1 82 THE CIRCULATION OF THE BLOOD 



on each side has already been filled and distended by the pressure of blood 

 from the veins. The systole of the auricle completes this filling and slightly 

 overdistends the ventricle. When the force of the auricular contraction is 

 spent, the ventricular walls rebound slightly toward their former position 

 and in so doing exert some pressure upon the ventricular side of the tricuspid 

 valve which floats the cusps upward toward the auricle. In this connection 

 another force comes into play, viz., vortex or back currents resulting from 

 the flow of the blood into the ventricle under the pressure of the auricular 

 systole. These currents aid in floating the valve cusps into apposition. 

 Thus the venous orifices of the ventricles are closed at the end of the auricular 

 systole; i.e., the end of the ventricular diastole. The ventricular systole 

 which follows simply serves to place the 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 



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



the ventricular systole is especially marked in the neighborhood of the 

 venous orifices, and this aids in rendering the tricuspid valve competent to 

 close the openings by greatly diminishing the diameter. The cusps of the 

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

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

 with one another by the simultaneous contraction of the papillary muscles, 

 whose tendinous chords have a special mode of attachment for this very 

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

 thus prevent the valve cusps from being everted into the auricles, an event 

 that does occur in certain valvular lesions. 



