160 THE CIRCULATION OF THE BLOOD 



lar pressure curve, the initial vibrations occur 0.01 second prior to the rise 

 in pressure, the main vibrations reaching their greatest amplitude before 

 the sphygmic period begins, and the final vibrations occurring during 

 the early part of the sphygmic period and therefore just before the aortic 

 pressure has reached its height. The main vibrations therefore occur 

 during the descending limb of the R wave of the electrocardiogram (be- 

 ginning 0.01 second before its completion), the small preliminary vibra- 

 tions occurring during the ascending limb. When taken from the aorta, 

 the record of the first sound is someAvhat different, there being no initial 

 vibrations and the main ones being of greater frequency and reaching 

 their maximum earlier than those taken from the ventricle. The sub- 

 sequent vibrations are also larger, especially when the aortic pressure 

 is high (Fig. 38). 



The record of the second sound at the ventricle is much simpler and 

 usually of less amplitude than the first, consisting of two to six vibrations 

 lasting 0.015 to 0.056 second. They begin a short time after the ventricu- 

 lar pressure begins to fall, approximately at the dicrotic notch of the aortic 

 curve, being completed in from 0.015 to 0.025 second after the bottom 

 of the notch. Their relationship to the T wave is variable. Taken from 

 the aorta, the record of the second sound shows vibrations of greater 

 amplitude and of a greater frequency than that from the ventricle. 



