THE HEART 293 



folds of endocardium, but these primitive valves are never 

 competent to prevent regurgitation. 



On the posterior wall of the right atrium, which is formed 

 by the interatrial (interauricular) septum, there is a definite 

 oval depression, termed the fossa ovalis. This depression 

 occupies the site of the foetal foramen ovale (p. 303), and a 

 small slit-like opening is often found in its upper part, but 

 usually it is not of sufficient size to have any pathological 

 or clinical significance. 



The right atrio-ventricular orifice, when normal in size, 

 admits the tips of three fingers. It is guarded by a valve of 

 three cusps, which consist of folds of redundant endocardium. 

 These folds hang down into the interior of the ventricle and 

 give attachment to a number of fine tendons, termed chorda 

 tendinecE) which are attached at their lower extremities to the 

 apices of the papillary muscles. A fibrous ring surrounds the 

 orifice and gives attachment to the upper borders of the cusps. 



The Right Ventricle receives the blood from the right 

 atrium and pumps it along the pulmonary artery into the 

 lungs. Its walls are roughened by numerous muscular bands 

 which are known as the trabccula carnece. The most important 

 of these bands are the papillary muscles, which are attached 

 to the ventricular walls by their bases and give origin, at 

 their apices, to the chorda tendinecz. It is owing to the action 

 of the papillary muscles that the tricuspid valve is able to 

 prevent the regurgitation of blood from the right ventricle into 

 the right atrium (p. 300). 



The Left Atrium receives the blood from the pul- 

 monary veins and expels it through the left atrio-ventricular 

 (mitral) orifice into the left ventricle. It is deeply placed 

 and is hidden from view anteriorly by the ascending aorta 

 and the pulmonary artery. 



The left atrio-ventricular orifice, when normal in size, 

 admits the tips of two fingers. It is guarded by the bicuspid 

 valve, which, save that it possesses only two cusps, corresponds 

 in every way to the tricuspid valve. 



