THE PERICARDIUM 183 



When the blood is forced into the elastic arteries by a contraction or 

 beating of the heart it stretches them. When the contraction is ended, the 

 wall of the artery recoils and there is a setting back of the blood for an instant 

 toward the heart, but it is stopped by the closing semilunar valves, which thus 

 make the second sound. 



Clinical note. If the valves of the heart are rough, the sounds are changed 

 by a "murmur." If they cannot close perfectly, a portion of the blood will 

 flow backward instead of going forward, and this is regurgitation. This, also, 

 changes the sound of a valve and causes a murmur. 



The pericardium (Fig. 124). A loose serous sac enclosing the 

 heart. The layer which closely covers the heart, or the visceral 

 layer, is the epicardium. It covers the aorta and pulmonary arte- 

 ries for about one inch, then leaves them to become the parietal 

 layer or lining of the fibrous sac which encloses the whole, and 

 which is closely attached to the diaphragm below and the great 

 vessels above. A small quantity of pericardial fluid prevents fric- 

 tion between the surfaces, as the smoothly covered heart beats in 

 the smoothly lined cavity; this increases in inflammation of the 

 pericardium, or pericarditis, and it is sometimes necessary to 

 remove it by tapping. 



REVIEW. PRINCIPAL POINTS OF INTEREST IN THE HEART 

 t RIGHT AURICLE 



Openings of two large veins bringing blood from the body. 

 Opening of coronary sinus bringing blood from the heart itself. 

 Oval fossa and annulus ovalis (or oval ring). 

 Eustachian valve (or valve of inferior vena'cava). 

 Tricuspid orifice with tricuspid valve. 



RIGHT VENTRICLE 



Tricuspid orifice and tricuspid valve. 

 Opening for pulmonary artery, and pulmonary valves. 

 Trabeculae carneae (fleshy bands) , and the tendinous cords con- 

 necting them with tricuspid valve. 



LEFT AURICLE 



Openings of three or four pulmonary veins. 

 Mitral orifice with bicuspid (or mitral) valve. 



