222 THE HUMAN BODY. 



on each side of them; but of course they might be forced 

 open without this by applying sufficient power to overcome 

 the higher water pressure on one side. It is in this latter 

 way that the semilunar valves are opened. The contracting 

 ventricle tightens its grip on the blood inside it and becomes 

 rigid to the touch. As it squeezes harder and harder, at 

 last the pressure on the blood in it becomes greater than 

 the pressure exerted on the other side of the valves by the 

 blood in the arteries, the flaps are pushed open, and the 

 blood begins to pass out: the ventricle continues its con- 

 traction until it has obliterated its cavity and completely 

 emptied itself. Then it commences to relax and blood 

 immediately to flow back into it from the highly stretched 

 arteries. This back current, however, catches the pockets 

 of the semilunar valves, drives them back and closes the 

 valve so as to form an impassable barrier; and so the blood 

 which has been forced out of either ventricle cannot flow 

 directly back into it. 



Use of the Papillary Muscles. In order that the con- 

 tracting ventricles may not force blood back into the 

 auricles it is essential that the flaps of the mitral and 

 tricuspid valves be maintained horizontally across the open- 

 ings which they close, and be not pushed back into the 

 auricles. At the commencement of the ventricular sys- 

 tole this is provided for by the cordae tendineae, which are 

 of such a length as to keep the edges of the flaps in appo- 

 sition, a position which is farther secured by the fact that 

 each set of cordae tendineae (Fig. 81*) radiating from a 

 point in the ventricle, is not attached around the edges of 

 one flap but on the contiguous edges of two flaps, and so 

 tends to pull them together. But as the contracting ven- 

 tricles shorten, the cordae tendineae, if directly fixed to 

 their interior, would be slackened and the valve-flaps 

 pushed up into the auricle. The little papillary muscles 

 prevent this. Shortening as the ventricular systole proceeds, 

 they keep the cordae taut and the valves closed. 



Sounds of the Heart. If the ear be placed on the chest 

 over the region of the heart during life, two distinguish- 

 able sounds will be heard during each cardiac cycle. They 

 are known respectively as the first and second sounds of the 



* P. 209. 



