The writer begins his communication by giving a brief description of the structures in which valves are found, and takes occasion to comment on the nature and properties of the veins and arteries, and on the arrangement of the muscular fibres in the ventricles, as these necessarily influence, to a greater or less extent, the action of the valves. He also adverts at some length to the shape of the venous, arterial, and auriculo-ventricular orifices, and to the fibro-cartilaginous rings by which the latter are surrounded; as well as to the dilatations or sinuses which are found behind or to the outside of the segments constituting the semilunar valves in the veins and arteries, and to the shape of the ventricular cavities, which, as he points out, bear an important relation to the valves, inasmuch as they determine the direction in which the blood acts upon them ; precise information on these points being, according to the author, indispensable to a just appreciation of the subject under investigation.