232 HUMAN PHYSIOLOGY 



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, fehe 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 setting the valves in vibration, produces the 

 sound. 



Aortic and Pulmonary 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 nature 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 best be heard 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 

 sternum in the second left interspace. The pulmonary 

 element of the second sound may best be heard here. 



The Cause of the First Sound is by no means so simple. 

 When it is heard, two changes are taking place in the heart, 

 either of which would produce a sound. 



1st. The muscular wall of the ventricles is contracting. 



2nd. The auriculo-ventricular valves are being closed and 

 subjected on the one side to the high ventricular pressure, 

 and on the other to the low auricular pressure. 



That the first factor plays an important part hi the pro- 

 duction of the first sound is proved by rapidly cutting out 

 the heart of an animal, and while it is still beating but 

 without any blood passing through it to stretch the valves 



