THE HEART SOUNDS 167 



their openings being in this way either narrowed considerably or actually 

 closed. 



B. THE SEMILUNAR VALVES 



Since the blood cannot flow back into the auricle during the ventricular 

 systole, it must pass into the great arteries. The mouths of the latter are 

 closed by means of valves, each consisting of three pouchlike flaps. 



These flaps are semicircular membranes, fastened by their curved edges to 

 the wall of the vessel, so that they stand out with their straight edges from the 

 wall and present concave surfaces toward the arteries. In this way pouches 

 are formed in which the blood is caught and dammed back, while at the same 

 time the wall of the pocket turned toward the lumen of the vessel is put on 

 the stretch. 



In both the aorta and the pulmonary artery the wall bulges outward directly 

 above the attached edges of the valve, forming in each three enlargements, the 

 sinuses of Valsalva. In the aorta one sinus is directed backward, two forward, 

 right and left. From the latter two the right and left coronary arteries take 

 their origin. 



When the pressure in the ventricle is lower than in the corresponding 

 artery, the semilunar valves are closed with their edges applied tightly to- 

 gether. When the pressure in ventricular systole increases enough to exceed 

 that in the aorta or pulmonary artery, the valves open and the 'blood flows 

 out. When the ventricle again passes into diastole the valve is closed once 

 more. 



What position the valves take in systole is not yet definitely determined, 

 although various observations make it probable that their free edges stand 

 out some distance from the sinuses, thus narrowing the arterial mouths. It 

 should be added that there are certain muscular folds springing from almost 

 all sides of the artery, which serve as supports for the valves (Krehl). 



In consequence of this arrangement the blood is pressed through a narrow 

 muscular cleft into a wider space above the valves. This must cause vortices 

 and eddies to be formed, which tend constantly to press the flaps of the valve 

 together, and they are unable so to do only because the blood is being forced 

 through at high pressure. When the outflow ceases, the valves are pressed 

 together suddenly, and without any re gurgitation. The closure is rigidly 

 maintained by the difference between aortic and ventricular pressures, a dif- 

 ference which is more than sufficient once the muscles of the ventricle relax 

 and the above-mentioned muscular supports of the valves give way. 



3. THE HEART SOUNDS 



If the ear be placed over the breast wall, with every heart beat one hears 

 a dull, long-drawn-out sound, and after this a shorter, clear sound. Then 

 follows a pause, then again the dull sound, and so on. The long sound is 

 called the first heart sound, the following one, the second. The first is heard 

 throughout the entire ventricular systole, and only then. The second follows 

 immediately after the first, i. e., immediately after the end of the ventricular 

 systole, and after it comes the pause. 



