Anatomy

Anatomy of the Heart

Right Atrium

Venous blood returns to the heart via the superior and inferior vena cave into the right atrium, where it is stored during right ventricular systole. During ventricular diastole, blood flows from the right atrium into the right ventricle (Figs. 1, 2, 4, 7, 8, 9 ). The right atrium forms the right lateral cardiac border and is above, behind, and to the right of the right ventricle (Figs. 4 and 7). Most of the right atrium is to the right and anterior to the left atrium (Figs. 4 and 7). Anteromedially, the right atrial appendage protrudes from the right atrium and overlaps the aortic root (Figs. 1 and 2). On the posterior external surface of the right atrium a ridge, the sulcus Terminalis (or terminal groove), extends vertically from the superior to the inferior vena cava. This corresponds to an internal muscular bundle, the crista terminalis, which runs along the edge of the entrance to the right atrial appendage to the front of the orifice of the superior vena cava and then to the right side of the inferior vena cava (Figs. 8, 9). The sinus node is usually located at the lateral margin of the junction of the superior vena cava with the right atrium and the atrial appendage, beneath or near the sulcus terminalis (terminal groove) (Figs. 1 and 2).

A. Diagram showing the normal relations of the pericardium, great vessels, ventricles, and the atria as viewed in the frontal position. R= right; L= left. (Diagram by McClaren Johnson, Jr., M.D.)

FIGURE 1

B. Frontal (AP) roentgenogram of the heart. The components that form the cardiac silhouette can be readily identified from A above. A= aortic valve ring; P= pulmonary valve ring; M= mitral valve ring; T= tricuspid valve ring. Reference: Hurst’s THE HEART, Eighth Edition, pages 60-61.

FIGURE 1

C. Schematic transverse section through the heart at approximately the level of the second intercostal space. The relation between the left and the right atria and the interatrial septum is illustrated. The relative positions of the aortic and pulmonary valves and their cusps are shown. AC = anterior cusp; RC = right cusp; LC = left cusp; RCC = right coronary cusp; NCC = noncoronary cusp of the aortic valve.

FIGURE 1

FIGURE 2: External views of the heart

A. Anterior surface showing epicardial fat*, which obscures the interventricular sulci containing the left anterior descending artery. Ao = aorta; LAA = left atrial appendage; LV = left ventricle; PT = pulmonary trunk; PV = pulmonary vein; RAA= right atrial appendage; RV= right ventricle; SVC = superior vena cava.

B. Posterior surface of heart showing location of the posterior descending artery (PDA), crux of the heart *, and inferior vena cava (IVC). LA = left atrium; RA= right atrium. Reference: Hurst’s THE HEART, Eighth Edition, page 61.

FIGURE 3: Transverse section through base of heart showing relationship of various chambers and great vessels. A = anterior; AO = aorta; AS = atrial septum; AV = aorta; LA = left atrium; LAA = left atrial appendage; MV = mitral valve; RA = right atrium; RAA = right atrial appendage; P = posterior; PT = pulmonary trunk; PV = pulmonary trunk; PV = pulmonic valve; TV = tricuspid valve. From Hurst’s THE Heart, Eighth edition, page 61.)

FIGURE 4: Basal view of heart showing relationship of great vessels and atria. The left atrium (LA) has a smooth endocardium while the right atrium (RA) is trabeculated. The aorta (Ao) is posterior to the pulmonary trunk (PT) but anterior to the atrial septum (AS).

The Morphologically Right Atrium

In the normal heart, this structure forms the rightward and anterior part of the cardiac mass, overlapping the right hand margin of the left atrium and communicating with the right ventricle to its right side (fig. 4.10a). Externally, the chamber consists of a posterior part which receives the superior and inferior venae cavae termed the sinus venarum (fig. 4.10b) and an anterior part which extends forwards in pouch-like fashion to encircle the right border of the aorta, the right atrial appendage (fig. 4.10c). The border between the two is marked by a groove, the sulcus terminalis (fig. 4.10b) which is variably developed and in some hearts may be inconspicuous. The left hand margin of the right atrium is marked posteriorly by the groove between the superior vena cava and the right pulmonary vein (fig.4.10d). Beneath the groove, the left border of the inferior vena cava is in the same plane as the atrial septum, running inferiorly to the crux cordis. Superiorly, the roof the atrium curves posteriorly behind the aorta, a small groove sometimes being seen at the site of the septum, to become continuous with the left atrial wall (fig.4.10c).

(The color pictures and their corresponding sketches in the following descriptions of the right and left atria are from Cardiac Anatomy,1980, by R.H.Anderson and Anton E. Becker)

Fig.4-10a: - The heart in its in situ position dissected to show the position of the right atrium and the right ventricle.

Diagram - Fig.4-10a-1

Figure 4.10b: The heart viewed from the right showing how the great veins open into the posterior part of the right atrial chamber. The anterior part, the atrial appendage, is seen extending round the aorta.

Schematic - Fig.4-10b-1

Figure 4.10c: The heart viewed from above showing how the atrial appendages encircle the roots of the great arteries (Left atrial appendage; aorta; pulmonary trunk; right atrial appendage)

Schematic - Figure 4.10c-1

Figure 4.10d: The heart viewed posteriorly and from the right showing the groove between the pulmonary veins and right atrium: sulcus terminalis, ‘Waterston's groove’, inferior vena cava, right pulmonary veins, superior vena cava.

Schematic - Figure 4.10d-1

Figure 4.10e: When the atrium is opened, the distinction between the posterior smooth-walled sinus venarum and the anterior trabeculated appendage is much more readily apparent (fig. 4.10e-1). The junction between the two is marked by a well-formed muscle bundle, the crista terminalis (fig. 4.10e-1). The trabeculae tend to run at right angles to the crista. The inside of the right atrial chamber presents a posterior surface, a septal surface, and an anterior surface. The floor of the chamber can be considered as tricuspid valve orifice orientated obliquely to the right (fig. 4.10e-2) although the inferior vena cava opens into the junction of the posterior wall and the floor.

Schematic - Figure 4.10e-1: Dissection of the right atrium viewed from the front showing the crista terminalis separating the posterior smooth wall sinus venarium and the trabeculated atrial appendage

Schematic - Figure 4.10e-2: The right atrium viewed from the right following removal of the parietal wall and showing its surfaces.

Figure 4.10f: - Section through the short axis of the atrial chambers showing oblique orientation of the atrial septum

When the atrium is opened, the distinction between the posterior smooth-walled sinus venarum and the anterior trabeculated appendage is much more readily apparent (fig. 4.10e-1). The junction between the two is marked by a well-formed muscle bundle, the crista terminalis (fig. 4.10e-1). The trabeculae tend to run at right angles to the crista. The inside of the right atrial chamber presents a posterior surface, a septal surface, and an anterior surface. The floor of the chamber can be considered as tricuspid valve orifice orientated obliquely to the right (fig. 4.10e-2) although the inferior vena cava opens into the junction of the posterior wall and the floor. The superior vena cava orifice is in the roof of the chamber, and the septal surface is obliquely orientated, running from a right posterior to left anterior position (fig 4.10f).

Diagram - 4.10f-1: - Figure 4.10f with labelling of structures.

The crista terminalis runs from the anterior part of the septal surface and swings in front of the orifice of the superior vena cava which enters the right atrium between the crista and the superior limbus of the fossa ovalis (figure4.10g).Having skirted the superior caval orifice, the crista turns down the right side of the inferior vena cava and curves in toward the tricuspid orifice, passing beneath the ostium of the coronary sinus (fig.4.10h. The margin of the crista terminalis is reinforced in the fetal life by sheet-like structures which separate the orifices of the inferior vena cava from the atrial appendage. These become the valves of the inferior vena cava (Eustachian valve) and the coronary sinus (Thebesian valve) and may be seen to variable extent in the adult heart. The Thebesian valve is usually reasonably formed, but the Eustachian valve is less well formed (fig.4.10g-2).Fibrous strands may exist between the various parts of the crista which extend across the cavity of the right atrium.They, like the valves, are remnants of the extensive right valves of the sinus venosus seen during development and are termed Chiari networks (figure 4.10h-1) Similar remnants may be seen across the fossa ovalis.They are remnants of the left sinus venosus valve (figure 4.10g-2).The atrial appendage usually shows a considerable pouch at its junction with the atrium anterior and inferior to the orifice of the inferior vena cava, the so-called sub-eustachian sinus (figure4.10h-2).The crista itself runs forwards onto the posterior margin of the tricuspid orifice as a muscular sheet which inserts into the inferior and septal leaflets of the tricuspid valve(4.10h-2).

Figure 4.10g

Diagram - Figure 4.10g-1: Dissection showing how the superior inferior vena cava enters the right atrium between the crista terminalis and the superior limbus of the fossa ovalis.

Diagram - Figure 4.10g-2: Dissection of the lateral wall of right atrium showing the relationship of the crista terminalis to the inferior vena cav, the fossa ovalis and the coronary sinus.

Fig.4.10h-1: Remnants of the extensive right valve of the sinus venosus termed the Chiari network.

Diagram - Figure 4.10h-1

Fig.4.10h-1a: Opened right atrium showing the extensive trabecular pouch found beneath the orifice of the inferior vena cava(the so-called sub-eustachian sinus).

Schematic - Figure 4.10h-1a

Figure 4.10i-1: Dissection of the sinus septum showing the tendon of Todaro. The heart has been transilluminated from the left ventricle to show the position of the membranous septum.

Diagram - Figure 4.10i-1.

Figure 4.10i-2: Dissection of the atrial septum from behind showing the limbus of the fossa ovalis is an infolding of the atrial roof and showing the position of the flap valve.

Diagram - Figure 4.10i-2

The posteroseptal surface of the right atrium is, at first sight, extensive and is characterized by the orifice of the coronary sinus, the third of the systemic venous channels which drain into to the right atrium (4.10i-1 and 4.10i-2).The fossa ovalis is the depression at the site of the fetal interatrial communication termed the foramen ovale. In fetal life, this hole permits richly oxgenated blood (coming from the placenta) to reacch the left atrium and has a well-marked rim or limbus.Superiorly, the limbus forms the 'septum secundum' between the superior vena cava and the pulmonary veins(fig.4.10g-2). Anteriorly, the limbus is the interatrial groove running behind the aorta. Inferiorly, the limbus overlies the central fibrous body and continues backwards as the structure separating the orifice of the coronary sinus from that of the inferior vena cava. this structure is termed the the sinus septum (fig.4.10g-2) the degree of accentuation of the limbic structure(compare figs.4.10g-1 and 4.10g--2) depends on the amount of fat in the interatrial groove. A tendinous structure extends through the sinus septum in most hearts, being a continuation of the commissure between the Eustachian and Thesebian valves. It runs intramyocardially to insert into the central fibrous body but can be easily demonstrated by superficial dissection(4.10i-1). It is termed the tendon of Todaro and is a vital structure in demarcating the position of the atrioventricular node.The posterior limbus of the fossa ovale is very variable in its formation. In some hearts, a well formed posterior lip is seen (fig.4.10e-2); In others, the posterior wall of the fossa is directly continuous with the left wall of the inferior vena cava ( fig.4.10g-2).The floor of fossa ovalis is a thin fibromuscular partition-the flap valve ( fig.4.10i-3,4).It can be easily transilluminated (fig.4.10j-1 and fig.4.10j-2). In normal hearts, the flap valve is of sufficiently large size to close completely the fossa ovalis. However, it is not always adherent at its superior margin, and in approximately 25% of the normal hearts, a probe can be passed through this site from right to left atrium producing a so-called probe-patent foramen ovale (fig.4.10k-1 and fig.4.10k-2).

However, because of this valve-like architecture, such a probe-patetnt foramen ovale does not permint an interatrial shunt as long as the left atrial pressure is higher than that of the right atrium.

The size of the opening of the coronary sinus is variable, but it is always placed between the sinus septum and the extension of the crista terminalis (figs.4.10l and 4.10l-1). An extensive band of atrial muscle is present inferior to the orifice which extends into the leaflets of the tricuspid valve. Although this is the wall of the right atrium, it also overlies the ventricular musculature due to the low attachment of the tricuspid leaflets. The area is not part of the atrial septum.

Anteriorly, this muscle band merges with the anterior limbus and sinus septum, forming the atrioventricular node. Frequently, small openings are present in this sheet from which venous channels extend into the conduction tissues of the atrioventricular junctional area. These, together with the tendon of Todaro, form better markers of the site of the atrioventricular node than the opening of the coronary sinus. The anterior wall of the right atrium is the atrial appendage. Seen externally, it has a characteristic blunt shape which serves to distinguish it from the left atrial appendage ( figs. 4.10m and 4.10m-1). Internally, the appendage, is lined by multiple trabeculae which extend at right aangles to the crista terminalis all along its length, continuing inferiorly into the subeustachian sinus (Figs. 4.10k, 4.10h, 4.10e). In the roof of the atrium, one of the trabeculae is frequently prominent and is sometimes termed the septum spurium.

Figure 4.10j-1: Transillumination fo the atrial septum showing the position of the fossa ovalis

Figure 4.10j-2: Figure 4.10j-1: and Schematic of Fig.4.10j-1

Figure 4.10k-1: The heart with a probe-patent foramenn ovale. The probe has been passed through th gap between th eflap valve and the superior limbus

Diagram - Figure 4.10k-2

Figure 4.10l: The septal surface of the right atrium showing the relationsip of the fossa ovalis to the ostium of the coronary sinus.

Diagram - Figure 4.10l-1

Figure 4.10m: The differing morphology of the right and left atrial appendages.

Schematic - Figure 4.10m-1

FIGURE 8 - Four-chamber view of the heart showing morphologic differences between the four chambers. The right atrium (RA) is more trabeculated than the left (LA), and the right ventricle is more heavily and coarsely trabeculated compared to the left ventricle (LV). AS = atrial septum; MV = mitral valve; TV = tricuspid valve; VS = ventricular septum. From Hurst’s THE Heart, Eighth edition, page 66) .

FIGURE 5 - Cross-sectional view of heart showing aortic valve (AV), pulmonary trunk (PT), origin of the right (R) and left main (LM), coronary arteries, tricuspid (TV) and mitral(MV) valves, and atrial septum(AS). A = anterior; P = posterior. (From Hurst’s THE Heart, Eighth edition, page 63.)

FIGURE 5A - Specimen showing the muscular ventricular septum (MVS)(black arrows) and membranous portion of the ventricular septum(MPVS)(white arrows).LV = left ventricle; MV = mitral valve.

FIGURE 6 - Long-axis view of the right side of heart right ventricle (RV), right atrium (RA), and tricuspid valve (TV). The RV walls are heavily trabeculated. (From Hurst’s THE Heart, Eighth edition, page 64).

FIGURE 7 - Closeup of right atrium showing atrial septum (AS), foramen ovale (FO), entrance to orifice of coronary sinus (arrow), and tricuspid valve (TV). From Hurst’s THE Heart, Eighth edition, page 65.

The inner surface of the posterior and medial (septal) walls of the right atrium is smooth, while the surfaces of the lateral wall and of the right atrial appendage are composed of parallel muscle bundles, the pectinate muscles (Figs. 4 and 7). The right atrial wall measures almost 2 mm in thickness. The superior and inferior venae cavae enter the right atrium posteriorly and medially at its superior and inferior aspects. The orifice of the superior vena cava usually has no valve; the orifice of the inferior vena cava is flanked anteriorly by an inconstant, rudimentary valve, the eustachian valve, formed by a crescentic fold. The caval orifices may vary in shape and diameter depending upon the phase of respiration, the cardiac cycle, and the contraction or relaxation of surrounding muscular hands. The variation in the orifice may play some role in promoting venous return or preventing atrial reflux. The medial wall of the right atrium includes the atrial septum and is also important because of its proximity to several structures (Figs. 5 and 6 to 7). Anteriorly, the posterior (noncoronary) cusp and the right coronary cusp of the aortic root lean against the medial right atrium, forming a normal slight bulge known as the torus aorticus, which is a useful landmark during transseptal catheterization of the left side of the heart. The proximal right coronary artery is in the immediate vicinity as it enters the coronary sulcus. The proximity of the aortic root to the right atrium permits an aneurysm of the sinus of Valsalva to rupture into the right atrium.

The atrial septum (Figs.1C, 3, 4, 5, and 6 to 7) is found in the posteroinferior portion of the medial wall of the right atrium and extends obliquely forward from right to left. Near the center of the atrial septum there is a shallow depression, the fossa ovalis, which often has a prominent fold, or limbus, anteriorly. The ostium of the coronary sinus is located between the inferior vena cava and the tricuspid valve (Figs. 4, 6 to 7). The orifice of the coronary sinus is guarded by a rudimentary flap of tissue, the Thebesian valve. The AV node is located in the lower atrial septum, anterior and medial to the coronary sinus, just above the septal leaflet of the tricuspid valve. The sinus and AV nodes, as well as the entire conducting pathways, are not grossly visible.

Right Ventricle

INTRODUCTION

Figure 1D: Diagrammatic representation of the three basic components of the right and left ventricles. Each has an inlet component containing atrioventricular valve; an apical trabecular zone; and an outlet component supporting an arterial valve.

Each ventricle has basically the same pattern, composed of an inlet atrioventricular valve and its tension apparatus, a body and an outlet arterial valve. As with the atria, there are important morphological differences between the ventricles which permit their clinical distinction. Therefore, each ventricle is described concentrating upon its inlet and outlet valves, and then follows a description of the interventricular septum. The ventricles are described in terms of three parts : an inlet, containing an atrioventricular valve and its tension apparatus; a trabecular body; and a outlet supporting an arterial valve. In similar fashion, the muscular ventricular septum is described as composed of inlet (separating the atrioventricular valves) ; trabecular (between the trabecular zones) ; and outlet (between the arterial valves) portions. This division is not meant to indicate that ventricular inlet and outlet portions lack trabeculations, although in many places they do have smooth walls. Rather it indicates that the apical trabecular zones are the most trabeculated and most distinctive atrioventricular valves is similar in each ventricle (figs. 1D-1 & 1D-2) although distinctive differences exist and will be described in the sections devoted to the tricuspid and mitral valves. Basically, each valve is made up of a number of leaflets consisting of a fibrous tissue core, the maior support of this being the atrioventricular annulus. The core is termed the fibrosa and is continuous distally with the chordae tendineae. The chordae tendineae, composed of dense collagen, are in turn attached to the ventricular myocardium, most coming from specialized papillary muscles but some chordae taking origin from the ventricular walls The chordae and papillary muscles make up the valvar tension apparatus.

Figure1D-1: The removed mitral (upper) and tricuspid (lower) valves viewed from their atrial aspect.

Figure1D-1a: Drawing and labelling of Figure1D-1.

Figure1D-2:

The removed mitral (upper) and tricuspid (lower) valves viewed from their ventricular aspect.

Figure 1D-2a : Drawing and labelling of Figure 1D-2.

Figure1D-3: Histology of an atrioventricular valve.

Figure 1D-3a: Labelling of structures in Figure 1D-3

The fibrosa forms the ventricular layer of the valve; on its atrial surface which is continuous with the atrial endocardium and which is separated from the fibrosa by a more loosely textured layer of fibrous tissue termed the spongiosa (fig. 1D-3). The distal end of the atrial myocardiukm may also extend for a distance between the atrialis and the fibrosa. Apart from the blood vessels present in the atrial musculature, the valve leaflets and chordae are avascular structures.

The major chordae supporting a leaflet insert either into its free edge, or the area beyond the free edge on the ventricular aspect up to the line of closure of the leaflet. This area between the free edge and the line of closure is termed the rough zone in contradistinction to the area between the line of closure and the basal attachment of the leaflet which is easily transilluminated and is smooth (figs.1- D4 and 1-D4a). It is important to remember that the line of closure of a leaflet is not its free edge(Fig.1-D5 and 1-D5a).

Figure1-D4: Transillumination mitral valve showing the rough zone from the free edge to the line of closure of the valve and the clear azone between the line of closure and the annulus.

Figure 1-D4a: Labelling of structures in fig.1-D4.

Figure 1-D5: The atrial surface of the mitral valve showing how the line of closure is some distance from the free edge of the valve.

Figure 1-D5a: Labelling of structures in fig.1-D5.

The chordae inserting into the rough zone are called rough zone chordae (fig.1-D6). They are distinguished from basal chordae which pass from the ventricular myocardium to the ventricular aspect of the leaflet close to its attachment (fig.1-D7) and commissural chordae which are the discrete fan-shaped chordae inserting into the free margin of the leaflet only and supporting two adjacent leaflets (figs.1-D8 & 1-D9). The artioventricular valves are, therefore, intricate and complicated structures, having several components. Each of these components must function correctly and in a cooridinated fashion if the valve itself is to be competent. From a functional standpoint, the atrioventricular valves should not be considered solely in terms of the leaflets and chordae. For this reason, the term atrioventricular valve apparatus’ is more apt.

Figure 1-D6: The ventricular aspect of the mitral valve showing the attachment of rough zone chordae.

Figure 1-D6a: Labelling of Fig.1-D6.

Figure 1-D7: Dissection of the mitral valve showing the morphology of a basal chorda.

Figure 1-D7a: Labelling of fig.1-D7a.

Figure 1-D8: Commissural chordae of the mitral valve.

FIGURE 1-D8a: Labelling of structures in figure 1-D8.

Figure 1-D9: Commissural chordae of the tricuspid valve.

Figure 1-D9a: Labelling of structures in figure 1-D9.

The right ventricle receives venous blood from the right atrium during ventricular diastole and propels blood into the pulmonary circulation during ventricular systole (Figs.4, 6, 7, 8, 9 to 12). The right ventricle is normally the most anterior cardiac chamber, lying directly beneath the sternum (Figs.1 and 2). Enlargement or hyperactivity of the right ventricle may often be detected by palpation of the sternum or the lower left sternal border. The right ventricle is partially below, in front of, and medial to the right atrium but anterior and to the right of the left ventricle. Most of the entire inferior border of the frontal roentgenogram view of the heart consists of the right ventricle (Fig.1).

FIGURE 9 - Family of ventricular slices from base to apex. A = anterior; LV= left ventricle; right ventricle. P= posterior; VS = ventricular septum. The LV cavity is more “circular” shaped compared to the more “triangular” shaped RV cavity. From Hurst’s THE Heart, Eighth edition, page 66.

FIGURE 9A - Closeup view of ventricular slice seen in FIG. 9. This view corresponds to the short axis echocardiographic views of the ventricular cavities. A = anterior; LV = left ventricle; RV =right ventricle; VS = ventricular septum.

The striking difference in configuration between the two ventricles is illustrated by a transverse section (See above (Figs. 8, 9 and 10). The left ventricular chamber is an ellipsoidal sphere surrounded by relatively thick (8 to 15 mm at autopsy) musculature, well suited to ejecting blood against the high resistance of the systemic vessels. The right ventricle, which normally contracts against very low resistance, has a crescent-shaped chamber and a thin outer wall, measuring 4 to 5 mm in thickness. The anterior right ventricular wall curves over the ventricular septum, which normally bulges into the right ventricular cavity. Although the ventricular septum forms the medial wall of both ventricles, it seems to contribute predominantly to left ventricle function in normal subjects. The anterior and inferior walls of the right ventricular cavity are lined by muscle bundles, the trabeculae carneae, which often form ridges along the inner surface of the wall or cross from one wall to the other (Figs. 6 to 8). A rather constant muscle, the moderator band, crosses from the lower ventricular septum to the anterior wall, where it joins the anterior papillary muscle (Figs.4 to 7). The right bundle branch, after traveling through the muscular ventricular septum, courses through the moderator muscle to the endocardium of the right ventricle.

Functionally, the right ventricle can be partitioned into an inflow tract, an outflow tract, and an apical trabecular component (body). The trabecular muscles in the apex of the right ventricle are much more coarse than those in the left ventricle. The inflow tract, consisting of the tricuspid valve and the trabecular muscles of the anterior and inferior walls, directs entering blood anteriorly, inferiorly, and to the left at an angle of 60° to the outflow tract (Fig. 6). The smooth-walled outflow tract, also referred to as the infundibulum, forms the superior portion of the right ventricle. It is separated from the inflow tract by a thick muscle, the crista supraventricularis, which arches from the anterolateral wall over the anterior leaflet of the tricuspid valve to the septal (medial) wall, where it joins other constrictor bands of muscle that encircle the outflow tract (Figs. 6 and 10). Blood entering the infundibulum is ejected superiorly and posteriorly into the pulmonary trunk.

FIGURE 6A: The heart positioned in its situ position with the anterior wall removed to show the extent of the morpholobgically right ventricle.

FIGURE 6A-1: Labelling of structures in Figure 6A above.

FIGURE 6B: Short axis section through the ventricular mass showing how the right ventricle wraps around the left ventricle.

FIGURE 6B-1: Labelling of structures in Figure 6B above.

FIGURE 6C: Short axis section through the ventricular mass showing how the right ventricle wraps itself around the left ventricle.

FIGURE 6C-1: Labelling of structures in Figure 6C above.

FIGURE 6D: Frontal section through the heart showing the junction between the inlet and trabecular portions of the right ventricle, with the inlet septum extending to the position of the crux (posterior junction of the arterial and ventricular septa).

FIGURE 6D-1: Labelling of structures in Figure 6D above.

FIGURE 6E: The inlet componen of the right ventricle viewed from behind showing the transition into the trabecular zone. Note also the leaflets of the valve separated by the commoissures.

FIGURE 6E-1: Labelling of structures in figure 6E.

FIGURE 6F: Section through the ventricular apex showing how thin both the right and the left ventricular myocardia are at this point.

FIGURE 6F-1: Labelling of structures in figure 6F.

FIGURE 6G: The ventricular mass of the heart viewed from its right side after removal of the inlet and part of the outlet components of the right ventricle. It shows how the trabecular zone is suspended like a piece of washing from the washing line made up of the inlet and outlet components.

FIGURE 6G-1: Labelling of structures in figure 6G.

FIGURE 6H: The right ventricle viewed from the front showing the structure of the infundibulum, a muscular tube which supports the pulmonary valve.

FIGURE 6H-1: Labelling of structures in figure 6H.

FIGURE 6-I: Dissection of the outflow part of the right ventricle showing the difference between the crista supraventricularis (the supraventricular crest separating the tricuspid from the pulmonary valve) and the trabecular septomarginalis (TSM) which is an extensive septal trabeculation. Note the distinct raphe between the two structrures.

FIGURE 6I-1: Labelling of the structures in figure 6-I above.

FIGURE 6J: Further dissection of the heart shown in figure 6-I above demonstrates that most of the crista supraventricularis is made up of the heart wall rather than septal structures. Note its relationshipp to the epicardial fat and the coronary artery.

FIGURE 6J-1: Labelling of the structures in figure 6-J above.

FIGURE 6K: Still further dissection confirms that the crista is made up in its larger part of the outer heart wall.

FIGURE 6K-1: Labelling of structures in Figure 6K above.

FIGURE 6L: Sectioning into the aorta in this heart shows that only the extreme septal insertion of the crista supraventricularis is made up of infundibular septum.

FIGURE 6L-1: Labelling of structures in Figure 6L above.

FIGURE 6M: The septal surface of the right ventricle. The raphe between trabecula septomarginalis and crista is less well seen in this heart than in the heart shown in figure 6-I.

FIGURE 6M-1: Labelling of structures in figure 6M.

FIGURE 6N: The moderator band of the right ventricle. It is an extension from the apex of the trabecula septomarginalis.

FIGURE 6N-1: Labelling of structures in figure 6N.

FIGURE 6O: The multiple muscular bars which line the anterior aspect of the infundibulum of the right ventricle.

FIGURE 6O-1: Labelling of structures in figure 6-O.

FIGURE 6P: The infundibulum of the right ventricle viewed from the front showing the muscular annulus formed by the crista supraventricularis and its parietal extension and the trabecula septomarginalis together with the moderator band.

FIGURE 6P-1: Labelling of structures in figure 6P.

FIGURE 6Q: The tricuspid valve viewed from behind with the heart in its in situ position.The three leaflets occupy septal, anterosuperior and inferior positions.

FIGURE 6Q-1: Labelling of structures in figure 6Q.

FIGURE 6R: Histology of the tricuspid ring. The leaflet does not spring from a strong well-formed annulus as in the mitral valve.

FIGURE 6R-1: Labelling of structures in figure 6R.

FIGURE 6S: The inlet part of the right ventricle showing the multiple papillary muscles supporting the septal and inferior leaflets. However, only one, the posterior papillary muscle, gives rise to a commissural chord.

FIGURE 6S-1: Labelling of structures in figure 6S.

FIGURE 6T: The anteroseptal commissure of the tricuspid valve viewed from behind having opened the valve through the inferior commissure is supported by the medial commissure. Note that the anteroseptal papillary muscle is superior to and to the right of the membranous septum.

FIGURE 6T-1: Labelling of structures in figure 6T.

FIGURE 6U: A heart with a cleft in the septal leaflet of the tricuspid valve at the site of the menbranous septum.

FIGURE 6U-1: Labelling of the structures in figure 6U.

FIGURE 6V: A frequent variant in tricuspid valve morphology is for the large anterior papillary muscle to support the midzone of the anterosuperior leaflet. The anteroinferior commissure in this heart is supported by an accessory anterior papillary muscle.

FIGURE 6V-1: Labelling of the structures in figure 6V.

FIGURE 6W: The opened inlet portion of the right ventricle showing the inferior commissure. Note that the other small muscles do not give rise to commissural chordae.

FIGURE 6W-1: Labelling of structures in figure 6W.

FIGURE 6X: The atrioventricular junction viewed from its atrial aspect after removal of the atrial chambers and the great arteries. It shows the relationships of the leaflets of pulmonary and aortic valves. The two leaflets of these valves always face each other, permitting the nomination of the right-facing and left-facing leaflets of the pulmonary valves.

FIGURE 6X-1: Labelling of the structures in figure 6X.

FIGURE 6Y: The infundibulum of the right ventricle opened from the front showing the morphology of the pulmonary valve.

FIGURE 6Y-1: Labelling of the structures in figure 6Y.

The Morphologically Left Ventricle

The left ventricle is a conical structure with tubular walls which narrow down to a rounded apex. It comprises an inlet portion, containing the mitral valve and its tension apparatus; an apical rabecular zone characterized by fine trabeculations and an outlet zone, supporting the aortic valve, which is incomplete posteriorly so that the aortic and mitral valves are in fibrous continuity.The left ventricle forms the greater part of diaphragmatic surface of the heart but is overlaid anteriorly and superiorly by the trabecular zone and outlet of the right ventricle . In contrast to the right ventricle where there is a gentle curve between inlet and outlet portions, the left ventricle shows an acute angle between these portions, both extending down into the trabecular zone separated by the anterior leaflet of the mitral valve. Usually there is no structure comparable to the crista supraventricularis in the left venricle owing to the fibrous continuity of the inlet and outlet valves, although in rare hearts, a muscular fold (ventriculo-infundibular fold) may interpose between the valves. The septal surface of the left ventricle is smooth, so that is no structure corresponding to the trabecula septomarginalis in the left ventricle.

FIGURE 9B: The left ventricle and atrium after removal of the right-sided structures and viewed from the front.

FIGURE 9B-1: Labelling of structures in figure 9B.

FIGURE 9C: Short axis section of the ventricular mass showing the tubular nature of the left ventricle.

FIGURE 9C-1: Labelling of structures in figure 9C.

FIGURE 9D: The opened left ventricle showing inlet,trabecular and outlet portions.

FIGURE 9D-1: Labelling of structures in figure 9D.

FIGURE 9E: A heart viewed from the front.

FIGURE 9E-1: Labelling of structures in figure 9E.

FIGURE 9F: Section through the left ventricle showing how the anterior mitral valve leaflet separates its inlet and outlet portions.

FIGURE 9F-1: Labelling of figure 9F.

FIGURE 9G: Mitral valve viewed from above showing the anterior or septal leaflet and the posterior or mural leaflet with its three scallops.

FIGURE 9G-1: Labelling of figure 9G.

The Mitral Valve

The mitral valve is characteristically described as having two leaflets, the anterior or septal and the posteror or mural leaflets. The leaflets are separated by the posteromadial and anterolateral commissures (fig.9G ). The anterior leaflet is attached to less than half the circumference of the mitral annulus but has considerable height and consequently presents as a large leaflet (fig.9H).

The posterior leaflet, in contrast, is attached to more than half the circumference (fig.G) but is less tall (fig.H), and occupies only about the same area as the anterior leaflet. Moreover, the posterior leaflet has a characteristic scalloped contour. In the usual case three scallops can be distinguished divided by clefts (fig.9G) These scallops are termed posteromedial, middle and anterolateral. However, it is not at all unusual to find aberrations from this pattern, two, four, five or more scallops being seen in otherwise normal valves (fig.9I ).The posterior leaflet throughout its length is attached to the mitral atrioventricular annulus (fig.9J). The anterior leaflet, in contrast, is in fibrous continuity with the aortic valve, the two valves having a common annulus (fig.K) strengthened at each end by the right and left fibrous trigones (fig.9L). The mitral valve leaflets are supported by two papillary muscles groups situated underneath the commissural areas in the posteromedial and anterolateral positions (fig.9M). Their position is such that the chordae between muscle and leaflet operate at the maximal mechanical efficiency (fig.9N). Each papillary muscle supports the adjacent part of both valve leaflets (fig.9N). There is considerable variation in the morpholgy of the papillary muscles themselves, particularly the posteromedial muscle. They may be single pillar-like muscles or be composed of several heads of differing size (compare figs.9P and 9Q). The different papillary muscle architecture affects the chordal distribution (vide infra) and also affects the mode of the arterial supply to the papillary muscle complex. Because of the different topography of the anterior and the posterior laeflets, there are corresponding differences in the mode of chordal support, which also show considerable individual variation. Thes variations may leave part of the leaflet less well supported than would be antcipated.The anterior leaflet is supported only by rough zone chordae together with the commissural chords (fig.9P).The rough zone chords may be strengthened by thicker tendinous structures, the so-called strut chordae (fig.9R), usually one for each half of the leaflet. The commissural chords spring from the tips of their papillary muscle and fan out to attach to the free margins of both leaflets. The posteromedial commissural chord usually fans out more than that of the anterolateral commissure (compare figs. 9S and 9T).

FIGURE 9H: The anterior or septal leaflet of the mitral valve viewed from behind after division of the valve through its commissure. The posterior or mural leaflet is shown in Figure9C. Note that the anterior leaflet is almost square.

FIGURE 9H-1: Labelling of figure 9H.

FIGURE 9I: The posterior or mural leaflet of the divided mitral valve shown in figure H viewed from the front. Note that the posterior leaflet is long and narrow.

FIGURE 9I-1: Labelling of figure 9I.

FIGURE 9J: Another normal mitral valve viewed from above showing the variation which exists in the number of scallops (compare with figure 9H).

FIGURE 9J-1: Labelling of structures in figure 9J.

FIGURE 9K: Histological section through the mitral ring. The leaflet takes origin from a well-formed annulus (compare with figure 6S).

FIGURE 9K-1: Labelling of structures in figure 9K.

FIGURE 9L: Section through the area of aortic-mitral fibrous continuity. The two valves have a common annulus.

FIGURE 9L-1: Labelling of structures in figure 9L.

FIGURE 9M: Dissection of the fibrous skeleton of the aortic and mitral valves viewed from above and behind showing the thickening at either end of the area of valve continuity.

FIGURE 9M -1: Labelling of structures in figure 9M.

FIGURE 9N: Cutaway of the left ventricle showing the papillary muscle groups of the mitral valve. Note how they arise adjacent to each other when in their in situ position (compare with figure 9P).

FIGURE 9N -1: Labelling of structures in figure 9N.

FIGURE 9O: Overall view of the mitral unit showing how the muscles act at maximum mechanical efficiency.

FIGURE 9O-1: Labelling of structures in figure 9O.

FIGURE 9P: The opened mitral valve showing how each papillary muscle supports the adjacent part of both valve leaflets. The apparent separation of the papillary muscles is artefactual. See fig. 9N for the in situ position of the mucles.

FIGURE 9P-1: Labelling of structures in figure 9P.

FIGURE 9Q: With the considerable variation possible in the papillary muscle, this group of fan-like muscles is in sharp contrast to the pillar-type muscles in figure 9P.

FIGURE 9Q-1: Labelling of structures in figure 9Q.

FIGURE 9R: The anterior leaflet of the mitral valve viewed from the outflow tract showing the strut chordae.

FIGURE 9R-1: Labelling of the structures in Figure 9R.

FIGURE 9S: The posteromedial commissural chordae of the mitral valve.

FIGURE 9S-1: Labelling of the structures in Figure 9S.

FIGURE 9Sa: The anterolateral commissural chordae of the mitral valve.

FIGURE 9Sa-1: Labelling of the structures in Figure 9Sa.

FIGURE 9Sb: Cleft chorda of the same valve as in figs.9S and 9Sa. Althuogh supporting a cleft between 2 scallops, it is virtually indistinguishable from the commissural chords.

FIGURE 9Sb-1: Labelling of the structures in Figure 9Sb.

FIGURE 9Sc: Detail of the attachments of the chordae to the papillary muscles. the blood disperses into the left ventricle through the interchordal spaces.

FIGURE 9Sc-1: Labelling of the structures in Figure 9Sc.

FIGURE 9Sd: Section of the ventricular apices showing how thin the myocardium is at this point.

FIGURE 9Sd-1: Labelling of the structures in Figure 9Sd.

FIGURE 9Se: The anterior half of the left ventricular outflow tract viewed from behind. The posterior part is shown in fig. 9Se. Note that the anterior quadrants are muscular.

FIGURE 9Se-1: Labelling of the structures in Figure 9Se.

FIGURE 9Sf: The posterior half of the left ventricular outflow tract shown in the fig.9Se. Note the continuity between aortic and mitral valves.

FIGURE 9Sf-1: Labelling of the structures in Figure 9Sf.

FIGURE 9Sg: Frontal section through a heart showing the considerable angle which exit between the trabecular and outlet parts of the left ventricle.

FIGURE 9Sg-1: Labelling of the structures in figure 9Sg.

FIGURE 9Sh: Increase in the angle between the trabecular and outlet portins leads to the sigmoid septum of old age (compare with figure 9Sg above).

FIGURE 9Sh-1: Labelling of the structures in figure 9Sh .

FIGURE 9Si: The opened left ventricular outflow tract. The relationship of the aortic cusps to the mitral valve anterior leaflet is variable.

FIGURE 9Si-1: Labelling of the structures in figure 9Si.

FIGURE 9Sj: Aortic leaflet viewed from above in the closed position. Note that the leaflets are not of the same size.

FIGURE 9Sj-1: Labelling of the structures in figure 9Sj.

FIGURE 9Sk: The origin of the coronary arteries enables the leaflets of the aortic valve to be designated right coronary, left coronary and non coronary leaflets.

FIGURE 9Sk-1: Labelling of the structures in figure 9Sk.

FIGURE 9Sl: Histologic section showing the origin of the parietal part of an aortic leaflet from ventricular muscle.

FIGURE 9Sl-1: Labelling of the structures in figure 9Sl.

FIGURE 9Sm: Bisection of the aorta through the origin of a coronary artery. Note that the valve leaflets closes against the aortic bar and that the coronary artery ostium is beneath the bar.

FIGURE 9Sm-1: Labelling of the structures in figure 9Sm.

FIGURE 9Sn: Section through the aortic outflow tract from the right side showing how the right coronary cusp is related to the infundibulum of the right ventricle.

FIGURE 9Sn-1: Labelling of the structures in figure 9Sn.

FIGURE 9So: A dissected atrioventricular junction viewed from above showing how the aortic valve wedges itself between the mitral and tricuspid valves.

FIGURE 9So-1: Labelling of structures in figure 9So.

FIGURE 9Sp: Section through the atrioventricular junction viewed from above and behind showing the relationship of the non-coronary cusp to the right atrium.

FIGURE 9Sp-1: Labelling of structures in figure 9Sp.

Left Atrium

The left atrium receives blood from the pulmonary veins and serves as the reservoir during left ventricular systole and as a conduit during left ventricular filling. In addition, left atrial contraction provides a significant increment of blood to the left ventricle, stretching the ventricle and priming it for ventricular ejection. This is sometimes referred to as the "atrial kick" or atrial component of ventricular filling.

The left atrium is located superiorly, in the midline, and posterior to the other cardiac chambers (Figs. 1C, 3, 7, and 8). As a consequence of this posterior position, the left atrium is not normally seen in the frontal roentgenogram (see figure 8.1 which follows).

Fig. 8.1

This photograph of an x-ray film of the chest showing a giant left atrium appeared on the front cover of the August 7, 2001, issue of Circulation. Note that the huge left atrium touches the right lateral wall of the chest and not the left.
( J. Willis Hurst, MD, Division of Cardiology, Emory University, 1462 Clifton Road, NE, Suite 301, Atlanta, GA 30322).

In the article by Doctor J. Willis Hurst, it is emphasizes that the above abnormalities noted on x-ray films of the chest can be diagnostic of giant left atrium. It also pointed out that a giant left atrium that occasionally occurs in patients with rheumatic mitral valve regurgitation does not occur in patients with mitral regurgitation due to other causes.

Fig. 8.10 Per J. Willis Hurst, MD, this photograph was published in the first edition (1931) of Heart Disease by Dr Paul White. It shows the heart of a patient with a giant left atrium due to predominate mitral regurgitation secondary to rheumatic heart disease. The left atrium held 1760 cc of fluid and the right atrium held 650 cc of fluid. Dr.Hurst felt this was the same heart that was preserved in a jar in the cardiac museum adjacent to the cardiac onference room in the Massachusetts General Hospital in the late 1940s.

Reproduced from White PD. Heart Disease. New York, NY: MacMillan Company; 1931: 460–461.

The publisher of Heart Disease is no longer in existence. For this reason and because the book was published 70 years ago, the photograph is used under the fair use rules of copyright protection.

Figure 8.11: Figure 1. Posteroanterior chest x-ray showing near-complete opacification of the right mid-to-lower lung zones, with a shift of the mediastinal structures and heart leftward. An underlying mass lesion could not be excluded. The remaining lung fields are without evidence of focal consolidation ( Paulo R. Schwartzman, MD and R.D. White,MD; Circ.2001;page104:e28.)

Figure 8.11: Figure 2. Cine images of enlarged left atrium due to mitral valve disease.
Coronal (A) and axial (B) images demonstrate almost complete filling of the right hemithorax by the left atrium secndary to mild mitral stenosis, and moderate-to-severe mitral insufficiency.

(Schwartzman PR, White RD. Giant left atrium. Circulation.. 2001; 104: e28–29).

Giant Left Atrium

The above images are from the N. ENGL. J. MED 358;19, MAY 8, 2008 by Garrick C. Stewart, M.D. and Anju Nohria, M.D., Brigham and Women's Hospital, Boston, Ma. 02115. The chest radiography (panel A) shows cardiomegaly, splaying of the carina, and an elevated left main bronchus(arrows). An echocardiogram illustrates massive biatrial enlargement(left greater than right), normal ventricular size and function, and moderate mitral and tricuspid regurgitation(Panel B;LA indicates left atrium, LV left ventricle, RA right atrium, and RV right ventricle). An esophagram( panel C) revealed a prominent impression of the left atrium on the esophagus(E),without evidence of obstruction.

These images are presented here to illustrate how the giant left atrium appears utilizing other modalities of imaging.

The esophagus abuts directly on its posterior surface, while the aortic root impinges on its anterior wall. The right atrium is located to the right and anterior (Fig. 1-C). The left ventricle is to the left, anterior, and inferior. The posterior position of the left atrium makes it impossible to palpate externally unless it is massively dilated. With severe mitral regurgitation, however, expansion of the left atrium from the regurgitation and the ejection recoil of the anteriorly located ventricles may force the heart anteriorly, producing a late systolic sternal lift. The left atrium usually enlarges posteriorly and laterally in mitral stenosis or regurgitation, occasionally even reaching the right or left lateral chest wall ( see figures 8.9 , 8.10 and 8.11 above)

The wall of the left atrium is 3 mm, slightly thicker than that of the right atrium. Two pulmonary veins enter posterolaterally on each side, conveying oxygenated blood from the lungs. Though there are no true valves at the junction of the pulmonary veins and the left atrium, "sleeves" of atrial muscle extend from the left atrial wall around the pulmonary veins for 1 or 2 cm and may exert a partial sphincter-like influence, tending to lessen reflux during atrial systole or mitral regurgitation (Fig. 8, 8.1a, 8.1b, and 8.1c ).

Fig. 8.1a, 8.1b, and 8.1c: Two pulmonary veins on each side (one superior and one inferior). Note the sleeves of atrial muscle extending into the pulmonary veins 1 to 2 cm. Pictures obtained by John Sutherland, M.D. Arizona Heart Institute, Phoenix , Az. using the G.E. 64 slice CT scanner.

Figure 8.1d Further view of the pulmonary veins entering the left atrium. Pictures obtained by John Sutherland, M.D. Arizona Heart Institute, Phoenix , Az. using the G.E. 64 slice CT scanner.

The endocardium of the left atrium is smooth and slightly opaque (Figs. 8 ). Pectinate muscles are present only in the left atrial appendage, which projects from the anterolateral left atrium, alongside the pulmonary artery. The atrial septum is smooth but may contain a central shallow area, corresponding to the fossa ovalis (Figs. 8).

The smooth-walled part of the left atrium is larger than the appendage (figure 8.4) and superiorly receives the four pulmonary veins, two to each side (figures 8.3, and 8.4).

Figure 8.3: Dissection viewed from behind showing the left-sided chambers. The atrium is the most posterior and receicves the four pulmonary veins at its four corners.The appedage passes forwards to hook round the great arteries.

Diagram - Figure 8.3-a: Drawing of figure 8.3 above with labelling.

Figure 8.4: The left atrium in an intact heart viewed from above and from the left. It shows the pulmonary veins entering the posterior aspect and the appendage hooking round the great arteries.

Diagram - Figure 8.4-a: Drawing of figure 8.4 above with labelling.

Figure 8.5: The heart viewed from behind and oriented in its in situ position. The position of the coronary sinus is shown in the left artioventricular groove between the left atrium and the left ventricle.

Diagram - Figure 8.5-a: Drawing of figure 8.5 above with labelling.

Inferiorly, the coronary sinus is found along the posterior wall of the left atrium occupying the left atriooventricular sulcus (figure 8.5). In hearts with a persistent left superior vena cava which drains to the coronary sinus, the left-sided cava forms a channel between the left atrial appendage and the left pulmonary veins (figure 8.6).

Figure 8.6: Posterior view of a heart with a persistent left superior vena cava. The vein runs down between the appendage and the pulmonary venous portion to drain into the right atrium via the coronary sinus.

Diagram - Figure 8.6-a: Drawing of figure 8.6 above with labelling.

Figure 8.7: View of the heart from behind showing the prominent groove (Waterston's groove) between the right pulmonary veins and the right atrium.

Diagram - Figure 8.7-a: Drawing of figure 8.7 above with labelling.

In some hearts, a fibrous strand representing the site of the left cava present during development can be observed in a similar position. The lower end of the strand is frequently patent,forming the oblique vein of the left atrium, which drains into the coronary sinus.To the right, the right pulmonary veins are separated from the right atrium by the sulcus marking the site of the limbus of the fossa ovalis (figure 8.7). Internally, the appendage of the left atrium is trabeculated as is the right appendage; but the junction of the trabeculae and the venous atrium on the left is not marked by the presence of any structure comparable to the crista terminalis, and the trabeculae are less pronounced (figure 8.8 and 8.8a).

Figure. 8.8: The left atrium has a roof, a floor, a posterior wall, an anterior wall and a septal surface.

The left atrium has a roof, a floor, a posterior wall, and a septal surface. The roof is formed by the tissue between the four pulmonary veins and this wall continues over into the posterior surface. The floor is the orifice of the mitral valve, considered along with the valve in its separate section. Anteriorly is thesmall ostium of the atrial appendage, abutting inferiorly on the mitral orifice. The left atrium also has an extensive anterior wall composed solely of roughened musculature which lies posterior to the aorta (figs. 12and 12a).

The septal surface is oblique and consists of the left atrial surface of the fossa ovalis.There is no rim to the fossa ovale on the left atrial side; but anteriorly, the flap valve is usually plastered down onto the anterior wall, the junction being marked by a characteristic rough area (fig.8.10a). When the septum is transilluminated, it is found that the floor of the fossa ovalis visible in the right atrium is posterior to the rugose area of the left atrial wall (compare figs.4.10i-1,4.10j-1, 8.11-a, 8.10). When the anterior part of the flap valve is not adherent to the anterior atrial wall, then probe-patent foramen ovale results(fig.8.11).

By comparison of the transilluminated figures(figs.4.10j-1 and 8.11-a ) and examination of the cross sections (figs.4.10f , 8.12 and 8.13) it can be seen that the interatrial septum occupies only a small part of the atrial walls described as septal 'surface'. Much of the superior limbus is simply the infolded sulcus between the superior vena cava and the right pulmonary veins(Fig.4.10i-1).The anterior limbus is mostly the anterior atrial wall and in this position is in direct relation to the anterior root of the aorta, being separated from it by the transverse sinus of the pericardium (fig.8-12).The inferior limbus is in part true atrial septum; but, owing to the origin of the tricusp valve from the septum being much more towards the ventricular apex than that of the mitral valve (fig.8.13), much of the inferior limbus is positioned between the right atrium and the inlet portion of the left ventricle. Similarly, the anterior part of the limbus overlying the the central fibrous body is continuous with the atrioventricular component of the membranous septum and is located between right atrium and the aortic outflow tract (fig.8.14). The area around the coronary sinus is related
directly to atrioventricular sulcus tissue.Consequently, it is not part of the septum (figs.8.15 & 8.16).

Finally, the area of the posterior limbus is directly continuous with the wall of the inferior vena cava and only a small part is true atrial septum.The small area of the true septum can be illustrated by inserting markers at its margins (figs.8.17 & 8.18) and by removing the septum (figs.8.19 and 8.20). The importance of this is largely surgical, since incisions placed outside the area of the septum will carry the surgeon outside the heart. Similarly,'septal' puncture performed at catheterization in a position anterior to the area of the fossa ovalis where there is frequently a recess in the anterior wall of the right atrium(figs.8.21 and 8.21a ) will produce cardiac rupture. A catheter lodged in this recess can easily simulate a position in the fossa ovalis. If pushed through the recess, the catheter will pass through the transverse sinus and into the aorta or pulmonary artery.

Diagram - Fig. 8.8a

Figure 8.10: The septal surface of the left atrium and the vestibule of the mitral valve. Note the roughened aspect of the septal surface which is the flap valve of the fossa ovalis.

Diagram - 8.10a. The same specimen as in fig.8.10 transilluminated from the right side. The fossa ovalis is well posterior and inferior to the flap valve noted on the left septal surface.