The left atrium (left auricle) is rather smaller than the right, but its walls are thicker, measuring about 3 mm.; it consists, like the right, of two parts, a principal cavity and an auricula.
The principal cavity is cuboidal in form, and concealed, in front, by the pulmonary artery and aorta; in front and to the right it is separated from the right atrium by the atrial septum; opening into it on either side are the two pulmonary veins.
The auricula (auricula sinistra; left auricular appendix) is somewhat constricted at its junction with the principal cavity; it is longer, narrower, and more curved than that of the right side, and its margins are more deeply indented. It is directed forward and toward the right and overlaps the root of the pulmonary artery.
The interior of the left atrium presents the following parts for examination :