THE THORAX. 355 



The opening of the coronary sinus is found between that 

 of the inferior vena cava and the auriculoventricular. It is 

 guarded by the valve of Thebesius, formed by a reduplica- 

 tion of the endocardium. 



The inferior caval opening is protected by the Eustachian 

 valve. 



The tubercle of Lower is found between the two caval 

 openings and at their right. 



The appendix and anterior surface of the auricle is thrown 

 into ridges by the musculi pectinati. 



The septum between the two auricles is depressed, form- 

 ing the fossa ovalis (in the foetus it is an opening), sur- 

 rounded by a border, the annulus ovalis. 



The Left Auricle. Openings. Four pulmonary veins 

 (two may open by one orifice), auriculoventricular, fora- 

 mina Thebesii. 



The musculi pectinati are found in the appendix. 



The Right Ventricle. Openings. The auriculoventri- 

 cular from the auricle, the pulmonary artery. The former 

 is guarded by the tricuspid valves, the latter by the semi- 

 lunar. 



The tricuspid valve is formed of three main and three 

 secondary triangular flaps of membrane. These flaps or 

 cusps are attached by their bases to the fibrous ring about 

 the auriculoventricular orifice, and also to each other for a 

 short distance from their bases. 



The semilunar valves, three in number, situated at the 

 beginning of the pulmonary artery (and aorta, also). They 

 are reduplicated folds of the inner coat of the vessel plus 

 some fibrous tissue. At the centre of the free margin is a 

 small nodule, the corpus Arantii. By the peculiar arrange- 

 ment of the fibrous tissue in the semilunar valves a lunated 

 space is left on either side of each corpus Arantii, consisting 



