CIRCULATION OF THE BLOOD. 



515 



action of these valves is proved by the alteration in its character which 

 occurs when either the mitral or bicuspid valves are diseased, when this 

 sound becomes obscure, altered, or replaced by murmurs. 



While the valves in their closure are the principal factors in the pro- 

 duction of the first sound of the heart, its characters are dependent upon 

 the presence of several other factors. It will be found that whenever 

 a muscle contracts a sound is produced which depends for its pitch upon 

 the number of contractions occurring per second. At the moment of 

 closure of the auriculo-ventricular valves the blood is forced out from 

 the ventricles into the great arteries. The rushing sound of this moving 

 column of blood is, therefore, another factor in the production of the first 

 sound of the heart. 



To recapitulate : It may be stated that the first sound of the heart is 

 produced by the sudden tightening of the auriculo-ventricular valves, 

 whatever view be accepted as to the nature of their action, to the sound 

 of muscular contraction, and to the rushing of the current of blood from 

 the ventricles into the pulmonary artery and aorta. In support of this view, 

 it may be mentioned that when the muscular contraction of the heart 

 becomes greatly weakened from any depressing cause, as in severe fevers, 

 the first sound of the heart becomes distinctly sharper and more purely 

 valvular in nature, evidently due to the diminished intensity of the con- 

 traction of the cardiac muscle. 



The second sound of the heart is short and sharp, and is due to the 

 sudden closure of the semi-lunar valves, which by their rapid increase in 

 tension, like every other elastic membrane, produce a sound. In living 

 animals, the second sound of the heart is best heard over the root of the 

 large vessels. When the semi-lunar valves are destroyed, as by inserting 

 a hook into these valves, the second sound disappears. The relative 

 lengths of the auricular and ventricular systole and diastole, the time of 

 the occurrence of the impulse, and the different sounds of the heart may 

 be diagrammatical^ represented by a line divided into five parts, which 

 represent the length of a cardiac revolution : 



1 | 2 | 3 I 4 | 5 



We may now extend, somewhat, the sketch which has already been 

 given as to the movement of the blood through the pulsating heart. 



