FIG 2.8 Intact morphologically right atrium photographed in comparable orientation to the opened chamber, as shown in Fig 2.7 The appendage forms the entirety of the anterior wall Note the prominent groove between the posterior extent of the appendage and the systemic venous sinus, marked by the dashed line The caval veins enter the systemic venous sinus superiorly and inferiorly 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.”2 As discussed in Chapter 1, this principle is the basis of logical analysis of congenitally malformed hearts.3 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.4 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,5 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.6 The infolding is particularly prominent superiorly, where it forms Waterston's, or Sondergaard's, groove (Fig 2.9) FIG 2.9 Adult heart sectioned through the long axis of the oval fossa and photographed from the front The section shows well that the superior rim of the fossa, often described as the septum secundum, is no more than the infolded walls between the right superior pulmonary veins and the superior caval vein The floor of the oval fossa is derived from the primary atrial septum, while its anteroinferior buttress is a second true septal structure, formed by muscularization during development of the mesenchymal cap carried on the primary septum and the vestibular spine (see Chapter 3) 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)