THE CIRCULATION OF THE BLOOD 279 



With the^cessation of the auricular systole the ventricular systole begins. 

 If the blood is not to be returned to the auricles at this moment, the tricuspid 

 and mitral valves must be suddenly and accurately closed. This is readily 

 accomplished by reason of the position of the valves, which have been 

 floated^up and placed almost in apposition by the blood itself. With the 

 beginning of the ventricular pressure the blood is forced upward against the 

 valves until their free edges are brought together and the orifices closed. 

 Reversal of these valves into the auricles is prevented by their attachment to 

 the chorda tendinece, and the latter are kept from moving bodily upward 

 during the ventricular contraction by the compensatory downward pull of 

 the papillary muscles. The blood now confined in the ventricle between the 

 closed auriculo- ventricular and semilunar valves is subjected to pressure 

 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. 



The approximate changes in the shape of the heart, the variations in the 

 size of its cavities and in the size of the blood-vessels arising from them, 

 and the relative position of the valves during systole and diastole are shown 

 in Fig. 123. 



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 with the systole, the first 

 phase of a new cardiac cycle, and is therefore termed the first sound; the 

 short clear sound occurs at the beginning of the diastole, with the second 

 phase of the cardiac cycle, and is therefore termed the second sound. ^ The 

 first sound is the systolic, the second 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 



x 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-ventncular 

 septum, due to the contraction of the papillary muscles. 



