THE HEART AND PERICARDIUM 205 



The coronary valve, or valve of Thcbesius, protects the open- 

 ing of the coronary sinus. 



Remnants of foetal structures. The Fossa ovalis is the 

 remains of the foramen ovale on the lower and posterior part 

 of the septum auricularum, just above and to the left of the 

 opening of the inferior vena cava. 



The annulus ovalis is the upper crescentic elevated margin 

 of the fossa. 



The musculi pectinati are the prominent muscular columns 

 running over the surface of the appendix auriculae. 



The endocardium is smooth in the main part of the 

 cavit)', but irregular over the musculi pectinati of the auricu- 

 lar appendix. 



The RIGHT VENTRICLE consists of a cavity, the upper and 

 left angle of which is prolonged into a funnel-shaped canal, 

 the infundibulum or conus arteriosus, leading to the pulmonary 

 artery. On the wall (except in the infundibulum, which is 

 smooth) are projections, column carnea, formed of muscular 

 bundles, of which there are three varieties : the first are 

 merely prominent ridges ; the second are attached at the ends, 

 being free in the middle ; the third are the musculi papillares, 

 which project inwards, and are attached by their bases to the 

 wall of the ventricle, the other end being connected with the 

 chorda tendinea, or cords attached to the flaps of the auriculo- 

 ventricular valve. 



The tricuspid valve, which guards the right auriculo-ventricu- 

 lar opening, consists of three cusps, formed by a reduplication 

 of the endocardium, with some fibrous tissue enclosed. The 

 bases of the flaps are attached to the fibrous ring of the 

 auriculo- ventricular orifice, while to their free ends are 

 attached the chordae tendineae. 



Position of cusps. Right one against right wall of ventricle 

 (marginal cusp). Posterior one between auriculo-ventricular 

 opening and septum (septal cusp). Left between auriculo 

 ventricular and pulmonary orifices (infundibular cusp). The 

 valve prevents regurgitation of blood into the auricle during 

 the heart's contraction. 



The opening of the pulmonary artery is at the summit of the 

 funnel-shaped infundibulum, and is guarded by the pulmonary 

 semilunar valves. The semilunar valves are three folds of 

 the lining membrane, aright, left, and posterior, which guard 

 the orifice of the pulmonary artery. The free margin of each 

 has in its middle a small fibrous nodule, the corpus Arantii, 

 and between each valve and the beginning of the pulmonary 

 artery is a dilatation called the pulmonary sinus, or sinus of 



