THE CIRCULATION OF THE BLOOD 143 



as to prevent the return of the blood into the auricles. 

 The valves are prevented from floating over into the auri- 

 cles by cords which tie them to the ventricles. These cords 

 may be drawn tight by the contraction of the little muscles 

 in the wall of the ventricles to which they are attached. 



Between the ventricles and the arteries are the half-moon 

 shaped, or semilunar, valves. 



FIG. 88. Muscular Fibers of the Auricles. 



L~A., left auricle ; R.A., right auricle ; A, opening of the inferior vena cava ; 

 B, superior vena cava ; C, cardiac vein of right auricle laid open ; D, left 

 pulmonary veins; E, right pulmonary veins; f, muscular fibers sur- 

 rounding the openings between the left auricle and left ventricle, and 

 the right auricle and right ventricle ; G, H, K, and L, muscular fibers 

 surrounding the great veins of the heart. 



201. The Work of the Heart. The heart is a wonderfully 

 busy machine, pumping away without getting tired, night and 

 day for eighty years or more, perhaps, at the rate of seventy 

 strokes every minute, over forty-two hundred times every hour, 

 and nearly thirty-seven million beats every year. At each 



