The atrial portion of the heart is situated behind, and slightly to the right of and above, the ventricular portion. The separation between the right and left atrium is not indicated behind except in distended hearts; in these it is marked by a slight groove connecting the left sides of the superior and inferior venae cavae. In front, the auricles are separated by the deep notch which lodges the aorta and pulmonary artery. A slight groove on the back of the right atrium which connects the right sides of the superior and inferior venae cavae, is the sulcus terminalis. This represents the right limit of what was, in the embryo, the sinus venosus. It also indicates that the embryonic sinus venosus has become an integral part of the adult right atrium. The superior and inferior cavae have each a nearly vertical direction and join the posterior part of the right atrium above and below, respectively. The coronary sinus runs downward, backward and to the right to join the lower part of the right atrium anterior to the inferior vena cava. The four pulmonary veins run nearly transversely and somewhat forward into the right and left sides of the left atrium.
The interior of the atrial portion of the heart is divided into right and left cavities by the septum atriorum. This septum is a composite structure, having been developed (see morphogenesis of the heart) in two independent parts, each forming an incomplete septum in itself. The two incomplete septa, however, partly overlap one another so that, by the lateral fusion at the time of birth, they together produce the impervious structure of the adult heart. Of these septa, the first to be formed is the membranous septum [pars membranacea septi atriorum]. Later there is formed to the right of this the muscular septum, the margin of which forms, in the adult atrium, the greater part of the limbus fossae ovalis. The margin of the membranous septum is recognizable as a fold of endocardium on the septal wall of the left atrium; it is called the valvula foraminis ovjilis.
Posteriorly into the right atrium [atrium dextrum], above and below, respectively, open the superior and the inferior vena cava. Upon the septal wall, immediately above the inferior cava is the fossa ovalis, a depression of which the floor is formed by the membranous septum. Surrounding the fossa ovalis except below (indeed producing the fossa) is the limbus fossae ovalis which is continuous anteriorly and below with the valvula venae cavae (inferioris Eustachii). Just anterior to the fossa ovalis is the orifice of the coronary sinus guarded by the valvula sinus coronarii (Thebesii). Leading from the front of the atrium forward and slightly downward and to the left is the ostium venosum (right atrioventricular orifice) guarded by the tricuspid valve. Above and behind this is the auricle, the exterior of which is in contact medially with the root of the aorta. To the right of the superior and inferior caval orifices there is a vertical ridge, the crista terminalis, which corresponds to the sulcus terminalis on the exterior.
The portion of the atrium medial to the crista is smooth and is called the sinus venarum; in the embryo it is separated from the atrial cavity proper by the right and left sinus valves. The crista terminalis marks the original line of attachment of the right sinus valve. The valve itself has disappeared, except at the lower part where it persists as the caval and coronary valves. These valves vary in size considerably in different specimens, and are frequently netlike from numerous perforations.
The conversion of a portion of a single valve into two separate valves, which meet at an acute angle, is brought about by an attachment between the sinus valve and an embryonic structure called the sinus-septum. This septum is a ridge dividing the right horn of the sinus venosus from the transverse portion of the sinus (the coronary of the adult); it probably con tributes somewhat to the formation of both the coronary and caval valves. The left sinus valve usually disappears by blending with the septum atriorum on which it unites with the limbus fossae ovalis; it occasionally remains partially separate in the adult.
The interior of the right auricle and of the portion of the atrium lateral to the crista terminalis is thrown into ridges (musculi pectinati) by prominent bands of the atrial myocardium. The musculi pectinati end abruptly by joining the crista. The orifice of the superior cava has no valve and is directed downward and somewhat forward; below it, on the posterior wall of the atrium, there has been described a tubercle or ridge, the tuberculum intervenostun (Loweri).
Apart from the posterior circumference of the superior cava itself and the limbus fossae ovalis, the human heart appears to contain nothing in this region that could be described as a tubercle. With regard to the segregation of the streams entering the fetal right atrium from the superior and inferior cavae, respectively, in which the tubercle of Lower has been supposed to participate, it is to be noted that the fossa ovalis is just above (almost within) the inferior caval orifice. Also that the caval opening and the fossal ovalis (containing the fetal foramen ovale) are, in hearts well hardened before removal, situated in a distinct diverticulum to the left of the remainder of the atrium. Between this diverticulum and the atrium proper, the caval valve and the limbus fossae ovalis form a prominent flange, better marked in the fetus than the adult. Opening into the right atrium, particularly upon the septal and right lateral walls, are numerous foromina venarum minimarum (Thebesii).
The left atrium [a. sinistrum] is to the left and somewhat posterior to the right. It is behind the root of the aorta and its auricle is to the left of the pulmonary root. Opening into it posteriorly on the right and left sides, respectively, are the right and left upper and lower pulmonary veins. The valvula foraminis ovalis forms a more or less distinct crescentic ridge on the septal wall. This may not be attached to the limbus fossas ovalis, in which case there is a communication between the two atria. Absence of lateral adhesion between the two septa atriorum does not necessarily lead to admixture of arterial and venous blood during life. The left ostium venosum (atrio-ventricular orifice) guarded by the mitral valve leads from the anterior part of the atrium forward and slightly downward and to the left. The interior of the left atrium is smooth except in the auricle, in which musculi pectinati are well marked.
The atrio-ventricular valves are attached around the venous ostia of the ventricles in such a way as to open freely into the ventricles, but to prevent regurgitation of the blood into the atria during ventricular systole. Each valve is continuous along its line of attachment, but its free edge is notched so as to produce an irregular margin; some of the notches are so deep as to partially divide the valve into cusps. The right atrio-ventricular valve is commonly divided by three deep notches into three cusps; this valve is therefore called the tricuspid [valvula tricuspidalis]. The left is similarly divided into two cusps and is called the bicuspid [v. bicuspidalis] or mitral. The depth of the notches, however, is very variable and there may be an increase or (more rarely) a diminution in the number of cusps; the addition of small subsidiary cusps is quite common. Each valve cusp is tied down to the papillary muscles [mm. papillares] of the ventricle by chordae tendinese. The latter are fibrous cords, generally branched, of varying thickness. The thinnest cords are attached to the free margin of the cusp; those of intermediate thickness to the ventricular surface a few millimeters from the margin, and the thickest to the ventricular surface near the attached margin. The valves are smooth and glistening on the atrial aspect, but rough and fasciculated, from the attachment of the chordae, on the ventricular. The cusps of the mitral valve are called anterior and posterior; those of the tricuspid, anterior, posterior and medial. Each cusp receives chorda from more than one papillary muscle and each papillary muscle sends chordse to more than one cusp. The chordae tendineae of the mitral valve are thicker than those of the tricuspid.