THE VALVES OF THE HEART 47 



having no chordfe tendinefe attached to them, are 

 called columnae carnese. 



It follows, from this arrangement, that the valves 

 oppose no obstacle to the passage of fluid from the 

 auricles to the ventricles ; but if any should be forced 

 the other way, it will at once get between the valve and 

 the waU of the heart, and drive the valve backwards and 

 upwards. Partly because they soon meet in the middle 

 and oppose one another's action, and partly because the 

 chordce temlineoi hold their edges and prevent them from 

 going back too far, the valves, thus forced back, give rise 

 to the formation of a complete transverse partition be- 

 tween the ventricle and the auricle, through which no 

 fluid can pass. . 



Where the aorta opens into the left ventricle, and 

 where the pulmonary artery opens into the right ventricle 

 another valvular apparatus is placed, consisting m each 

 case of three pouch-like valves called the semilunar 

 valves (Fig. 16, s.v. ; Figs. 18 and 19, Ao. P. A.), which 

 are similar to those of the veins. Since they are placed on 

 the same level and meet in the middle line, they completely 

 stop the passage when any fluid is forced along the artery 

 towards the heart. On the other hand, these valves flap 

 back and allow any fluid to pass from the heart into the 

 artery, with the utmost readiness. 



The action of the auriculo-ventricular valves may be 

 demonstrated with great ease on a sheep's heart, in which 

 the aorta and pulmonary artery have been tied and the 

 greater part of the auricles cut away, by pouring water 

 into the ventricles through the auriculo-ventricular aper- 

 ture. The tricuspid and mitral valves then usually 

 become closed by the upward pressure of the water which 

 gets behind them. Or, if the ventricles be nearly filled, 

 the valves may be made to come together at once by 

 gently squeezing the ventricles. In like manner, if the 

 base of the aorta, or pulmonary artery, be cut out of the 

 heart, so as not to injure the semilunar valves, water 



