ATRIAL PORTION 



513 



front of the atrium forward and slightly downward and to the left is the ostium 

 venosum (right atrio-ventricular 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 terminahs on 

 the exterior (figs. 425, 428). 



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 terminahs 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 

 netUke 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- 



FiG. 427. — The Interior of the Ventricles, Anterior Half. (After His.) 



Pulmonary artery 



Opening into ventricle 



Aortic semilunar 

 valves 



Anterior papillary 

 of left vent. 



Muscular ventricu- 

 lar seplum 



Conus arteriosus 



Membranous ven- 

 tricular septum 

 Crista supraven- 



tricularis 

 ■Papillary of conus 



Right ventricle 



Anterior papillary 

 muscle 



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 terminahs 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 intervenosum (Loweri). 



Apart from the posterior circumference of the superior cava itseK 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 foetal 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 ovahs is just above (almost within) the inferior 



33 



