160 THE CIRCULATION OF THE BLOOD 



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

 the record of the first sound is somewhat 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. 37). 



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. 



The absolute and the relative intensity of the heart sounds is of clin- 

 ical value in forming a judgment of the dynamic state of the heart muscle 

 and of the tension or pressure in the aorta and pulmonary artery. Ac- 

 centuation of the second sound, for example, is considered to indicate a 

 high pressure in the aorta. Wjggers 46 has recently confirmed this by ex- 

 perimental methods. He recorded the sounds graphically by attaching 

 receivers to the shaved skin of the thorax over the apex and pulmonary 

 artery in anesthetized dogs, the receivers being connected with sound- 

 recording capsules. After taking normal tracings, various alterations were 

 brought about in the pumping action of the heart or in the blood pressure, 

 and the effect was noted on the amplitude and number of vibrations en- 

 tering into the sound tracings. It was found that the intensity of the 

 first sound varied with the tension which was developed within the ventri- 

 cles during systole, particularly during. the presphygmic period. When 

 the vagus was stimulated for example, the first sound diminished because 

 of the lower diastolic pressure to be overcome during the presphygmic 

 period. Increase in the diastolic pressure without change in heart rate 

 (by reflex vasoconstriction, compression of aorta, etc.) caused the second 

 sound to become accentuated, and decrease of pressure (by nitrites) 

 caused it to become less. Of course in both these experiments the first 

 sound was also affected. It was possible to show, further, that when the 

 pressure changes were more marked in one circuit of the circulation than 

 the other, the sounds were more decidedly altered in the circuit in which 

 the changes predominated (see also Lewis 47 ). 



