178 THE CIRCULATION OF THE BLOOD 



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 

 valve takes place at once. The valve remains 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 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 is to dilate 

 the wall of the first portion of the artery in 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 soon as it relaxes, and 

 the dilated arterial walls can 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 effective closure. 



The Sounds of the Heart. When the ear is placed over the region 

 of 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 somew r hat flapping character. 

 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 o . 3 of a second, while between the second and the suc- 

 ceeding first it is nearly 0.5 of a second, see figures 153, 154, and 158. 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, compara- 

 tively slow contraction of a large mass of muscular fibers, urging forward 

 a certain quantity of fluid against considerable resistance; while in the other 

 it is a strong but shorter and sharper recoil of the elastic coat of the large 

 arteries shorter because there is no resistance to the flapping back of the 

 semilunar cusps as there was to their opening. The sounds may be ex- 



