A fairly careful search of the literature failed to reveal photomicrographic reproduction of the cross striations of the muscle fibers of the sino-auricular node. These striations have been well described and drawings have been made. In the average section they are often invisible; in some, however, they are visible with the ordinary high power objective. The fibers bear a striking resemblance to the cardiac muscle fibers but are much smaller. They do not have any characteristics of nerve tissue. I1have described a simple routine method of examining the sino-auricular node as well as the conduction system of the heart. The accompanying illustration shows the cross striations distinctly in the muscle fibers of the sino-auricular node of a human heart, with a magnification of 1,200. The nucleus of the fiber running obliquely across the figure is in the lower left hand corner. The black tissue is a mass of