The paper contains observations upon the structure and connections of the auriculo-ventricular and arterial valves of the human heart, which the author thinks will assist in explaining their nature and functions. The relation of the “four orifices” in the base of the heart is examined, and it is shown that the aortic and left auriculo-ventricular apertures are not separated as the others are; that no muscular tissue of the ventricle intervenes between them, but that when the auricles and great vessels are separated from the ventricles (which may be accomplished with facility after prolonged boiling), the aortic aperture is separated from the left auriculo-ventricular only by the anterior mitral valve; and when this is removed (or even while it remains), it is plainly seen that only one aperture exists whose borders are formed by the muscular tissue of the ventricle, and in shape somewhat resembling the figure 8. This is divided into two portions, an anterior (aortic) and posterior (auriculo-ventricular) by the anterior mitral valve, and above it, by the posterior wall of the aorta, into which is inserted a large portion of the anterior wall of the left auricle, but no muscular tissue of the ventricle intervenes.