The ventricles form the greater portion of the heart. In the adult, the relation of the ventricles to one another is as follows. The left [ventriculus sinister] has the form of a narrow cone, the apex of which is the apex of the heart. The right ventricle [ventriculus dexter] is crescentic in section and appears to be partially wrapped around the right or lower wall of the left ventricle which forms the septum ventriculorum. The left ventricle forms the margo obtusus of the heart, about half the diaphragmatic surface, and a shght part of the sterno-costal surface. The right ventricle forms about half the diaphragmatic surface and the major part of the sterno-costal surface; it takes no share in the formation of the apex of the heart.
The interventricular septum [septum ventriculorum] is thick and muscular except for a small area near the root of the aorta which is membranous [septum membranaceum ventriculorum]. The latter can be seen from the left ventricle in the angle between the attached edges of the right and posterior aortic valves. The membranous septum is partly concealed from the right heart by the medial cusp of the tricuspid valve which is attached to it near its upper part. The portion of the membranous septum above the medial tricuspid cusp is therefore atrio-ventricular, i. e., between the right atrium and left ventricle.
The membranous septum is the extreme lower part of the independent septum (s. aorticum) which divides the aortic root from the pulmonary artery and conus arteriosus (and partially subdivides, also, the right ventricle by separating the conus arteriosus from the remainder of the ventricle). The relation of the part of the aortic septum between the conus arteriosus and aortic root to the septum ventriculorum is beautifully shown by His.
The greater part of the interior of the ventricles is thrown into ridges by myocardial bundles of large size. These fasciculi [trabeculae cordis] either stand out in relief only, or, by being undermined, form bands covered except at either end by endothelium. A careful examination of the endocardium of fresh hearts will reveal a plexiform network of Purkinje fibers. These fibers, belonging to the atrio-ventricular conducting system, become very obvious when the endocardium has been exposed to the air long enough to become partially dry.
The wall of the right ventricle [ventriculus dexter] is much thicker than that of the atria, but less so than that of the left ventricle. The upper and anterior part of the right ventricle is in relation posteriorly with the root of the aorta. This portion of the ventricle is called the conus arteriosus and is separated from the remainder of the right ventricle by a muscular spur which extends from the back of the conus to the right venous ostium. The spur is the crista supraventricularis; its relation to the partition between the conus and aorta, and to the septum membranaceum, shows that it is the free edge of the embryonic aortic septum (see morphogenesis of the heart).
Two papillary muscles in the right ventricle are constant in position, the large anterior papillary muscle, and the small papillary muscle of the conus (Luschka). The anterior papillary is situated on the sterno-costal wall, near the junction of this with the septal wall. The papillary of the conus is placed just below the septal end of the crista supraventricularis. The posterior papillary muscles form an irregular group springing from the diaphragmatic wall. Some chordae tendinese stretch directly from the septal wall (with or without small muscular elevations at their bases) to the medial cusp of the tricuspid valve. The chordae tendinese from the anterior papillary go to the anterior and posterior cusps; those from the conus papillary to the medial and anterior, and those from the posterior papillary muscles to the medial and posterior cusps of the tricuspid valve, respectively.
There is frequently a band of myocardium extending from the septal wall of the right ventricle to the anterior papillary muscle near its middle. This is the moderator band, which contains a part of the right limb of the atrioventricular bundle. If the moderator band joins the anterior papillary near its base, as it frequently does, it is difficult to distinguish it from the ordinary trabeculae in this situation.
The term moderator band was originally applied to this bridge or band of muscle under the impression that it prevented overdistention of the ventricle. Subsequent discovery of the conducting system of the heart makes it plain that there is always a band conducting the right limb of the atrioventricular bundle from the septum to the anterior papillary muscle. Whether the band is isolated from the other trabecules, and therefore readily recognizable, appears to depend somewhat upon the relation of the base of the papillary muscle to the septum ventriculorum.
The wall of the left ventricle [veiitriculus sinister] is very thick except at the extreme apex, and at the membranous septum. In the left ventricle are two large papillary muscles, generally known as anterior and posterior; both send chordae tendineae to each cusp of the mitral valve. On the septal wall of the ventricle the left limb of the atrioventricular bundle can usually be seen as a broad, flattened band beneath the endocardium. The band appears just below the septum membranaceum and divides into strands which go to the two papillary muscles. The strands in many places bridge across part of the ventricle to reach the papillary muscles covered only by tubes of endocardium.
These bridging strands connecting the papillary muscles with the septum ventriculorum, which were formerly called "false chordae tendinese," are exactly comparable to the moderator band of the right ventricle which occasionally consists of atrioventricular bundle and endocardium only.
The semilunar valves [valvulse semilunares] guard the arterial ostia of the ventricles. The aortic ostium is directed upward and slightly forward and to the right; the pulmonary backward and slightly upward and to the left. Each valve, of which there are three to each ostium, is a pocket-like fold of endocardium strengthened by fibrous tissue. The free edge of each valve is directed away from the ventricle, so that excess of pressure within the great vessels brings the three valves of either ostium into mutual apposition. In the middle of the free edge of each valve there is a small fibro-cartilaginous nodule; radiating from this toward the entire fundus, and along the extreme free edge of the valve, are fibrous thickenings. On either side of the nodule, between the thicker margin and fundus, the valve is thin over a crescentic area called the lunula.
The aortic valves are called the right, left, and posterior; the pulmonary valves, the right, left, and anterior.* The aortic semilunar valves are stronger than the pulmonary; opposite them there are three dilatations in the aortic wall, the aortic sinuses [sinus aortae] or sinuses of Valsalva. From the right and left sinuses, the right and left coronary arteries, respectively, arise.
After ventricular systole, the increased pressure in the great vessels distends the valves with blood. The noduli meet in the center and the lunulas, coming into mutual contact, produce a tri-radiate line of contact between the valves.
* The BNA names of the aortic and pulmonary valves are not based upon their relative positions in the body. From transverse sections through the thorax (see any good atlas) it may be seen that one aortic valve is anterior, one pulmonary valve posterior, and the other aortic and pulmonary valves are right and left. If the removed heart is held so that the ventricles are on the right and left of the septum, respectively, the valves take the positions indicated by the BNA. The names given by the BNA to the valves, although conventional (Uke many other terms of orientation applied to parts of the heart), are convenient, particularly from a developmental standpoint.