The muscle tissue of the heart differs both in structure and mode of action from the other contractile tissues of the body. The elements are firmly united with one another to form irregular close networks, which, however, can be broken up into masses easily recognizable as peculiar cells. These cells are irregular, prismoidal blocks, the blunt ends of which are often split, allowing connection with two contiguous cells. They contain a nucleus, situated in the central axis of the cell. The cells are not surrounded by a distinct sheath of sarcolemma.

Striated Muscle Tissue of the Heart, showing the trelliswork formed by the short branching cells, with central nuclei.

Fig.115. Striated Muscle Tissue of the Heart, showing the trelliswork formed by the short branching cells, with central nuclei.

Though striated, the action of the heart muscle is peculiarly independent of the higher nervous centres, being quite involuntary; it is characterized by a definite periodicity and is incapable of tetanus. The duration of its contraction is very long when compared with that of the skeletal muscles, but is much shorter than that of the contracting tissues of most hollow viscera.