238 PHYSIOLOGY OF THE HEART. [CH. xx. 



The contraction of the auricles is inaudible. 



The first sound is heard most distinctly at the apex beat in 

 the fifth interspace ; the second sound is best heard over the 

 second right costal cartilage that is, the place where the aorta 

 lies nearest to the surface. The pulmonary and aortic valves 

 generally close simultaneously. In some cases, however, the 

 aortic may close slightly before the pulmonary valves, giving rise 

 to a 'reduplicated second sound.' The pulmonary contribution 

 to this sound is best heard over the second left cartilage. 



The Coronary Arteries. 



The coronary arteries are the first branches of the aorta ; they 

 originate from the sinuses of Valsalva, and are destined for the 

 supply of the heart itself ; the entrance of the coronary vein, into 

 the right auricle, we have already seen (p. 206). 



Ligature of the coronary arteries causes almost immediate 

 death ; the heart, deprived of its normal blood supply, beats 

 irregularly, twitches, and then ceases to contract altogether. 



In fatty degeneration of the heart in man, sudden death is by 

 no means infrequent. This is in many cases due to a growth in 

 thickness of the walls of the coronary arteries called atheroma, 

 which progresses until the lumen of these arteries is obliterated, 

 and the man dies almost as if they had been ligatured. 



Self-steering Action of the Heart. This expression was originated by 

 Briicke. He supposed that the semi-lunar valves closed the orifices of the 

 coronary arteries during the systole of the heart. Unlike all the other 

 arteries of the body, the coronary arteries would therefore fill only during 

 diastole, and this increased fulness of the vessels in the heart walls during 

 diastole would assist the ventricle to dilate. This, however, is incorrect ; 

 the valves do not cover the mouths of the arteries ; and at the beginning 

 of systole the velocity and pressure in the coronary arteries increase ; but 

 later on during systole the ventricular wall is so strongly contracted that 

 the muscular tension becomes greater than the coronary pressuie, and so the 

 coronary arteries and their tributaries are compressed and the blood driven 

 back into the aorta ; the coronary arteries are then again filled with the 

 commencing diastole. Self-steering action of the heart therefore exists, but 

 it is brought about in a different way from what Bracke supposed. 



Cardiographs. 



A cardiograph is an instrument for obtaining a graphic record 

 of the heart's movements. In animals the heart may be exposed, 

 and levers placed in connection with its various parts may be 

 employed to write on a revolving blackened surface. 



