THE CIRCULATION. 243 



experimenter is ready to determine, among the complex actions which 

 make up a cardiac cycle, the one which gives rise to each of the two 

 sounds. 



SECOND SOUND. The cause of the second sound is the sudden 

 closure of the sigmoid (semilunar) valves of the aorta and pulmonary 

 artery during relaxation of the ventricle. The sudden closing of the 

 valves is produced during the effort of the arterial blood to escape 

 backward from the elastic reaction of the aorta and pulmonary artery. 



Proofs abound in support of this theory. If the valvular move- 

 ments be hindered in one of the above-mentioned arteries by placing 

 a clamp close to its base, immediately the second sound is suppressed 

 at that point. If the valvular action of both vessels be suppressed, 

 the second sound may be completely extinguished. 



Fig. 73. The Action of the. Semilunar Valves. (CHAUVEAU.) 



Pv, Tracing of the variations of pressure in the left ventricle. 2, Means 

 second sound. 8, Tracing by the signal magnet showing the action of the valve 

 which by its movements closes and opens an electric current to the signal 

 magnet. The second sound (closure of the semilunar valves) corresponds to the 

 moment when the ventricle relaxes, that is, at the beginning of the ventricular 

 diastole. 



Again, should the apex of the heart be cut off and the ventricular 

 blood be made to escape to the outside, no second sound occurs. In 

 this experiment the sigmoid valves have neither been lifted up nor 

 allowed to fall back and stretch themselves out with a sound. 



Physically, one is able to account for the production of the second 

 sound on the principle that it is produced by the clicking of the 

 sigmoid valves. In fact, similar sounds are obtained by producing 

 sudden tension of a membrane under the action of a column of liquid. 



When the initial stump of an aorta, whose valves are still intact, 

 is attached to a tube and the reflux of the liquid closes the valves, a 

 clear, snappy click is produced. 



When pathological conditions occur, the sound is altered, being 

 accompanied by or even altogether replaced by a blowing sound, 

 known as a "murmur." 



