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a cellular element found in the conduction system of ventricular myocardium; described in 1845 by J. E. Purkinje. Purkinje fibers are particularly large in ruminants. They have been studied mainly in mammals but appear to exist in other vertebrates as well. Although they are of muscular origin, Purkinje fibers have largely lost their contractile function and have become specialized in the conduction of excitation to contractile elements of the myocardium; this accounts for their meager number of myofibrils. Purkinje fibers are usually thicker than the contractile fibers of the myocardium and are rich in mitochondria and glycogen.