THE HEART AND THE BLOOD VESSELS 



139 



preventing the passage of any blood back into the auricle. 

 These valves between the chambers of the right side of the 

 heart are called the tricuspid valves. 



Next, the muscles in the walls of the ventricle contract, 

 pressing upon the blood until the ventricle is emptied. In 

 what direction will the blood flow? It would go back into 

 the auricle if the tricuspid valve had not closed the opening 

 in that direction. These valves are only soft membranes, 

 and one would suppose that they might give way under the 

 pressure of blood in the ventricle and turn back into the 

 auricle. To prevent this, stout cords (chordae tendinae) are 

 attached to the edges of the valves (Fig. 74), their other 

 ends being fastened below to the walls of the ventricle. 

 These cords are of such lengths that when the valves are 

 stretched across the opening the cords are tight; Fig. 74 B. 

 It would not be possible to push the 

 valves up into the auricle without 

 breaking these cords; moreover they 

 can be drawn downward somewhat 

 by little muscles attached to their 

 lower ends, the so-called papillary 

 muscles. As the ventricle contracts, 

 then, the blood must find another 

 outlet. 



The only real outlet from the right 

 ventricle is a large artery shown in 

 Figure 74, and called the pulmonary 

 artery, since it leads to the lungs. 

 This artery is already filled with FIG. 75. THE PULMONARY 

 blood, also under pressure, blood that ARTERY CUT OPEN TO 

 would readily flow back into the SHOW THE SEMILUNAR 

 heart were it not for a set of valves 



preventing such a return; these, called the semilunar valves, 

 consist of three soft folds in the shape of half-cups or 

 pockets with their open ends directed away from the cavity 



