cardiac muscle, a special striated muscle of the myocardium, containing dark intercalated disks at the junctions of abutting fibers. Cardiac muscle is an exception among involuntary muscles, which are characteristically smooth. Its contractile fibers resemble those of skeletal muscle but are only one third as large in diameter, are richer in sarcoplasm, are abundant in mitochondria, and contain centrally located nuclei. Compare smooth muscle, striated muscle.
