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Figure 4 Selected segment of polygraph tracings of February 27, 1882, from Figure 4, Plate III of study by Gaskell (13), from the right atrium and ventricle of a tortoise with our laddergram added, showing in panel I a heart rate of 7.5 beats/min and stable 1:1 AV conduction. In panel II, recorded after cutting deeply into the right atrium, 2:1 AV block develops; the As pulse rate remains 7.5 beats/min, the Av and V pulse rates are 3.75 beats/min, and alternate Av pulse waves are absent. The laddergram depicts 1:1 AV conduction in panel I and 2:1 AV conduction in panel II. On the atrial tracing, numbers 1 through 9 represent sequential As pulse waves, and the numbered Vs under the V pulse waves refer to connected V waves; Av pulse waves follow odd numbered As pulse waves but are absent after each even numbered As pulse wave. In panels I and II, As, Av and V under the tracing are printing by Gaskell at the end of the respective pulse waves. On the ventricular tracings, the numbered Vs represent sequential ventricular pulse waves and correspond to the numbered Vs of the atrial tracing. On the laddergram, numbers 1 through 9 above the top line refer to sequential As pulse waves and correspond to the numbers above the atrial tracing. A = atrial pulse wave; AV = interval between the atrial and ventricular pulse waves; V = ventricular pulse wave; see Figure 3 for other abbreviations. Time line above the laddergram has notches at 2 s intervals; arrows point in the direction of impulse propagation; paper speed by our measurement is 1.15 mm/s. The figure has been lightly retouched for clarity.