106 THE CIRCULATION. 



tricular valves. The pause immediately follows the second 

 sound, and corresponds in its first part with the completed dis- 

 tension of the auricles, and in its second with their contraction, 

 and the distension of the ventricles, the auriculo-ventricular 

 valves being all the time open, and the arterial valves closed. 



The chief cause of the first sound of the heart appears to be 

 the vibration of the auriculo-ventricular valves, and also, but 

 to a less extent, of the ventricular walls, and coats of the aorta 

 and pulmonary artery, all of which parts are suddenly put into 

 a state of tension at the moment of ventricular contraction. 



This view, long ago advanced by Dr. Billing, is supported 

 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 intestine 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 calculated to produce sound. The in- 

 fluence 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 re- 

 spective 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 dis- 

 tinctness 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 composing the walls of 

 the heart to such an extent as are the auriculo-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 



