Cardiac or heart muscle stands, both structurally and functionally, in an intermediate position relative to the two other varieties of muscle. It is imperfectly striated and branches freely so that there is muscular continuity throughout the atria as one unit and throughout the ventricles as another. In the mammalian heart the atria are separated from the ventricles by a connective-tissue ring. Similar to smooth muscle it is involuntary and contracts rhythmically, irrespective of its extrinsic nerve supply.
