THE SOUNDS OF THE HEART 183 



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 valve 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. The valves remain open as long as this 

 difference continues. When the diastole of the ventricles begins and the 

 arterial blood pressure exceeds the intraventricular pressure, there is an 

 initial reflux of blood toward the heart which closes the semilunar valve. 



The dilatation of the arteries is peculiarly adapted to bring this about. 

 The lower borders of the semilunar valves are attached to the inner surface 

 of the tendinous ring which bounds the orifice of the artery. The tissue of 

 this ring is tough and inelastic and the valves are equally inextensible, 

 being formed mainly of tough fibrous tissue with strong interwoven cords, 

 the effect, therefore, of each propulsion of blood from the ventricle into the 

 artery is to dilate the wall of the first portion of the artery and the three 

 pouches behind the valve cusps while the free margins of the cusps are 

 drawn inward toward the center. This position of the valves and arterial 

 walls is maintained while the ventricle continues in contraction; but as it 

 relaxes, and the dilated arterial walls recoil by their elasticity, the blood is 

 forced backward toward the ventricles and onward in the course of the 

 circulation. Part of the blood thus forced back lies in the pouches (sinuses 

 of Valsalva) between the valve cusps and the arterial walls; and the cusps 

 are pressed together till their thin lunated margins meet in three lines 

 radiating from the center to the circumference of the artery, figure 151. 

 The corpora Arantii at the middle of the free margins insure a more effec- 

 tive closure. 



The Sounds of the Heart. When the ear is placed on the chest over 

 the heart, two sounds may be heard at every beat. They follow in quick 

 succession and are succeeded by a pause or period of silence. The first 

 sound is dull and prolonged; its commencement coincides with the impulse 

 of the heart against the chest wall, and just precedes the pulse at the wrist. 

 The second sound is shorter and sharper, with a somewhat flapping char- 

 acter. The periods of time occupied, respectively, by the two sounds 

 taken together and by the pause between the second and the first are 

 unequal. According to Foster, the interval of time between the beginning 

 of the first sound and the second sound is 0.3 of a second, while between 

 the second and the succeeding first it is nearly 0.5 of a second, see figures 

 153, 154, and 167. The relative length of time occupied by each sound, as 

 compared with the other, may be best appreciated by considering the 

 different forces concerned in the production of the two sounds. In one 

 case there is a strong, comparatively slow contraction of a large mass of 



