158 THE CIRCULATION OF THE BLOOD 



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 

 valves, while the free margins of the valves are drawn inward toward its 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 valves 

 and the arterial walls; and the valves are pressed together till their thin 

 lunated margins meet in three lines radiating from the center to the circum- 

 ference of the artery, 7 and 8, figure 136. 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. 



FIG. 153. Simultaneous Tracings of the Cardiac Impact, or Cardiogram (lower), and the 

 Heart Tones (upper), of Man. The cross strokes at the beginning of the cardiac sound tracing 

 and on the cardiogram mark the synchronous events. (Hurt hie.) 



The second is shorter and sharper, with a somewhat 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 0.3 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 



