pages 533

by tendinous fibers, and the former presents, at its middle, a thickened nodule (corpus Arantii). From this nodule tendinous fibers radiate through the segment to its attached margin, but are absent from two narrow crescentic portions, the lunulæ, placed one on either side of the nodule immediately adjoining the free margin. Between the semilunar valves and the wall of the pulmonary artery are three pouches or sinuses (sinuses of Valsalva).

Left Atrium (atrium sinistum; left auricle).—The left atrium 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.

Auricula (auricula sinistra; left auricular appendix).—The auricula 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.

FIG. 496– Interior of left side of heart. (See enlarged image)

The interior of the left atrium (Fig. 496) presents the following parts for examination:

Openings of the four pulmonary veins.

Left atrioventricular opening.

Musculi pectinati.

The pulmonary veins, four in number, open into the upper part of the posterior surface of the left atrium—two on either side of its middle line: they are not provided with valves. The two left veins frequently end by a common opening.

The left atrioventricular opening is the aperture between the left atrium and ventricle, and is rather smaller than the corresponding opening on the right side.

Contents -SUBJECT INDEX -BIBLIOGRAPHIC RECORD -ILLUSTRATION INDEX