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In five dogs (18 to 28 kg) under pentobarbital anesthesia metal clips were placed on the free margin of the anterior and posterior mitral valve leaflets, and on the epicardial surface of the left ventricle at the apex and at the base. After full recovery cineangiograms were taken in the right anterior oblique and right lateral position and the length from the apex of the left ventricle to the free margins of the mitral valve leaflet was measured. The average distance between the apex and the free margin of the anterior leaflet in two dogs and on a clip placed on the chordae tendineae above the anterior leaflet in two dogs was the same during slow ventricular filling, atrial systole, and during ventricular ejection. The average distance between the apex and the free margin of the posterior leaflet in three dogs was also the same during the same time periods in the cardiac cycle. The maintenance of the same distance between the free margin of the mitral valve leaflet and the apex during diastole when the papillary muscle is relaxed and during systole when it is contracted suggests that the papillary muscle contracts isometrically.

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