Surface topology and re-entry. (A) Uninterrupted plane (outer-boundary only). (B) Interrupted plane (disconnected inner-boundary). (C) Several time-steps after a lesion has connected wave-tip to tissue-edge (eliminating inner-boundary). Note in (A) with nothing to divide activation a single wave traverses the plane (and will extinguish at the right edge); in (B) functional block at the wave-tip (star) causes interruption of the plane, creating a circuit and allowing re-entry. In (C) a linear lesion, by connecting the edge to the inner-boundary, converts the surface topology into an uninterrupted (though geometrically more convoluted) plane and increases the total boundary length and length-to-area ratio. (A) (uninterrupted plane) and (C) (wave-tip connected to outer edge) are homomorphic, whereas (A) and (B), and (B) and (C), are heteromorphic.