EXTERNAL ANATOMY OF THE HEART 



183 



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ventricles. In the partition between the right auricle and 

 right ventricle is an opening guarded by a three-cornered 

 valve known as the tricuspid valve. The left auricle and 

 left ventricle communicate by a similar opening, and the 

 valve which guards this opening is called the mitral valve 

 from its resemb- 

 lance to a miter. 

 The auricles re- 

 ceive the blood 

 from' veins, and 

 this blood passes 

 from the auricles 

 into the ventri- 

 cles through the, 

 valves. The ven- 

 tricles in turn, by 

 contraction of 

 their muscular 

 walls, force this 

 blood out of the 

 heart into arter- 

 ies and thus over 

 the body. It is 

 the pumping ac- 

 tion of these ven- 

 tricles which con- 

 stitutes what is 

 known as the heart beat. The two sides of the heart 

 beat in unison. 



External anatomy of the heart. — (See Fig. 67.) When 

 the heart is examined from the outside, the position of 

 these four internal cavities is seen to be outlined by grooves 



Fig. 67 — External view of the heart; 1, right ventricle; 

 2, left ventricle ; 3, right auricle ; 4, left auricle ; 5, 

 aorta ; 6, pulmonary artery ; 7, innominate artery ; 

 8, carotid arteries ; 9, subclavian artery ; 10, supe- 

 rior vena cava 11, pulmonary veins. 



