46 THE CIRCULATION OF THE BLOOD 



Sounds of the Heart. Listening to the heart's action through 

 the thoracic wall we hear two distinct sounds. The first is a 

 slightly elongated sound and comes immediately after the beat 

 of the radial pulse. It is characterized by the syllable lub. 

 The cause of this sound is supposedly the closure of the auriculo- 

 ventricular valves combined with the sound made by the con- 

 tracting muscle. It can best be heard over the apex of the 

 heart. 



The second sound is shorter and sharper than the first and is 

 heard just before the impulse of the radial pulse. It is char- 

 acterized by the shorter syllable dap. 



The cause of this sound is supposedly the closure of the aortic 

 semi-lunar valves along with those of the pulmonary artery. It 

 is best heard in the right second intercostal space, as the aortic 

 current transmits it. 



Certain diseases affect the heart valves and the sounds then 

 depart from the normal. Thus it is of importance to know the 

 cause and sound of the normal vibrations so as to detect the 

 diseased conditions. 



Heart Innervation.- The nerves that inhibit the action of 

 the heart are the two vagi ; cutting these results in an increase 

 of the frequency of the heart beats. 



The nerves that accelerate the action of the heart are the 

 nervi accelerantes, which are branches of the sympathetic 

 system. Stimulation of these causes increase in force and fre- 

 quency of heart beats. 



II. CIRCULATION IN BLOOD-VESSELS. 



Taking the heart as a central station for supplying force, we 

 find the blood current constantly going from a place of higher 

 pressure to a place of lower pressure. 



The highest pressure is in the muscular center, the heart. 

 Blood-vessels connect with both auricles and ventricles. Those 



