THE HEART AND VASCULAR SYSTEM. 171 



Opening of coronary sinus, protected by a valve of two un- 

 equal segments the coronary valve ; 



Foramina Thebesii, minute foramina returning the blood 

 from the heart-muscle; 



Eustachian valve, the remains of a foetal structure, extend- 

 ing from the right of the orifice of the inferior vena cava to 

 the outer border of the oval foramen; 



Fossa ovalis, the obliterated foramen ovale of foetal life; 



Annulus ovalis, the oval margin of the preceding structure; 



Tubercle of Lower, a small rudimental projection on the 

 right wall, directing the blood toward the auriculo- ventricular 

 opening : 



FIG. 104. 



Right side of heart: a, apex; b, right ventricle; d, pulmonary 

 artery; e, f, chordae tendinae; p, aorta; 3, superior vena cava; 5, 

 inferior vena cava. 



Musculi pectinati, elevated muscular columns before re- 

 ferred to; 



Auriculo-ventricular opening, an oval aperture about one 

 inch in diameter, surrounded by a fibrous ring, and protected 

 by the tricuspid valve. 



THE LEFT AURICLE. The left auricle is smaller, and its 

 walls somewhat thicker, than the right, and consists of a cuboidal 

 sinus and an elongated appendix auriculae. Its interior surface 

 is smooth, except the auricular appendage, which is provided 

 with musculi pectinati, and presents the following openings: 



Pulmonary veins, four in number, two on either side; they 

 are without valves; 



