THE CIRCULATORY SYSTEM 167 



mouths of the great arteries are closed; the tricuspid and 

 mitral valves are open. The ventricles now begin to dilate, 

 and the blood flows freely into them from the auricles. 

 As they fill, currents in the blood along the walls carry up 

 the flaps of the valves till they are nearly closed. The 

 auricles now contract, the contraction beginning at the 

 mouths of the great veins, which narrows the openings, 

 and more blood is sent into the ventricles. 'This swifter 

 flow into the ventricles sets up stronger back currents 

 along their walls, and, the muscular walls beginning to 

 contract, the valves are completely closed. The blood 

 then has no escape but by the arteries issuing from the 

 ventricles. The strong walls of the ventricles press more 

 and more upon the imprisoned blood, it is forced swiftly 

 through the mouths of the arteries, pressing back the 

 semilunar valves, and the ventricles themselves are empty 

 before they begin to relax again. When relaxation sets 

 in, the return of arterial blood into the ventricles is pre- 

 vented by the semilunar valves. 



231. Sounds of the Heart. Two distinct sounds from the 

 heart may be detected by placing the ear over the region 

 of that organ. The first is dull and somewhat prolonged, 

 the second is shorter and sharper. The first may be heard 

 immediately before the pulse is felt at the wrist, the sec- 

 ond immediately after it. They are followed by a brief 

 silence. The exact cause of the first sound in not fully 

 determined, but it is thought that it may be partly due to 

 vibrations of the tendinous cords attached to the heart 

 valves themselves, and partly to a muscular sound pro- 

 duced by contraction in the mass of muscular fibers in 

 the ventricles. The second sound occurs at the moment 

 of closure of the semilunar valves, and is due to the strik- 

 ing together of those valves. 



