The gross and microscopic anatomy of the sinus node (SN), atrioventricular node (AVN) and specialized fibers in the atrium were studied in 19 horses and eight mules. The SN is supplied with blood by a branch from the left circumflex artery which subdivides within the node. The SN has a body and long tapering cranial and caudal crura which encircle the lateral margin of the precaval orifice. The AVN, which has no large artery consistently present within its structure, is located within the fibrous septum above the septal leaflet of the tricuspid valve and anterior to the coronary sinus.

Histologically most of the fibers in the SN and AVN are similar to those in other species. They are smaller, paler staining and much more interwoven than ordinary myocardial fibers. In the caudal crus of the SN, the fibers are clumped together with loss of individual characteristics.

Large, glycogen-rich cells morphologically similar to ventricular Purkinje fibers are found in the right atrial subendocardium. The distribution of these fibers suggest that they may be concerned with intraatrial spread of excitation. Muscular pathways between the SN and AVN are composed of ordinary myocardial fibers. The large atrial myocardial fibers do not connect directly with SN or AVN fibers.