THE LEFT ATRIUM—THE LEFT VENTRICLE 625 
the fourth rib. It is commonly about three or four inches (ca. 6-9 cm.) above the sternal ends 
of the ribs. 
It is guarded by the pulmonary valve, composed of three semilunar cusps 
(Valvule semilunares arteriz pulmonalis); of these, one is medial, one lateral, 
and the third posterior.1_ The convex peripheral border of each cusp is attached 
to the fibrous ring at the junction of the pulmonary artery and the conus arteriosus. 
The central border is free and concave. Each cusp consists of a layer of endo- 
eardium on its ventricular surface, a continuation of the inner coat of the artery on 
its arterial surface, and an intermediate layer of fibrous tissue. The edge of the 
conus arteriosus forms three arches with intermediate projecting angles or horns, 
to all of which the cusps are attached; and the artery forms opposite each cusp a 
pouch, the sinus of the pulmonary artery.” 
The walls of the ventricle (except in the conus arteriosus) bear muscular ridges 
and bands, termed trabecule carnee. These are of three kinds, viz., (1) ridges or 
columns in relief; (2) musculi papillares, somewhat conical flattened projections, 
continuous at the base with the wall and giving off the chorde tendinee to the 
tricuspid valve; (8) moderator bands (Musculi transversi cordis) which extend 
from the septum to the opposite wall. The latter are partly muscular, partly 
tendinous, and vary in different subjects. The strongest one is usually about 
midway between the base and apex and extends from the septum to the base of 
the lateral musculus papillaris (Fig. 547). It is considered that they tend to prevent 
overdistention. 
THE LEFT ATRIUM 
The left atrium (Atrium sinistrum)* forms the posterior part of the base of the 
heart. It hes behind the pulmonary artery and the aorta and above the left 
ventricle. The auricle (Auricula sinistra) + extends outward and forward on the left 
side, and its pointed, 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, 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 foetus. The left 
atrio-ventricular opening (Ostium atrio-ventriculare sinistrum) is situated below 
and in front; it usually appears smaller than the right one on account of the con- 
traction of the left ventricle in the dead subject. The apertures of small veins of 
the heart are found in the spaces enclosed by the musculi pectinati. 
The number and the arrangement of the pulmonary veims are variable. They may be 
five tonineinnumber. The largest vein is posterior; it is formed by the union of veins from both 
lungs. Usually three veins of considerable size, which lie above the posterior vena cava, enter 
close together on the right, and three or four open close to the ridge which projects from the roof 
at the base of the auricle. 
THE LEFT VENTRICLE 
The left ventricle (Ventriculus sinister) forms the left posterior part of the 
ventricular mass. It is more regularly conical than the right ventricle, and its 
wall is much thicker except at the apex. It forms all of the posterior contour of 
the ventricular part and the apex of the heart. Its base is largely continuous with 
the left atrium, with which it communicates through the left atrio-ventricular 
opening, but its anterior part opens into the aorta. The cavity usually appears 
smaller than that of the right ventricle in the dead subject, on account of the greater 
contraction of its wall. It is almost circular in cross-section. 
1 Tn some cases there are four cusps, and very rarely only two. 
? These are also known as the sinuses of Valsalva. 
- Pie termed the left auricle. * Also termed the auricular appendix. 
