THE LEFT ATRIUM THE LEFI VENTRICLE 625 



the fourth rib. It is commonly about three or four inolies (ca. 6-9 cm.) above the sternal ends 

 of the ribs. 



It is guarded by the pulmonary valve, composed of three semilunar cusps 

 (Valvulje semilunares arteriae pulmonalis); of these, one is medial, one lateral, 

 and the third posterior. ^ 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- 

 cardium 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 trabeculae cameae. 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 chordae tendineae to the 

 tricuspid valve; (3) 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 (A'trium sinistrum)' 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 (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 veins are variable. They may be 

 five to nine in number. 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 anterierHPart -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 In some cases there are four cusps, and very rarely only two. 



2 These are also known as the sinuses of Valsalva. 



3 Also termed the left auricle. * Also termed the auricular appendix. 



40 



