132 THE CIRCULATION. 



of the first. It is probably due almost entirely to the 

 sudden closure and consequent vibration of the semilunar 

 valves when they are pressed down across the orifice of 

 the aorta and pulmonary artery ; for, of the other events 

 which take place during the second sound, none is cal- 

 culated to produce sound. The influence of the valves 

 in producing the sound, is illustrated by the experiment 

 already quoted from Valentin, and by others performed on 

 large animals, such as calves, in which the results could 

 be fully appreciated. 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 had ceased to be audible. 

 Disease of these valves, when so extensive as to interfere 

 with their efficient action, also often demonstrates the same 

 fact by modifying or destroying the distinctness of the 

 second sound. 



One reason for the second sound being a clearer and 

 sharper one than the first jnay be, that the semilunar 

 valves are not covered in by the thick layer of fibres 

 composing the walls of the heart to such an extent as are 

 the auricula-ventricular. It might be expected therefore 

 that their vibration would be more easily heard through a 

 stethoscope applied to the walls of the chest. 



The contraction of the auricles which takes place in the 

 end of the pause is inaudible outside the chest, but may be 

 heard, when the heart is exposed and the stethoscope 

 placed on it, as a slight sound preceding and continued 

 into the louder sound of the ventricular contraction. 



The Impulse of the Heart. At the commencement of each 

 ventricular contraction, the heart may be felt to beat with 



