Introduction
It is axiomatic that to understand abnormal anatomy, and to describe it adequately, it is necessary first to understand the normal arrangement, including the relationships of the conduction tissues and coronary arteries to the various components of the heart. We review these features in this chapter and emphasize the significance of describing the various cardiac components as they are seen with the heart normally positioned within the chest, the attitudinally appropriate approach. We show how it is now possible also to illustrate the significant features during life by appropriate interrogation of multidetector-row computed tomographic datasets. The significant aspects of abnormal anatomy will then receive attention in the various chapters within the book concerned with specific lesions.
Heart Within the Chest
The normal interrelationships of the chambers within the heart, and the location of the heart itself within the chest, are now readily apparent to the clinician since they are revealed by the various three-dimensional techniques increasingly used during the diagnostic cascade. The heart normally occupies the middle compartment of the mediastinum, with the larger part of its bulk to the left of the midline ( Fig. 2.1 ).
The cardiac long axis shows a considerable obliquity relative to the long axis of the body, extending roughly along a line drawn through the right shoulder to the left hemidiaphragm. Despite this discrepancy between the planes of the body and those of the heart, we should still describe the cardiac structures relative to the bodily coordinates. Such attitudinally appropriate description is a basic rule of human anatomy, but has not always been followed by those describing the heart. The cardiac silhouette, when projected to the frontal surface, is more or less trapezoidal with horizontal upper and lower borders, a relatively vertical right border just outside the right edge of the sternum, and a sloping left border extending out to the apex in the fifth intercostal space (see Fig. 2.1 ).
The true base of the heart is the posterior aspect of the atrial chambers, where the organ is anchored within the mediastinum. More frequently, it is the ventricular mass that is described as having a base. All four cardiac valves are seen when this area is viewed from the atrial aspect ( Fig. 2.2 , left ).
When viewed from the front in attitudinally appropriate fashion, the pulmonary valve is located superiorly and to the left, with the aortic, mitral, and tricuspid valves overlapping when traced in rightward and inferior direction (see Fig. 2.2 , right ). The key feature from the stance of understanding normal cardiac anatomy is the central location of the aortic root with the aortic valvar leaflets and their supporting aortic sinuses, being related to all the cardiac chambers (see Fig. 2.2 , left ).
The heart itself, occupying the middle part of the mediastinum, is encased within the pericardial sac. The fibrous outer layer of the pericardium functions as the cardiac seat belt, with its attachments to the diaphragm, along with the reflections at the entrances and exits of the great veins and arterial trunks, anchoring the heart within the mediastinum ( Fig. 2.3A ). The tough fibrous component is lined with a serous layer, the parietal pericardium, which is itself reflected onto the surface of the heart as the epicardium. Within the cavity thus formed are two recesses, the transverse and oblique sinuses (see Fig. 2.3B ).
Embedded within the fibrous pericardium are the vagus and phrenic nerves. Both sets of nerves traverse the length of the heart on each side, with the phrenic nerves anterior ( Fig. 2.4 ) and the vagus nerves posterior to the hilums of the lungs.
The recurrent laryngeal nerves branch from the vagus nerves within the mediastinum, recurring around the brachiocephalic trunk on the right side, and around the arterial duct, or its ligamentous remnant, on the left. The thymus gland is also a prominent structure related to the anterior and lateral aspects of the pericardial sac in the region of exit of the great arterial trunks. It is particularly prominent in neonates and infants. The esophagus, tracheal bifurcation, and descending aorta are additional important mediastinal structures related posteriorly to the heart, with their locations now well demonstrated using computed tomography ( Fig. 2.5 ).
Chambers Within the Heart
It is a mistake to assess the heart in upright “valentine” fashion. As already emphasized, the cardiac long axis extends from right to left with considerable obliquity (see Fig. 2.1 ). When seen in frontal projection, the anterior surface of the silhouette is occupied for the most part by the right atrium and ventricle. The left atrium is almost entirely a posterior structure, with only its appendage projecting to the left upper border, while only a strip of left ventricle is seen down the sloping left border. The so-called right chambers of the heart, therefore, are basically anterior, with the ventricles situated to the left and inferiorly relative to their atrial counterparts. The aortic and mitral valves are closely related one to the other within the base of the left ventricle, while the pulmonary and tricuspid valves are separated in the roof of the right ventricle by the extensive supraventricular crest, known classically in its Latin form as the crista supraventricularis. The crest itself is intimately related on its posterior aspect to the aortic root (see Fig. 2.2 , left ). The diaphragmatic border of the ventricular mass, made up of the right ventricle, has a sharp angle between its sternocostal and inferior surfaces, known as the acute margin ( Fig. 2.6 ). In contrast, the left border of the ventricular mass formed by the left ventricle has a much gentler curve, and is the obtuse margin. Important grooves are found within the various surfaces, with the atrioventricular, or coronary, grooves marking the cardiac short axis, and the interventricular grooves demonstrating the location of the ventricular septum. A particularly important point is found on the diaphragmatic surface, positioned inferiorly rather than posteriorly when the heart is located within the body (see Fig. 2.6 ). This is the cardiac crux, which marks the crossing of the interventricular and atrioventricular grooves.
Morphologically Right Atrium
The right atrium in the normal heart is recognized most readily as the chamber receiving the systemic venous return through the superior and inferior caval veins, along with the venous return from the heart itself through the coronary sinus ( Fig. 2.7 ).
These channels open into the smooth-walled venous component of the atrium. The atrial chamber possesses a second smooth-walled area, the vestibule of the tricuspid valve. The atrium also has a characteristic septal surface, which is located posteriorly. It is the appendage with its pectinated wall that is the most prominent component of the morphologically right atrium. It forms the entirety of the anterior wall ( Fig. 2.8 ).
A small part of the initial atrial component of the developing heart is retained within the definitive atrium. This part, the body, is not usually recognizable in most normal hearts since it is located between the left venous valve and the septum. The left venous valve itself does not remain as a discrete structure in most postnatal individuals. It is the appendage that is the most constant part. Therefore this feature should be used to permit recognition of the chamber as the morphologically right structure when the heart is congenitally malformed. Recognition of structures according to their morphology rather than their location, and using their most constant part in final arbitration, is called the “morphological method.” As discussed in Chapter 1 , this principle is the basis of logical analysis of congenitally malformed hearts. The characteristic external feature of the right appendage is its broad triangular shape (see Fig. 2.8 ), along with its extensive junction with the smooth-walled venous component, this being marked by the terminal groove. Internally, the groove matches with the strap-like terminal crest (see Fig. 2.7 ).
Taking origin in parallel fashion from the crest, and extending laterally into the appendage, are the pectinate muscles. In the morphologically right atrium, these muscles extend all around the atrioventricular junction, reaching into the recess located inferior to the orifice of the coronary sinus. This recess, also described as a sinus, is sub-thebesian when the heart is seen in the attitudinally appropriate position (see Fig. 2.7 ). The extent of the pectinate muscles relative to the vestibule of the right atrioventricular valve is the single most characteristic feature of the right atrium in congenitally malformed hearts. In many hearts, the flap-like muscular or fibrous eustachian and thebesian valves, remnants of the embryonic right venous valve, take origin from the terminal crest and guard the orifices of the inferior caval vein and the coronary sinus, respectively (see Fig. 2.7 ). Their size varies markedly, and they are not uniformly present. An important structure can usually be found in continuation with the eustachian valve. This is the tendon of Todaro, which runs through the wall that separates the coronary sinus from the oval fossa, the so-called sinus septum, to insert into the fibrous root of the aorta. It forms one of the borders of the triangle of Koch (see below). In some hearts it is possible to recognize remnants of the left venous valve, which when present is usually adherent to the rims of the atrial septum.
At first sight, the right atrium seems to have an extensive septum, surrounding the oval fossa and incorporating the orifices of the superior caval vein and coronary sinus. This appearance is deceptive. Only the floor of the oval fossa, derived from the primary atrial septum, along with its anteroinferior rim, separate the cavities of the two atriums. The remaining rims of the oval fossa, often described as the septum secundum, or the secondary septum, are the infolded walls of the atrial chambers. The infolding is particularly prominent superiorly, where it forms Waterston’s, or Sondergaard’s, groove ( Fig. 2.9 ).
The anteroinferior margin of the fossa representing the part formed by muscularization of the vestibular spine, also known as the dorsal mesenchymal protrusion, is an additional true septal structure (see Chapter 3 ). It continues anteriorly as the atrial component of the atrioventricular muscular sandwich. This exists because of the more apical attachment of the leaflet of the tricuspid relative to the mitral valve (see below). The meat in the sandwich is an extension of the inferior atrioventricular groove, which separates the overlapping segments of atrial and ventricular muscle, and is particularly well demonstrated by virtual dissection ( Fig. 2.10 ).
The atrial wall of the sandwich forms the floor of the triangle of Koch and is confluent with the so-called sinus septum, which separates the orifices of the coronary sinus and the inferior caval vein (see Fig. 2.8 ). The sinus septum, however, is no more than the adjacent walls of the two venous structures.
Morphologically Left Atrium
The left atrium, like its right-sided counterpart, possesses a venous component, an appendage, and a vestibule ( Fig. 2.11 ).
The body, which is the remnant of the atrial component of the developing heart tube, is largely incorporated into the morphologically left atrium. It is well seen in the setting of totally anomalous pulmonary venous connection. Even when the pulmonary venous component is lacking, the smooth-walled body forms the site of union for the appendage, vestibule, and septum. The appendage is again the most characteristic and constant component. It is a tubular structure, usually with several constrictions along its length. Its opening with the venous component is restricted, but its most characteristic feature is that its pectinate muscles are contained within the appendage, or else spill only marginally into the smooth-walled component. The vestibule, which surrounds the posterior part of the left atrioventricular groove, is part of this smooth-walled component. The coronary sinus is located within the atrioventricular groove, and hence is an integral component of the morphologically left atrioventricular junction, even though it opens into the cavity of the morphologically right atrium ( Fig. 2.12 ).
The walls of the coronary sinus are separate from those of the left atrium itself. The pulmonary veins open into the corners of the roof of the extensive smooth-walled venous component. The septal surface is formed by the flap valve of the oval fossa, which is derived developmentally from the primary atrial septum. It overlaps the infolded superior rim ( Fig. 2.13 ) with two horns which anchor it on the left atrial side.
Morphologically Right Ventricle
The muscular walls of the right ventricle extend from the discrete atrioventricular junction to their union with the fibroelastic walls of the pulmonary trunk at the anatomic ventriculoarterial junction. The cavity thus demarcated has three components: the inlet, the apical trabecular part, and the outlet ( Fig. 2.14 ).
The inlet component surrounds the leaflets of the tricuspid valve, extending to the attachments of the valvar tension apparatus. The three leaflets of the valve take origin from the septal, inferior or mural, and anterosuperior margins of the atrioventricular junction ( Fig. 2.15 ).
The septal leaflet has multiple cordal attachments to the septum. The inferior leaflet runs along the diaphragmatic surface of the ventricle. Its margin with the anterosuperior leaflet is often indistinct, but its pattern of closure reveals the trifoliate configuration of the valve. The anterosuperior leaflet is the most extensive of the three. It extends from its zone of apposition with the septal leaflet, an area supported by the medial papillary muscle, to the acute margin of the ventricle. The prominent anterior papillary muscle arises from the septomarginal trabeculation to support this leaflet, but not always at its site of apposition with the inferior leaflet.
The apical part of the ventricle has particularly coarse trabeculations, this being the most constant feature of the ventricle in malformed hearts. One of these trabeculations on the septal surface, the septomarginal trabeculation, is particularly prominent. It diverges into two limbs at the base, which clasp the supraventricular crest, also known as the septal band ( Fig. 2.16 ).
The medial papillary muscle arises from its posterocaudal limb, while the anterior papillary muscle springs from its body toward the ventricular apex. The moderator band continues on from the anterior papillary muscle to reach the parietal ventricular wall. A further series of trabeculations extend from the anterior surface of the septomarginal trabeculation. They run into the parietal margin of the trabecular zone as the septoparietal trabeculations.
The outlet component of the right ventricle is relatively smooth walled. It forms the free-standing sleeve of musculature that supports the leaflets of the pulmonary valve. The leaflets of the valve themselves are attached in semilunar fashion within the sleeve, crossing the circular junction between ventricular muscle and the fibroelastic wall of the pulmonary trunk ( Fig. 2.17 ).
