THE CHAMBERS OF THE HEART. 875 



is the remains of an aperture, the foramen ovale. through which the two atria 

 communicated with one another before birth. Even in the adult a portion of the 

 1 aperture persists at the upper part of the fossa in about one in five cases. Between 

 'the apertures of the superior and inferior venae cavae, and posterior to the upper 

 part of the fossa ovalis, a small eminence may be distinguished, which is called 

 the tuberculum intervenosum ; it probably directs the blood from the superior vena 

 cava to the tricuspid orifice, during foetal life. 



The valvula venae cavae inferioris is a thin and sometimes fenestrated fold of 

 endocardium and subendocardial tissue, which extends from the anterior and 

 lower margin of the orifice of the inferior vena cava to the anterior part of 

 the limbus ovalis. It varies very much in size, and is usually of falciform shape ; 

 its apex is attached to the limbus fossae ovalis and its base to the margin of the 

 inferior caval orifice. It is an important structure in the foetus, directing the 

 blood from the inferior vena cava through the foramen ovale into the left atrium. 



The valvula sinus coronarii is usually a single fold of endocardium which is 



Right anterior cusp of pulmonary valve 

 Left anterior cusp of pulmonary valve Anterior cusp of aortic valve 



Pulmonary ^ery^^^JJ^^ Orifice of right coronary artery 

 Posterior cusp , '^ ' - ' -^ ^ ' Conus arteriosus 



Interventricular branch \^^Bfiii^'^^^S^^B^^^^^^B^k.-'^^ coronary artery 



Left coronary artery 



Orifice of left jC^^P^HIEk^lS ^SP\ \ ^ Anterior cusp of tricuspid valve 



coronary artery " 



Circumflex branch of _ 

 left coronary artery 



Left posterior cusp _ 

 of aortic valve ' 



Inferior cusp of 

 tricuspid valve 

 Posterior cusp of ,1 

 mitral valve 



Medial cusp of tricuspid valve 

 Right posterior cusp of aortic valve Interventricular branch of right coronary artery 



FIG. 754. THE BASES OF THE VENTRICLES OF THE HEART, showing the auriculo-ventricular, aortic, 

 and pulmonary orifices and their valves. 



placed at the right margin of the orifice of the coronary sinus. It is almost 

 invariably incompetent. 



Atrium Sinistmm. The left atrium is in relation posteriorly with the 

 descending thoracic aorta and the oesophagus, but is separated from them by the 

 pericardium and the oblique sinus of the pericardium. Below and anteriorly it is 

 continuous with the left ventricle. Its sterno-costal surface is concave, and lies in 

 , close relation to the ascending aorta, the pulmonary artery, and the left coronary 

 artery. Its right side, formed by the interatrial septum, is directed anteriorly and 

 , to the right. Its left side forms a very small portion of the left margin of the 

 heart, and from it, at its junction with the antero-superior surface, the long and 

 .narrow left auricle is prolonged, forwards, round the left side of the ascending 

 , portion of the aorta and the trunk of the pulmonary artery. 



The four pulmonary veins enter the upper part of the posterior surface, two on 

 each side. 



Che interior of the left atrium is lined with endocardium, and its walls are 



)th, except in the auricle, where musculi pectinati are present, and on the 

 im, in a position corresponding with the upper part of the fossa ovalis on 

 e right side, where there are several musculo-fibrous bundles radiating anteriorly 

 ,and upwards. These septal bundles are separated at their bases by small semi- 

 lunar depressions, in the largest of which remains of the foramen ovale may 



lound. Foramina venarum minimarum, and the apertures of venae cordis 

 minimse, are scattered irregularly over the inner aspect, whilst in the inferior 



