280 TEXT-BOOK OF PHYSIOLOGY 



from all sides* As the pressure rises proportionately to the vigor of the con- 

 traction, there comes a moment when the intra-ventricular pressure exceeds 

 the pressure in the aorta and in the pulmonic artery. As soon as this 

 occurs the semilunar valves of both vessels are thrown open and the blood 

 discharged. 1 Both contraction and outflow continue until the ventricles 

 are practically empty, after which ventricular relaxation sets in. With 

 the discharge of the blood the pressure in both the pulmonic artery and aorta 

 rises, passing from a minimal to a maximal value. Coincidently the pres- 

 sure in the ventricles rises and even exceeds that in the pulmonic artery and 

 aorta and so continues until near the close of the systole when the two 

 opposing pressures are approximately equal. With the onset of the 

 ventricular relaxation the intra-ventricular pressure suddenly falls, and so 

 soon as it falls below the positive pressure of the blood in the sinuses of 

 Valsalva the semilunar valves are again closed, the column of blood is 

 supported, and regurgitation is prevented. In the meantime and while 

 the ventricles are contracting, blood is again flowing into, and accumulat- 

 ing in the auricles and thereby distending them preparatory to the next 

 systole. With the accumulation of blood in the auricles and ventricles 

 the cardiac cycle is completed. 



Heart-sounds. Two sounds accompany each pulsation of the heart, 

 both of which may be heard by applying the ear or the stethoscope to the 

 chest walls, especially over the region of the heart. One of these sounds is 

 low in pitch, dull and prolonged; the other is high in pitch, clear and short. 

 These sounds can be approximately reproduced by pronouncing the syllables 

 lubb-dup, lubbdup. The long dull sound occurs at the beginning of the 

 ventricular systole, and is therefore termed the first sound; the short 

 clear sound occurs at the beginning of the ventricular diastole, and 

 is therefore termed the second sound. The first sound is the systolic, the 

 second is the diastolic. With the ear it can readily be determined that there 

 is a brief pause between the first and second sounds, and a longer pause 

 between the second and the first sounds. The duration of the first sound 

 is almost equal to the duration of the systole viz., 0.3 second; the duration 

 of the second sound is not more than o.i second. The systolic sound is 

 heard most distinctly over the body of the heart; the diastolic sound is 

 heard most distinctly in the neighborhood of the third rib to the right of 

 the sternum. 



The causes of the heart-sounds have enlisted the attention of clinicians 

 and physiologists for years, and many factors have been assigned for their 

 production. At present it is generally believed that the first sound is the 

 product of at least two, possibly three, factors: viz., the contraction of the 

 muscle walls of the ventricles, the simultaneous closure and subsequent 

 vibration of the tricuspid and mitral valves, and the sudden increase of 

 pressure of the apex of the heart against the chest wall. 



That the contraction of the ventricular muscle gives rise to a sound is 

 certain from the fact that it is perceptible in an excised heart when the 

 cavities are free from blood and when the valves are prevented from closing. 

 The explanation of this sound is extremely difficult, as the contraction, 



1 The discharge of the blood by the contraction of the ventricular walls is probably aided by 

 the simultaneous downward displacement of the more central portion of the auriculo-ventricular 

 septum, due to the contraction of the papillary muscles. 



