The action potential generated in the sinoatrial node (SAN) propagates through the atria to the atrioventricular node (AVN) and then onto the ventricles. The action potential is conducted from the SAN to the AVN via two ‘internodal pathways’: posteriorly down the crista terminalis and anteriorly down the interatrial septum (Spach et al., 1971; Li et al, 2014). Although the septal pathway is the dominant (i.e. faster) pathway during normal sinus rhythm (Spach et al., 1971; Li et al, 2014), there is no anatomical evidence for it. We have now studied the interatrial septum in New Zealand White rabbits. After iodine contrast enhancement, micro-CT was used to visualise the high resolution (~20 µm spatial resolution) 3D anatomy of the right atrium between the SAN and AVN, including the interatrial septum. From this dataset, we extracted the orientation of myocytes. This showed the presence of a continuous pathway from the SAN to the AVN via the interatrial septum – the SAN in the right atrium is located next to the upper part of the crista terminalis and this is continuous with Bachmann’s bundle (which conducts the action potential to the left atrium). We now show that Bachmann’s bundle splits and one branch travels down the interatrial septum to the ‘fast pathway’ into the AV node. Along this internodal pathway, myocytes are arranged longitudinally, which means that conduction will be relatively fast (longitudinal conduction can be ~3 faster than transverse conduction). Masson’s trichrome staining confirmed the presence of the novel bundle down the interatrial septum. Immunohistochemistry showed that the myocytes making up the bundle do not express the nodal cell marker, neurofilament, indicating that the myocytes are not nodal in origin. Conduction velocity is also dependent on cell diameter and electrical coupling between cells (provided by connexins). However, the myocytes making up the bundle are similar in diameter to neighbouring less well-aligned myocytes (10.7±0.4 versus 9.7±0.9 µm, P>0.05) and expression of Cx40 and Cx43 (connexins responsible for fast conduction) is also similar. Is the longitudinal orientation of myocytes in the novel bundle sufficient to explain the importance of the septal internodal tract? Computer simulation of action potential conduction from the SAN to the AVN was carried out using the micro-CT generated structure of the right atrium. With incorporation of the novel septal bundle, simulations showed that the action potential was conducted to the region of the fast pathway of AVN via the interatrial septum as expected. In conclusion, the identified longitudinally arranged myocytes in the interatrial septum, although histochemically indifferent to the surrounding atrial myocytes, are arranged in a way that favours faster conduction of the action potential to the AVN.