104 THE HEART AND CIRCULATION. 



tricuspid valve, while that on the left has two flaps, larger 

 and thicker than those of the tricuspid, and is known as 

 the bicuspid or mitral valve. The flaps of either valve are 

 kept from being forced into the auricle in closing by fine 

 tendinous cords, the chordce tendinece, which are attached 

 to the columnce carnece, muscular bands or columns pro- 

 jecting from the walls of the ventricle, which contract 

 and hold the chordae tendineae taut. The opening into 

 the pulmonary artery is from the posterior part of the 

 right ventricle and is guarded by the semilunar or pulmo- 

 nary valve, while the aortic opening from the left ventricle 

 is guarded by a similar valve, the aortic valve, the most 

 important valve in the body. All these valves are planned 

 primarily to prevent regurgitation of the blood during 

 contraction of the heart muscle. Pressure in the ven- 

 tricle must exceed that in the arteries before the semi- 

 lunar valves will open and the blood can be driven out, 

 just as the auriculo-ventricular valves remain closed until 

 the pressure in the auricles exceeds that in the 

 ventricles. 



The heart beat is caused by the twisting of the heart 

 upon its axis during contraction of the muscle. Nor- 

 mally it beats rhythmically and regularly, whatever a 

 person does, at a rate of about seventy-two contractions 

 to the minute in the adult. To the regular cardiac cycle, 

 as it is called, there are two periods, the systole and the 

 diastole, the former representing the period of contraction 

 of the ventricles, when the blood is sent to the lungs and 

 over the body, and the latter representing the period of 

 rest following the emptying of the ventricles, during 

 which they are refilled. Contraction of the heart occu- 

 pies one-fifth of the time of one beat, dilatation two-fifths, 

 and the pause two-fifths. There are really two systoles, 

 one of the auricles and one of the ventricles, but they 

 come so close together that they are practically simul- 

 taneous so far as sound is concerned, though they can be 

 distinguished by sight. During systole the tricuspid 

 and mitral valves close sharply to prevent regurgitation 



