ACTION OF THE VALVES 



157 



valves under greater tension, thus closing them still more firmly. It should 

 be recollected that the diminution in the breadth of the base of the heart in 

 its transverse diameters during the ventricular systole is especially marked 

 in the neighborhood of the auriculo-ventricular rings, and this aids in render- 

 ing the auriculo-ventricular valves competent to close the openings by greatly 

 diminishing the diameter. The cusps of the auriculo-ventricular valves 

 meet not by their edges only, but by the opposed surfaces of their thin outer 

 borders. The margins of the valves are still more secured in apposition 

 with one another by the simultaneous contraction of the muscular papillae, 



FIG. 152. The Tricuspid Valves of the Ox, Closed. Vertical section. (Krehl.) 



whose chordae tendineae have a special mode of attachment for this very 

 object. They compensate for the shortening of the ventricular walls and 

 thus prevent the valves from being everted into the auricle, an event that 

 does occur in certain valvular lesions. 



The actions of the tricuspid and mitral valves on the right and left 

 sides of the heart are essentially the same. 



The Semilunar Valves. The commencement of the ventricular systole 

 precedes the opening of the semilunar valves by a fraction of a second. The 

 intraventricular pressure increases with the progress of the systole until 

 there is a distinct increase over the arterial pressure, then the opening of the 

 valves takes place at once. They remain open as long as this difference 

 continues. When the diastole of the ventricle begins and the arterial blood 

 pressure exceeds the intraventricular pressure, there is an initial reflux of 

 blood toward the heart which closes the semilunar valves. 



