CH. XXI.] THE CORONARY ARTERIES 237 



There is, on the other hand, much to be said against the 

 view that the cause of the first sound is entirely due to vibra- 

 tion of the auriculo - ventricular valves. Any sound produced 

 by the valves must be very quickly damped by the high pressure 

 acting on their ventricular surfaces only. The sustained character 

 of the sound (throughout practically the whole of the ventricular 

 systole) is on the other hand exactly what is to be expected if it is 

 of muscular origin. The argument that the extent to which the 

 muscle sound contributes to the production of the first sound can 

 be judged from the sound heard in an isolated and empty heart is 

 quite fallacious, since under these conditions the muscle is contract- 

 ing against no resistance. 



The cause of the second sound is more simple than that of the 

 first. It is entirely due to the vibration consequent on the sudden 

 stretching of the semilunar valves when they are pressed down across 

 the orifices of the aorta and pulmonary artery. The influence of 

 these valves in producing the sound was first demonstrated by Hope, 

 who experimented with the hearts of calves. In these experiments 

 two delicate curved needles were inserted, one into the aorta, and 

 another into the pulmonary artery, below the line of attachment of 

 the semilunar valves, and, after being carried upwards about half an 

 inch, were brought out again through the coats of the respective 

 vessels, so that in each vessel one valve was included between the 

 arterial walls and the wire. Upon applying the stethoscope to the 

 vessels, after such an operation, the second sound ceased to be 

 audible. Disease of these valves, when sufficient to interfere with 

 their efficient action, also demonstrates the same fact by modifying 

 the second sound or destroying its distinctness. 



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 simul- 

 taneously. 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. 209). 



Ligature of the coronary arteries causes almost immediate 



