242 PHYSIOLOGY. 



SOUNDS OF THE HEART. 



When the ear is placed over the cardiac region, >r to a stetho- 

 scope applied to the precordial area, two characteristic sounds are 

 heard. The two sounds are known as the first sound and second 

 sound, and are emitted during every cardiac revolution. Though the 

 sounds occur in quick succession, yet they are each separated by silences. 



The first sound is the stronger of the two. In nature it is dull. 

 It coincides with the shock of the heart. The first sound is fol- 

 lowed by the first, or short, silence. 



The second sound is shorter in duration and clearer in character 

 than the first. It comes an instant afterward, at the moment when the 

 whole heart is in relaxation. In pitch, the second sound is from 

 one-fourth to one-third higher than that of the first sound. 



Following the second sound of the heart there occurs the second, 

 or long, silence. In reality the pause occupies but a fraction of a 

 second, yet it is said to be "long" as compared with the first silence. 



It must be borne in mind by the student that there occur in reality 

 four sounds during each cardiac cycle. However, the first two 

 normally occur in unison, as do the second two, so that but two sounds 

 are heard by the examiner. 



From their difference in pitch the two heart-sounds may be 

 expressed graphically upon the musical staff. To the ear they simulate 

 the sounds which are produced in pronouncing the words, "lubb," 

 "dup," the former corresponding to the first heart-sound, the latter to 

 the second. 



If the two sounds be listened to at some distance from the heart, 

 the first may nearly always be distinguished from the second by com- 

 paring the intervals between them. The time elapsing between the 

 first and second sounds is generally much shorter than that which 

 separates the second sound from the first in the succeeding revolution 

 of the heart. But, in medical practice, too much importance must 

 not be attached to these intervals, since their respective duration is 

 extremely variable. In the absence of the impulse it is better to 

 depend upon the differences of pitch. 



Causes of the Sounds. The nature and causes of the cardiac 

 sounds are best studied in a large mammal whose heart-action is 

 comparatively slow. For this purpose the horse is used. Its pulse 

 averages but forty. The animal is properly prepared by anaesthetizing, 

 curarizing, and exposing the viscus to view by placing a window in the 

 thorax. With stethoscope and by observation and palpation, the 



