RESUMEN
The location, shape and internal architecture of the sinoatrial node (SAN) were studied histologically in serial section in 15 hearts (7 adults, 8 foetuses). Based on our observation, the human SAN may be divided into four layers, namely, (1) thesinoatrial node artery layer: its outer longitudinal muscular layer of its media partly lacked in the SAN concentrically; (2) the periarterial layer: the cells of this layer surround the artery concentrically; (3) the central layer, making up the main mass of the node: most P cells in which are arranged parallel to the node artery. In adult the diameter of P cells in this layer is 5.18 ?m; (4) the peripheral layer: most transitional cells in this layer radiate from SAN into the atrial muscle. These "tongues of transitional cells" extend mostly into right atrial-muscle and a few into the musculature of superior vena cava. The diameter of transitional cells of this layer in the adult is 7.28 ?m. In the adult the arrangement of dense collagen fibers appears circular in the periarterial layer, longitudinal in the central layer and radiative in the peripheral layer. According to the location, layers of sinoatrial node and the arrangement of the connective tissue, the functional significance and morphological basis of the re-entry are discussed.