Abstract

Sinoatrial (S-A) nodes and atrioventricular (A-V) nodes of 13 human hearts were examined by regular step sections and morphometry. Eleven cases were from fetuses, infants and children, and the others were from adult cases. S-A nodes were examined in 3 ways. The first one was resection, horizontal trisection and regular step sections by 100 micrometers. The second one was resection, longitudinal bisection and regular step sections, and the third one was regular step section in toto. Fifty sections were obtained from each node, and length of tissue in paraffin block was calculated from the total count of sections. Location, area and volume of S-A nodes were measured and corrected. The major sources of errors were contraction during formalin fixation, contraction during dehydration and paraffin embedding, and irregular change of length during microtoming. The latter two were corrected by use of total section count and measurement of length of paraffin on slide. The following results were obtained. 1) S-A node is located at one fourth (26.8/100) of the length of sulcus terminalis, below the summit of atrial auricle. S-A node is located at subepicardium of sulcus terminalis. (Crista terminalis is at some distance.) 2) Relative range of S-A node is 16.8/100 of the length of sulcus terminalis. And it decreases as cardiac weight increases. Relative volume of S-A node is 1/20,000 of the volume of heart. And it decreases as cardiac weight increases. 3) S-A node is located at the right atrial subendocardium, at some distance above the center of tricuspid septal leaflet. The node runs antero-inferiorly to the His bundle and pierces the cardiac skeleton.