consider only the parts that can be removed without creating a communication with the extracardiac space to be true septal structures Thus, when one views the septal surface of the right atrium, it seems at first sight that a large expanse of the atrial walls between the orifices of the superior and inferior caval veins and the attachment of the septal leaflet of the tricuspid valve are potentially interposed between the cavities of the right and left atria (Fig 29.2, left) FIG 29.2 Septal surface of the right atrium The yellow line shows the plane of section taken to produce the image at right, which shows that the anterior and posterior rims of the oval fossa are infoldings of the atrial walls (An orthogonal section of the same heart is shown in Fig 29.3.) Note that the tendon of Todaro is the cranial continuation of the eustachian valve It extends through the anteroinferior buttress of the fossa to insert into the central fibrous body ICV, Inferior caval vein; SCV, superior caval vein When the heart is sectioned across the oval fossa, its anterior and posterior rims are seen as folds between the atrial walls (see Fig 29.2, right) Sections taken through the atrial chambers in the frontal plane show that the superior rim of the fossa, often considered to represent the septum secundum, is similarly a fold (see Fig 29.1) This fold is the area known to surgeons as the Waterston or Sondergaard groove Inferiorly, the rim separates the fossa from the orifice of the coronary sinus, with the mouth of the coronary sinus itself separated from the mouth of the inferior caval vein by an additional fold, the sinus septum The tendon of Todaro, one of the important landmarks of the triangle of Koch, runs through this area to insert into the central fibrous body (see Fig 29.2, left) This anteroinferior muscular buttress, traversed by the tendon, is another true septal structure (Fig 29.3) FIG 29.3 The heart shown in Fig 29.2 has been sectioned again to show that the superior rim of the oval fossa is also a fold between the atrial walls Note the walls of the coronary sinus in the left atrioventricular groove The star shows its mouth in the right atrium There is an additional fold between the walls of the sinus and the inferior caval vein, which is known as the sinus septum ICV, Inferior caval vein; SCV, superior caval vein Thus only the floor of the oval fossa and the anteroinferior muscular buttress constitute true interatrial septal structures, which can be removed without exiting the cavities of the heart (see Fig 29.1) Types of Interatrial Communication Recognition of the extent of the normal septum underscores the fact that not all interatrial communications are septal deficiencies.5 Of the channels that permit interatrial shunting, those within the confines of the oval fossa, along with the much rarer deficiencies of the anteroinferior muscular buttress, are true septal defects The ostium primum defects, along with the sinus venosus defects and those found at the site of the mouth of the coronary sinus, are interatrial communications, but they lie outside the confines of the atrial septum (Fig 29.4) FIG 29.4 Location of the various lesions that permit interatrial shunting The oval fossa and vestibular defects are true atrial septal deficiencies The remaining lesions are properly described as interatrial communications rather than atrial septal defects Defects Within the Oval Fossa These lesions, which are true septal defects, are by far the commonest type of interatrial communication Although most frequently termed secundum defects, this term is justified only because the defect represents persistence of the second interatrial communication to be formed during cardiac development Should they be found postnatally, it is because of deficiencies of the floor of the fossa, which