462 THE BLOOD- VASCULAR SYSTEM. 



It presents the following parts for examination : 

 [" Superior cava. 



Inferior caya. v , / Eustachian. 



Openings <j Coronary sinus. Valves | Cor 



Foramina Thebesn. J 



Auriculo-ventricular. 



Fossa ovalis. 



Annulus ovalis. 



Tuberculum Loweri. 



Musculi pectinati. 



The superior vena cava returns the blood from the upper half of the body, and 

 opens into the upper and back part of the auricle, the direction of its orifice being 

 downward and forward. 



The inferior vena cava, larger than the superior, returns the blood from the 

 lower half of the body, and opens into the lowest part of the auricle near the 

 septum, the direction of its orifice being upward and inward. The direction of 

 a current of blood through the superior vena cava would consequently be toward 

 the auriculo-ventricular orifice, whilst the direction of the blood through the 

 inferior cava would be toward the auricular septum. This is the normal direction 

 of the two currents in fostal life. 



The coronary sinus opens into the auricle, between the inferior vena cava and 

 the auriculo-ventricular opening. It returns the blood from the substance 

 of the heart, and is protected by a semicircular fold of the lining membrane 

 of the auricle, the coronary valve (valve of Thebesius). The sinus, before enter- 

 ing the auricle, is considerably dilated nearly to the size of the end of the little 

 finger. Its wall is partly muscular, and at its junction with the great coronary 

 vein is somewhat constricted and furnished with a valve consisting of two unequal 

 segments. 



Tlae foramini Thebesii are numerous minute apertures, the mouths of small 

 veins (venae cordis minimce), which open on various parts of the inner surface of 

 the auricle. They return the blood directly from the muscular substance of the 

 heart. Some of these foramina are minute depressions in the walls of the heart, 

 presenting a closed extremity. 



The auriculo-ventricular opening is the large oval aperture of communication 

 between the auricle and the ventricle, to be presently described. 



The Eustachian valve is situated between the anterior margin of the inferior 

 vena cava and the auriculo-ventricular orifice. It is semilunar in form, its convex 

 margin being attached to the wall of the vein ; its concave margin, which is free, 

 terminating in two cornua, of which the left is attached to the anterior edge of 

 the annulus ovalis, the right being lost on the wall of the auricle. The valve is 

 formed by a duplicature of the lining membrane of the auricle containing a few 

 muscular fibres. 



In the foetus this valve is of large size, and serves to direct the blood from the 

 inferior vena cava, through the foramen ovale, into the left auricle. 



In the adult it is occasionally persistent, and may assist in preventing the 

 reflux of blood into the inferior vena cava ; more commonly it is small, and its 

 free margin presents a cribriform or filamentous appearance ; occasionally it is 

 altogether wanting. 



The coronary valve (valve of Thebesius) is a semicircular fold of the lining 

 membrane of the auricle, protecting the orifice of the coronary sinus. It prevents 

 the regurgitation of blood into the sinus during the contraction of the auricle. 

 This valve is occasionally double. 



The fossa ovalis is an oval depression corresponding to the situation of the 

 foramen ovale in the foetus. It is situated at the lower part of the septum auricu- 

 larum, above and to the left of the orifice of the inferior vena cava. 



The annulus ovalis is the prominent oval margin of the foramen ovale. It is 



