THE CHAMBERS OF THE HEART. 



695 



Fig. 658. 



Pulmonary aorta 



uricular 

 appendage 



708). Between the superior and inferior venae cavae there may sometimes be seen 

 a more or less marked prominence of the posterior wall, the tubercle of Lower 

 (tuberculum intervenosum), the remains of a structure also of importance in the foetal 

 circulation. Below and somewhat median to the opening of the inferior vena cava 

 is the circular orifice of the coronary' sinus,, measuring about 12 mm. in diameter, 

 and guarded, like the inferior caval orifice, by a crescentic valve which surrounds its 

 lateral margin and is termed the Thebesian valve (valvula sinus coronarii). 



The median wall, in addition to a number of Thebesian orifices, presents at about 

 its centre an oval depression, the fossa ovalis, whose superior and anterior borders 

 are surrounded by a thickening or slight fold termed the amiulus ovalis (limbus fossae 

 ovalis). 



The fossa ovalis indicates the position of what was in fcetal life the foramen ovale, through 

 which the blood entering the right auricle from the inferior vena cava passed directly into the 

 left auricle and so joined at once the systemic circulation (page 929). This foramen traversed 

 the auricular septum obliquely, the septum really consisting of two folds, one of which projected 

 backward from the anterior wall of the auricular portion of the heart, and the other forward from 

 the posterior wall, the plane of the latter fold lying sHghtly to the left of that of the former one. 

 After birth these two folds increase in size so that their free margins overlap and event- 

 ually fuse, closing the foramen, and 

 the original free edge of the ante- 

 rior fold becomes the annulus of 

 Vieussens, while the floor of the 

 fossa ovalis is formed by the pos- 

 terior fold. 



It occasionally happens that 

 the foramen ovale fails to close 

 after birth, remaining sufficiently 

 open to permit of serious disturb- 

 ances of the circulation which are 

 usually, although not always, early 

 fatal. Very frequently, however, 

 the fusion of  the overlapping sur- 

 faces of the two folds is not quite 

 complete, and a small, oblique, slit- 

 like opening persists between the 

 two auricles. In such cases during 

 the contraction of the auricles the 

 pressure of the blood on the over- 

 lapping walls of the slit brings 

 them into close apposition and effectually closes the slit, so that no disturbances of the circula- 

 tion result from its presence. This slit-like opening has been found to be present in somewhat 

 over 30 per cent, of the adult hearts examined. 



The Left Auricle. — The left auricle (atrium sinistrum) has the same general 

 external form as the right one, and, as in the latter, its antero-lateral wall is prolonged 

 into an auricular appendix which curves forward around the left side of the proximal 

 portion of the pulmonary aorta. Upon its posterior surface the auricle receives the four 

 pulmonary veins arranged in pairs, one of which is situated nearer the medial and the 

 other towards the lateral edge of the surface, and passing obliquely over this surface 

 towards the coronary sinus is a small vein, known as the oblique vei7i of the left 

 auricle (vena obliqua atrii sinistri [Marshalli] ), which represents the proximal end of 

 the left vena cava superior present during early embryonic life (page 927), 



Viewed from the interior, the walls of the left auricle, like those of the right one, 

 are everywhere lined by a smooth, shining endocardium ; in the appendix the spongy 

 structure due to the existence of anastomosing musculi pectinati also occurs, and 

 occasional depressions of the surface mark the openings of vencs Thebesii, which are, 

 however, much less abundant than in the right auricle. The openings of the pul- 

 monary veins on the posterior wall are circular, and each measures from 14-15 mm. 

 in diameter ; they are unguarded by valves, although a slight horizontal fold sepa- 

 rates the portion of the auricular cavity into which the left veins open from the 

 entrance into the auricular appendix. 



Upon the median wall, over the area occupied by the fossa ovalis of the right 

 auricle, a slight depression is frequently to be observed, and immediately anterior to 

 it there is usually a small crescentic fold, the semilunar fold, whose concavity is 



Superior 

 vena cava 



Interauric- 

 ular septum 



Foramen 



ovale 



Eustachian 

 valve 



Inferior 

 vena cava 



Right 



ventricle 

 Auriculo- 



ventricular 



valve 



Coronary 

 sinus 



Heart of foetus just before birth ; wall of right auricle 

 has been cut away, showing foramen ovale. 



