THE FRESH-WATER MUSSEL. 



105 



heart. Peri moans around, and kardia the heart. The 

 mussel's heart has three rooms, one at each side and 

 one in the middle (Fig. 99). The side-rooms are called 

 auricles, because in your heart they are ear-shaped, but 

 in the mussel they are more like wings fastened to the 

 central chamber. The central chamber is the ven- 

 tricle, but your heart has two ventricles, which with 

 the two auricles make four rooms. If you look care- 

 fully while the heart is working, you can see that the 

 ventricle, or middle-room, draws itself together and 

 then swells out again ; next, the two side-rooms, or au- 

 ricles, draw themselves together and swell out, and, 

 while they are swelling out, the ventricle begins to 

 draw itself together again. This regular motion of the 

 little rooms, the auricles and ventricles, is called the 

 heating of the heart. If you count the beats, you find 

 they are about fifteen in a minute, less than one- 

 quarter as fast as yours. Why does the heart beat ? In 

 order to receive the blood and send it through the body, 

 and the flowing of the blood through the body is called 

 the circulation of the blood. When the middle-room, 

 or ventricle, closes or draws itself together, it sends 

 the blood into little tubes called arteries. Artery 

 means air-container, or air-tube ; for long ago, when 

 doctors opened dead bodies, they found these tubes 

 empty, and they supposed wrongly that they were filled 



