USE OF PAPILLARY MUSCLES. 219 



the arteries, the valves are forced open, and the blood begins 

 to pass out; the ventricle continues to contract until it has 

 obliterated its cavity and completely emptied itself. Then 

 it commences to relax, and blood to flow back into it from 

 the arteries. This back current, however, catches the pock- 

 ets 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 the ventricle is hindered from 

 flowing directly back into it. 



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

 tracting ventricles may not force blood back into the auri- 

 cles, it is essential that the flaps of the mitral and tricuspid 

 valves be held together across the openings which they 

 close, and not pushed back into the auricles. If they 

 were like swinging doors and opened both ways they 

 would be useless ; they must so far resemble an ordi- 

 nary door as only to open in one direction, namely, from 

 the auricle to the ventricle. At the commencement of the 

 ventricular systole this is provided for by the chordae tendin- 

 eas, which are of such a length and so arranged as to keep 

 the valve-flaps shut across the opening, and to maintain 

 their edges in contact. But, as the contracting ventricles 

 shorten, the chordae tendineae would be slackened and the 

 valve-flaps pushed up into the auricle. The little papil- 

 lary muscles prevent this. Shortening as the ventricular 

 systole proceeds, they keep the chordae taut and the valves 

 closed. 



What then happens? How long does the ventricle continue to 

 contract? What then follows? How are the semilunar valves closed? 



What is essential In order that blood may not be forced back from 

 ventricle to auricle? Illustrate. How is the pushing back of the 

 valve-flaps between auricle and ventricle prevented at the beginning 

 of a ventricular systole? When would the chordae tendineae be slack- 

 ened? What would result? How is the slackening prevented? 



