The Left Auricle when distended presents somewhat the form of a four-sided pyramid, the base of which is situated at its right side and forms the septum auricu-larum; while the truncated apex constitutes the left wall or side of this cavity. At the anterior and upper portion of this latter wall, where it joins the superior, we find the opening of the left auricular appendix; and farther back, where the left wall unites with the posterior, we find the openings of the left pulmonary veins. The posterior wall is directed a little upwards; and at its right extremity, and upper angle, immediately behind the septum auricularum, we find the openings of the right pulmonary veins. The anterior wall looks somewhat downwards; it corresponds to the left ventricle, and presents to our view the left auriculo-ventricular opening. The superior wall looks a little forward. Lastly, the inferior wall is very smooth, and forms with the posterior wall a continuous convex surface which corresponds, with the interposition of the pericardium, to the oesophagus and descending aorta.

The left pulmonary artery may be seen crossing from before backwards, so as to get behind the left auricular appendix.

In order to see the interior of the left auricle, an incision should be made vertically through its posterior and superior walls, so as to separate the pulmonary veins of the right and left sides. We may now observe that the septum of the auricles is convex towards the left side; in the foetus it presents the valve already described in connection with the fossa ovalis, but in the adult it is by no means so distinctly marked. The auriculo-ventricular opening situated in its anterior wall is smaller than that on the right side, and its long axis is directed somewhat transversely. The superior portion of the left wall presents the orifice of the auricular appendix, which is smoother internally than that on the right side: and lastly, opening into the posterior wall, we observe superiorly, the four pulmonary veins, the orifices of which are unprovided with valves: sometimes the two veins of the left side have a common opening; but when there are four, the two inferior veins have the larger openings, and the two left veins are nearer to each other than the two right. From the above account it appears that there are seven openings into the left auricle of the foetus, viz., the four openings of the pulmonary veins, the opening of the left auricular appendix, the left auriculo-ventricular opening, and the foramen ovale. There are commonly but six in the adult, the foramen ovale being ordinarily closed: a small valvular opening, however, occasionally exists in the adult at the upper part of the fossa ovalis. The interior of this auricle, with the exception of its appendix, is destitute of musculi pectinati; it is therefore smoother than the interior of the right auricle; it is also stronger in its muscular structure, and its capacity is about one-fifth less.