230 THE HUMAN BODY. 



the systole the ventricle commences to relax and blood imme- 

 diately to flow back towards it from the highly stretched ar- 

 teries. This return 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 tricnspid valves 

 be maintained in position across the openings which they 

 close, and be not pushed back into the auricles. At the com- 

 mencement of the ventricular systole this is provided for by 

 the cordae tendineae, which are of such a length as to keep 

 the edges of the flaps in apposition, a position which is 

 farther secured by the fact that each set of cordae tendineae 

 (Fig. 92) radiating from a point in the ventricle, is not at- 

 tached 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 ventricles 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. 



The mechanism is indeed even better working than this. 

 The area of the valve-flaps is greater than is sufficient to 

 stretch across the auriculo-ventricular orifice, so that when 

 their edges are in apposition they form a cone projecting into 

 the ventricle. Towards the ends of the systole the papillary 

 muscles pull this cone down into the ventricular cavity so as 

 to practically obliterate it and force out from it nearly every 

 drop of blood. \ 



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

 over the region of the heart during life, two distinguishable 

 sounds will be heard during each cardiac cycle. They are 

 known respectively as the first and second sounds of the 

 heart. The first is of lower pitch and lasts longer than the 

 second and sharper sound: vocally their character may be 

 tolerably imitated by the words lubb, dup. The cause of the 

 second sound is the closure, or, as one might say, the"*' click- 

 ing up," of the semilunar -valves, since it occurs at the 

 moment of their closure and ceases if they be hooked back in 



