THE ARRANGEMENT OF THE MUSCULATURE OF THE HEART 271 



however, the ventricular wall recoils and exerts a static pressure upon 

 the blood with which this cavity is now filled. Secondary currents 

 are set up which strike the surfaces of the valve flaps and push them 

 upward in the direction of their position of closure. This static 

 back pressure, however, is not the only factor upon which the approxi- 

 mation of the valve flaps depends ; in fact, it merely serves the purpose 

 of " floating" them into their initial position of closure, while the actual 

 snapping together of their marginal areas is accomplished by the 

 suction which must necessarily result in the wake of the column of 

 auricular blood as it clears the auriculoventricular orifice. 1 When 

 the contraction of the auricles ceases, the driving force is suddenly 

 withdrawn. The column of blood, however, rushes on, with the 

 result that an area of negative pressure is developed in the rear of it 

 which immediately draws the flaps 

 almost transversely across the center 

 of the orifice. Thus, it will be seen 

 that the- final closure of the valves is 

 accomplished by the "breaking" of 

 the column of auricular blood and 

 clearly, as the flaps swing in from the 

 side, the blood is cut off very sharply 

 so that a backward movement of it is 

 impossible under ordinary conditions. FIG. 134. LONGITUDINAL SECTION 



The Semilunar Valves. The con- THROUGH THE ROOT OF THE AORTA TO 



,. , . . ., ,, , . , SHOW CUP-LIKE SHAPE OF SEMILUNAR 



ditions met with at the aortic and VALVE FLAPS. 



pulmonary orifices, are quite different 



from those encountered at the auriculoventricular openings. In ac- 

 cordance with the high degree of pressure developed by the ventricles, 

 their exits are narrow and surrounded by solid walls. Each orifice is 

 guarded by three separate segments which are fastened end to end 

 against the internal surface of these blood-vessels. Each segment 

 exhibits a cup-like shape, its convex surface being directed toward the 

 heart. The basal portions of the flaps rest upon a solid cushion of 

 the ventricular substance, while their free ends project far into the 

 lumen of the blood-vessel. No special structures are present to hold 

 them in place. 



When the ventricles contract and drive the blood through these 

 slit-like orifices into the arteries, the tips of the different valve-flaps 

 are pushed far apart, but it should be emphasized that they are not 

 forced into contact with the wall of the blood-vessel. 2 Such a result 

 is practically impossible, because the basal portions of the flaps are 

 well protected against the ventricular stream by the heavy cushion 

 of muscle tissue to which they are fastened, and because the beginning 

 segment of each blood-vessel is very much larger than its more periph- 



1 Henderson, Am. Jour, of Physiol., xvi, 1906, 325; also see: Henderson and 

 Johnson, Heart, iv, 1912, 69. 



2 Ceradini, Der Mechanismus der halbmondf. Klappen, Leipzig, 1872. 



