The =left atrium= (Atrium sinistrum) or auricle forms the posterior part
of the base of the heart. It lies behind the pulmonary artery and the
aorta and above the left ventricle. The auricle (or appendix) extends
outward and forward on the left side, and its blind end is behind the
origin of the pulmonary artery. The =pulmonary veins=, usually seven or
eight in number, open into the atrium behind and on the right side. The
cavity of the atrium is smooth, with the exception of the auricle (or
appendix), in which the musculi pectinati are present. In some cases
there is a depression on the septal wall opposite the fossa ovalis,
bounded above by a fold which is the remnant of the valve of the foramen
ovale of the fœtus. The =atrio-ventricular opening= is situated below
and in front; it usually appears smaller than the right one on account
of the contraction of the left ventricle in the dead subject. The
apertures of small veins of the heart are found in the spaces inclosed
by the musculi pectinati.
The number and the arrangement of the pulmonary veins are variable.
They may be five to nine in number. The largest orifice is posterior.
Usually three veins of considerable size enter close together on the
right above the posterior vena cava, and three or four open close to
the ridge which projects from the roof at the base of the auricle
(appendix).