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Figure 3 Appearance and measurements of the tortoise heart when excised and suspended, February 27, 1882 (13). (I) Measurements before Gaskell began the experiment. (II) Measurements after experimental cutting of the right atrium by Gaskell, dividing it into two parts, As and Av. We measured pulse rates, cardiac intervals, atrial and ventricular pulse wave durations and ventricular pulse wave amplitudes from the atrial and ventricular tracings of Figure 4 before and after the experiment. After cutting deeply into the right atrium, 2:1 AV block develops between As and Av. The As pulse rate remains 7.5 beats/min, but the Av and V pulse rates fall to 3.75 beats/min. The V pulse wave amplitude and duration increase with the ventricular bradycardia due to the increased ventricular stroke volume. The As-Av interval increases 0.7 s after the atrial cut, whereas the Av-V interval increases 1.0 s and alternate Av and V pulse waves are blocked. The beginning of the As pulse wave is obscured by the V pulse wave; it is assumed that the As pulse wave duration is the same as the measured Av pulse wave duration. A = right atrium; As = pulse wave of the atrium near the sinus venosus; Av = pulse wave of the atrium near the AV groove; As-Av interval = time from the onset of the As pulse wave to the onset of the Av pulse wave; Av-V interval = time from the onset of the Av pulse wave to the onset of the V pulse wave; As-V interval = time from the onset of the As pulse wave to the onset of the V pulse wave. B = upper part of the tortoise body; F = forceps attached to the aorta; L = two silk ligatures with hooks at one end (one ligature is tied to the right atrium and the other is tied to the apex of the ventricle; the two hooks are attached to levers of the kymograph); S = sinus venosus; V = ventricle. The figure has been lightly retouched for clarity.