The concept of re-entry was first demonstrated by several investigators at the beginning of the 20th century who studied strips of atrial and ventricular muscles of the frog heart and the circular mantle of jelly fish.
To visualize re-entry, picture a ring of homogeneous excitable tissue, such as ventricular myocardium, with a stimulating electrode at the top of the ring. A stimulus will induce wave fronts traveling in both a clockwise and counterclockwise direction. These wave fronts will collide and extinguish themselves at the bottom of the ring because the tissue ahead of both wave fronts, having just been excited, will be refractory.