THE MECHANISM OF THE HEART PUMP 1007 



Whenever the pressure in the arteries is greater than that in the 

 ventricles, these valves are closed, and the thin margins come in con- 

 tact with similar portions of the adjacent valves, so preventing the 

 reflux of a single drop of blood. The borders of the valves under 

 these circumstances come together in the form of a star composed of 



1... 



14- 



FIG. 389. Left auricle and ventricle, with outer side cut away to show chief 



points in anatomy of heart. (TESTUT.) 



1, aorta ; 2, pulmonary artery ; 3, ant. coronary vessels ; 5, 5', pulmonary 

 veins ; 6, left auricle ; 7, auricular appendage ; 10, cavity of left ventricle ; 

 11, 12, mitral valves ; 13, 14, papillary muscles ; 16, arrow pointing to aortic 

 orifice. 



three lines at angles of 120, the three corpora Arantii being pressed 

 together at the centre of the star. 



No valves are found at the orifices of the great veins into the auricles, 

 a reflux of blood in this situation during contraction of the heart 

 being prevented by the contraction of the muscular rings round the 

 veins, which always accompanies the auricular contraction. 



The heart and the roots of the great vessels lie almost free in a 

 special cavity, the wall of which is formed by a tough fibrous mem- 

 brane, the pericardium. This is attached below to the central tendon 



