THORACIC CAVITY 69 



the coronary sinus (O.T. Thebesian), which turns the blood, 

 flowing from left to right in the sinus, anteriorly into the atrio- 

 ventricular orifice. The venae minimae cordis and the anterior 

 cardiac veins open directly into the atrium by small orifices 

 scattered irregularly over the walls. 



The tricuspid orifice is in the lower and anterior part of the 

 atrium. It opens anteriorly into the lower and posterior part of 

 the cavity of the right ventricle, and is sufficiently large to 

 admit the tips of three fingers. It is bounded by a fibrous 

 ring to which the cusps of the right atrio-ventricular valve are 

 attached. These cusps will be examined when the right 

 ventricle is opened. 



Pulmonary artery Aorta 



Right end of transverse sinus 



Left end of 

 transverse sinus 

 Left auricle 



Left atrium ^ Right atrium 



^y Transverse sinus 

 Inter-atrial septum 



FIG. 35. Transverse section through the Upper Part of the Heart. 



The Septum Atriorum and the Fossa Ovalis. The inter- 

 atrial septum is a nbro-muscular partition which intervenes 

 between the right and left atria. In the foetus it is pierced 

 by an obliquely directed foramen, the foramen ovale, already 

 referred to ; and in the adult it is marked on the lower and 

 posterior part of its right side by a shallow depression, the fossa 

 ovalis, which is bounded anteriorly and above by a muscular 

 ridge, the limbus ovalis^ whilst below and posteriorly it fades 

 away into the orifice of the inferior vena cava. 



The floor of the fossa ovalis is very thin ; it marks the 

 situation of the lower part of the foramen ovale of the foetus, 

 and is formed by a portion of the inter-atrial wall which, 

 during foetal life, acted as a flap valve and prevented regurgi- 

 tation of blood from the left to the right atrium. 



The Vena Cava Superior. The superior vena cava returns 

 to the right atrium the blood from the head and neck, the 

 ii 5 a 



