Transmission electron micrograph of longitudinal section of rat heart cardiac muscle from the atrium. A darkly stained intercalated disc runs from upper left to lower right. Also prominent are groups of myofibrils running diagonally across the image and several mitochondria. This 1960 TEM image has historical value as one of the earliest views of muscle ultrastructure. Image made available by James D. Jamieson and the Department of Cell Biology, Yale University School of Medicine.