By careful microscopic identification, sections from the S-A and A-V nodes in the dog were selected for electron microscopy. The main portion of each node appears to be characterized by a complicated arrangement of small, irregularly shaped muscle cells and the presence of abundant nerve elements in the interstitium. The nodal cells, joining by apposition of the respective plasma membranes, tend to be arranged in a few rows and sometimes may give the appearance of an agglomerate of a number of cells. The myofibrils, being rather scanty and variable in direction in the sarcoplasm, terminate at the cell junction apparently in the same manner as in other heart muscle cells. Besides abundant sarcosomes, a considerable number of profiles of the sarcoplasmic reticulum can be seen in the sarcoplasm, some being associated with small dense granules. Occasionally a Golgi apparatus is seen in the peri-nuclear sarcoplasm. Nuclei in the nodal cells often appear to be large in relation to the size of the cells. Besides numerous non-myelinated nerve fibers, some profiles of myelinated fibers are also seen in the nodal regions. Although a number of axons, which are wholly or partially enveloped by Schwann cells and contain numerous small vesicles, may lie very closely to the muscle cells, no evidence has yet been shown of any connection corre-sponding to the neuromuscular junction in other muscle types. The intrinsic ganglion cells located close to the A-V node also contain Nissl bodies, agranular reticulum, mitochondria, and dense bodies in the perikaryon. The nodal artery in the S-A region was also studied with the electron microscope.

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A case of antemortem diagnosis of coronary embolism was reported. The clinical features of this case strongly suggested embolism, but not thrombosis of coronary artery. At the cardiac operation about two weeks before death, there were no noticeable findings of coronary sclerosis, and a clinical diagnosis was confirmed at autopsy.

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