308 THE HUMAN BODY 



ably due to vibrations of the tense ventricular wall at that time. 

 It is not due, at least not entirely, to the auriculoventricular 

 valves, since it may still be heard in a beating heart empty of 

 blood, and in which there could be no closure or tension of those 

 valves. In various forms of heart disease these sounds are mod- 

 ified or cloaked by additional "murmurs" which arise when the 

 cardiac orifices are roughened or narrowed or dilated, or the 

 valves inefficient. By paying attention to the character of the 

 new sound then heard, the exact period in the cardiac cycle at 

 which it occurs, and the region of the chest-wall at which it is 

 heard most distinctly, the physician can often get important in- 

 formation as to its cause. 



Action of the Heart Valves. The valves of the heart are en- 

 tirely without rigidity. They consist of tough, but perfectly 

 flaccid membranes, so that they respond perfectly to the forces 

 which act upon them. This structure makes it inevitable that the 

 valves will open whenever the pressure behind them is greater 

 than that in front, and will close whenever the pressure in front 

 is greater than that behind. During the whole diastole of the heart 

 the pressure behind the auriculoventricular valves is greater than 

 that in front of them; for in front is only the gradually filling 

 cavity of the ventricle, while behind is the onward flow of blood 

 from the great veins. During this time, therefore, these valves 

 stand open. The systole of the auricle, by increasing the pressure 

 behind, keeps them open until its end. During this same time the 

 aortic valves are shut, because in front of them are arteries whose 

 walls are stretched with their load of blood and which, therefore, 

 exert high pressure upon the valves, while behind are only the 

 ventricular cavities, filling with blood. At the instant the ven- 

 tricles begin to contract the situation with regard to the auriculo- 

 ventricular valves changes. The relaxing auricles make room for 

 the blood coming in from the great veins and so release the pressure 

 behind these valves; the contracting ventricle exerts pressure in 

 front of them ; they therefore close instantly. Since the semilunar 

 valves remain closed until the rising pressure in the ventricle be- 

 comes greater than that in the aorta there is an instant at the be- 

 ginning of ventricular systole when all the valves are shut. Again, 

 at the beginning of ventricular diastole there is an instant when the 

 ventricular pressure has fallen below that in the aorta but is still 



