70 SCIENCE PRIMERS, . [§ V. 



called you will understand if you lay open the right 

 ventricle by cutting with a pair of scissors from the 

 auricle into the ventricle along the side of the heart, 

 or by cutting away the front of the ventricle as has 

 been done in Fig. 8. You will then see that the 

 valve is made up of three little triangular flaps, which 

 grow together round the opening with their points 

 hanging down into the cavity of the ventricle (Fig. 

 10, /. V.) They do not, however, hang quite loosely. 

 You will notice fastened to the sides of the flaps, thin 

 delicate threads, the other ends of which are fastened 

 to the sides of the ventricle, and often to little fleshy 

 projections called papillary muscles (Fig. 8, i^.-P.) 



How do these valves act? In this way. When 

 the ventricle is empty, and blood or water or any 

 other fluid is poured into it from the auricle, the 

 valves are pushed on one side against the walls of 

 the ventricle, and thus there is a great wide opening 

 from the auricle into the ventricle. But as the ven- 

 tricle fills, the blood or water gets behind the flaps 

 and floats them up towards the auricle. The more 

 fluid in the ventricle the higher they float, until 

 when the ventricle is quite full they all meet to- 

 gether in the middle of the opening between the 

 auricle and ventricle and completely block it up. 

 But why do they not turn right over into the auricle, 

 and so open up again the wrong way? Because of 

 those little threads (the cordae tendinese, as they 

 are called) which fasten them to the walls of the 

 ventricle. The flaps float back until these threads are 

 stretched quite tight, and the threads are just long 

 enough to let the flaps reach to the middle of the 

 opening, but no further. The tighter the threads are 





