Figure 1: Top: photographs from experiments in which the left ventricle (left) and right ventricle (right) were dissected and treated with Lugol’s solution, which stained the Purkinje network darker than surrounding tissue. The Purkinje network is visible to the eye. The left bundle branch is visible in the center at the top of the photograph, which corresponds to the septum wall. Middle: visible Purkinje fibers are manually traced in an image-editing program; the resulting network structure is overlaid for comparison onto the photograph from which it was extracted. Left: left ventricle. Right: right ventricle. Bottom: the resulting digitized Purkinje structures for the left (left) and right (right) ventricles. Coupling sites, located at the ends of the fibers, are indicated by orange circles.