricuspid valve and 3) an oblique line joining these two landmarks. B. A square window has been cut, including part of the atrial septum and extending well into the interventricular septum, as shown in the photograph. C. After removal of this block of tissue, the overall structure of the heart is not destroyed and can be easily preserved for further study. D. The block of tissue removed can be measured and sliced into 3 to 5 thin segments, depending on its size. The scale shown applies to parts D and E. E. The morphology of the slices from the atrioventricular tissue block is fairly typical. The atrial septum, the tricuspid valve and the mitral valves can be easily recognized for orientation. The insertion of the mitral valve into the fibrous annulus is always more cephalad than that of the tricuspid valve. The most anterior slice should always include the fibrous portion of the interventricular septum close to the posterior cusp of the aortic valve. F. Five histologic sections are show with the typical morphology of the atrioventricular blocks of tissue. The most posterior section is shown on the left side of the image and the most anterior portion including the fibrous septum and part of the mitro-aortic continuity, is shown in the far right. The AV node is delineated by brackets. The most posterior section shows a very large mass of AV nodal tissue, which progressively decreases in size as it approaches the central fibrous body and condenses into the bundle of His (middle section). From this point forward the bundle is divided into perforating branches forming the right and left bundle branches. The last section shows the anterior fascicle of the left bundle branch. The disproportionately large size of this structure in human hearts of these age groups has been noted by James {12}.