THE ACTION OF THE HEART 307 



tion until it has obliterated its cavity and completely emptied 

 itself; this total emptying appears, at least, to occur in the nor- 

 mally beating heart, but in some pathological conditions and 

 under the influence of certain drugs the emptying of the ventri- 

 cles is incomplete. After the systole the ventricle commences to 

 relax and blood immediately to flow back towards it from the 

 highly stretched arteries. 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 contracting 

 ventricles may not force blood back into the auricles it is essential 

 that the flaps of the mitral and tricuspid valves be maintained 

 in position across the openings which they close, and be not 

 pushed back into the auricles. At the commencement of the 

 ventricular systole this is provided for by the corda3 tendineae, 

 which are of such a length as to keep the edges of the flaps in ap- 

 position, a position which is further secured by the fact that each 

 set of cordse tendineaB (Fig. 105) radiating from a point in 

 the ventricle, is not attache'd 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 

 tendinese, 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 cordse 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 distinguishable sounds 

 will be heard during each cardiac cycle. They are known re- 

 spectively 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 "clicking up," of the semilunar valves, since it 

 occurs at the moment of their closure and ceases if they be hooked 

 back in a living animal. The origin of the first sound is still un- 

 certain: it takes place during the ventricular systole and is prob- 



