The hearts of thirty-four apparently healthy adult rabbits were carefully studied microscopically. Twenty, or 60 per cent, contained lesions consisting of lymphocytes and endothelial leucocytes invarying proportions, to which were sometimes added polymorphonuclear eosinophils, plasma cells, and fibroblasts in small numbers. The lesions occurred most frequently between the muscle fibers of the papillary muscles and ventricular walls, and occasionally beneath the endocardium and epicardium. No microorganisms or cell inclusion bodies were found in the lesions.

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