ATRIAL PORTION 



511 



ATRIAL PORTION 



The atrial portion of the heart is situated behind, and shghtly 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 (such as that shown in 

 fig. 424) ; in these it is marked by a slight groove connecting the left sides of the 

 superior and inferior vena? 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 vense cavse, is the sulcus terminalis (figs. 424, 425). This represents the 

 right limit of what was, in the embrj^o, 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 cavse have each a nearly vertical direction 

 and join the posterior part of the right atrium above and below, respectivelj'. 

 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. 



Fig. 425. — Atria Opened Posteriorly to show the Septum Atriorum. 



Pulmonary artery 

 Aorta 



Left superior pulmonary 

 vein 



Valvula f or a 

 minis ovalis 



Vena cava superior 



Crista terminalis 



Limbus fossse 

 ovalis 



^section) 



Valvula 

 venee cavse 



Vena cava inferior 



Fades diaphragmatica 



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, bj- the lateral fusion at the time of iDirth, they 

 together produce the impervious structure of the adult heart (fig. 425). 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 cf the limbus 

 fossae ovalis. The margin of the membranous septum is recognizable as a fold 



