SOUXDS OF THE HEART. 12 1 



are suddenly put into a state of tension at the moment of 

 ventricular contraction. 



This view, long ago advanced by Dr. Billing, is sup- 

 ported by the fact observed by Valentin, that if a portion 

 of a horse's intestine, tied at one end, be moderately filled 

 with water, without any admixture of air, and have a 

 syringe containing water fitted to the other end, the first 

 sound of the heart is exactly imitated by forcing in more 

 water, and thus suddenly rendering the walls of the intes- 

 tine more tense. 



The cause of the second sound is more simple than 

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

 sudden closure and consequent vibration of the semilunar 

 valves when they are pressed down across the orifices 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 from 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 may be, that the semilunar 

 valves are not covered in by the thick layer of fibres 



