This cavity also is of a conical form: its apex constitutes, in the adult, the apex of the heart: and its base has an arterial, and an auricular opening. The interior of the left ventricle may be exposed by an incision similar to that recommended when speaking of the right: the arterial opening thus exposed, will be found in front of the auriculo-ventricular aperture, and a little to its right side. The auriculo-ventricular opening is guarded by two triangular folds of the lining membrane, which constitute the mitral valve of Vesalius. The anterior lamina of this valve is much larger than the posterior, and has been correctly termed the valvular septum of Lieutaud of the left ventricle ; that anatomist supposed that it was applied against the orifice of the aorta while the ventricle was filling: this appears possible, as the aortic opening is in front of the auriculo-ventricular, and the substance of this valve separates the two openings from each other. The mitral valve is similar in structure to the tricuspid : it does not, however, admit of regurgitation of the blood from the left ventricle into the left auricle; and the two surfaces of the anterior division of the valve are equally smooth;—the posterior surface for the purpose of facilitating the flow of blood from the auricle into the ventricle ; and the anterior, the flow from the ventricle into the aorta. In this respect this portion of the valve differs from the posterior, and from the three portions of the tricuspid valve. The muscular structure of this ventricle is much thicker and stronger than that of the right ventricle.