46 THE CIRCULATION OF THE BLOOD 



syllable lub. The cause of this sound is supposedly the 

 closure of the auriculo-ventricular valves combined with 

 the sound made by the contracting 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 

 characterized by the shorter syllable dup. 



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 in- 

 crease 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 frequency 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 connecting with the ventricles and carrying blood 

 away from the heart are called arteries and the pressure in 

 these is high, but lower than in the heart. Those vessels 

 connecting with the auricles and carrying blood back to the 

 heart are called veins and the pressure is lowest of all in 

 these. 



