Cardiac muscle. Light micrograph of a longitudinal section through cardiac (heart) muscle. The muscle cells each contain one or two elongated nuclei (for example the purple rectangle at upper left) and an extensive, branching cytoplasm (pink), giving the appearance of a continuous network of tissue. Junctions between cells in this network are known as intercalated discs (narrow vertical pink lines). Here, the electrical resistance of the tissue is low, allowing the rapid spread of electrical signals from the heart's pacemaker throughout the muscle. Haematoxylin and eosin stained. Magnification: x 300 at 6x7cm x150 at 35mm.