70 HUMAN PHYSIOLOGY 



be described as consisting of flaps of the heart's lining 

 membrane, two flaps existing on the left side, and 

 three on the right. These flaps hang down into the 

 cavity of the ventricle, to the walls of which they are 

 attached by thin yet strong cords (Fig. 20, 9.) When 

 blood passes downwards from the auricle into the 

 ventricle, these flaps, falling down, are pressed closely 

 against the walls of the ventricle and so allow the free 

 downflow of the blood. When the ventricle is filled, 

 the work of an instant, the flaps of the valves are 

 floated up on the top of the blood so that their edges 

 meet accurately and are brought into perfect apposi- 

 tion (Fig. 22, a) t thus forming a complete partition for 

 the time being between auricle and ventricle. The 

 function of the cords attached to the flaps can now 

 be understood. As the blood under the flaps of the 

 ventricle is pressing upwards before the ventricle 

 empties itself, these cords attached to muscular 

 prominences on the walls of the ventricle (Figs. 20, P, 

 and 22, a) pull the flaps down in opposition to the up- 

 ward pressure of the blood and thus prevent any 

 back flow. When the blood has left the ventricle and 

 the ventricle dilates, the flaps once more assume a 

 dependent position, and thus permit of a fresh flow of 

 blood from the auricle above to the ventricle below. 

 The action of these valves has been not inappropri- 

 ately compared in a rough way to the effect of run- 

 ning water into an empty pond. The leaves of the 

 duckweed lying in the pond are floated up by the in- 

 rush of the water, and in a similar fashion the flaps 

 or valves are raised so as to constitute a perfect tem- 

 porary partition between the two cavities of the heart. 

 The other valves of the heart exist at the entrance to 

 the great bloodvessels leaving each ventricle. They 

 are called semilunar valves (Fig. 20, 10} and consist 



