108 ANATOMY FOE NUESES. [CHAP. IX. 



contract and shorten at the same time, and thus keep them 

 taut. 



Besides the openings between the auricles and ventricles, each 

 auricle has two or more veins opening into it, and each ventricle 

 has a large artery opening out of it. The openings of the veins do 

 not require valves, but both the arterial openings are provided 

 with a set of valves. These valves, called semilunar valves, con- 

 sist of three semicircular flaps, each flap being attached at its 

 base to the inside of the artery where it joins the ventricle, 

 while its free edge projects into the interior of the vessel. The 

 A B 



vent. vent. 



FIG. 83. DIAGRAM TO ILLUSTRATE THE ACTION OF THE HEART, aur, auricle; 

 vent, ventricle ; v, veins ; a, aorta ; m, mitral valve ; s, semilunar valves. In A, 

 auricle is seen contracting, ventricle dilated, mitral valve open, semilunar valves 

 closed. In B, auricle is seen dilated, ventricle contracting, mitral valve closed, 

 semilunar valves open. 



flaps of these valves form a complete barrier, when closed, to 

 the passage of the blood from the arteries into the heart, but 

 offer no resistance to the flow from the heart into the arteries. 

 The beat of the heart. So long as life lasts, the* muscular 

 tissue of the heart contracts and relaxes unceasingly. We may 

 call the heart a muscular pump, the force of whose strokes is 

 supplied by the contraction of muscular fibres, the strokes being 

 repeated so many times a minute. It is constructed and fur- 

 nished with valves in such a way that, at each stroke, it drives 

 a certain quantity of blood with a certain force and a certain 

 rapidity from the ventricles into the arteries, receiving, during 

 the stroke, and the interval between that stroke and the next, 

 the same quantity of blood from the veins into the auricles. 



