338 PRACTICAL ANATOMY. 



heart, and is protected by the coronary valve, a semi- 

 luriar fold of the endocardium. It opens into the auricle. 

 The sinus will admit the end of the little finger, is about 

 one and one-half inches long, and is formed by the right 

 and left coronary and oblique veins of the heart. 



The foramina thebesii are numerous minute openings 

 of the small veins of the auricles. 



The right auricula-ventricular or tricuspid opening 

 is the opening from the auricle into the ventricle ; it will 

 admit the ends of three fingers, and measures accu- 

 rately four and five-sixths inches ; it is the largest open- 

 ing at the base of the heart, and is strengthened by a 

 strong ring of white fibrous tissue, which aifords attach- 

 ment to the tricuspid valves and gives origin to the 

 muscular layers of the heart. 



The Eustachian valve is a semilunar fold of the 

 endocardium, extending between the anterior margin 

 of the ascending vena cava and the tricuspid opening. 

 In the foetus it helps to direct the column of blood to- 

 ward the foramen ovale. 



On the septum between the auricles is an oval 

 depression, the fossa ovalis, which corresponds to the 

 position of the foramen ovale in the foetus ; it is sur- 

 rounded by an elevated margin, the annulis ovalis. 



The auricular appendix is an ear-like extension of 

 the right auricle. It passes to the left and lies upon the 

 root of the pulmonary artery. It presents a number of 

 cylindrical interlacing bundles of muscular fibres, form- 

 ing the musculi pectinati. 



The Right Ventricle. 



The right ventricle is placed immediately below the 

 right auricle. It is conoidal in form ; its walls are about 

 three-sixteenths of an inch thick. It presents two open- 



