246 VETERINARY PHYSIOLOGY 



to these sounds it is easy to determine that the first sound 

 occurs synchronously with the cardiac impulse i.e. synchron- 

 ously with the ventricular contraction. 



It develops suddenly, and dies away more slowly. In 

 character it is dull and rumbling, and may be imitated by 

 pronouncing the syllable lub. In pitch it is lower than the 

 second sound. 



The second sound is heard at the moment of ventricular 

 diastole. Its exact time in the cardiac cycle has been deter- 

 mined by recording it on the cardiac tracing by means of a 

 microphone. It develops suddenly and dies away suddenly. 

 It is a clearer, sharper, and higher-pitched sound than the 

 first. It may be imitated by pronouncing the syllable 

 dupp. 



According to the part of the chest upon which the ear is 

 placed, these sounds vary in intensity. Over the apical region 

 the first sound is louder and more accentuated ; over the base 

 the second sound is more distinctly heard. 



The Cause of the Second Sound is simple. At the moment 

 of ventricular diastole, when this sound develops, the only 

 occurrence which is capable of producing a sound is the sudden 

 stretching of the semilunar valves by the high arterial pressure 

 above them and the low intra-ventricular pressure below them. 

 The high arterial pressure comes on them suddenly like the 

 blow of a drum-stick on a drum-head, and, by seating the valves 

 in vibration, produces the sound. 



Aortic and Pulmonar.y Areas. The second sound has thus 

 a dual origin from the aortic valve and from the pulmonary 

 valve ; and it is possible by listening in suitable positions to 

 distinguish the character of each of these. 



The aortic valve is placed behind the sternum at the level of 

 the lower border of the third costal cartilage. But it is deeply 

 situated. The aorta, passing upwards and forwards, lies in 

 close relationship to the chest wall at the junction of the right 

 side of the sternum and the right second costal cartilage. The 

 sound produced by the valve is conducted up the aorta, and 

 may be heard best in this " aortic area. " 



On the other hand, the pulmonary valve lies in close re- 

 lationship to the anterior chest wall being covered only by 

 the anterior border of the left lung close to the edge of the 



