118 HAND-BOOK OF PHYSIOLOGY. 



the semilunar valves, as there was to their opening. The sounds may 

 be expressed by saying the words lubb dup (C. J. B. Williams). 



The events which correspond, in point of time, with the first sound, 

 are (1) the contraction of the ventricles, (2) the first part of the dilatation 

 of the auricles, (3) the closure of the auriculo-ventricular valves, (4) the 

 opening of the semilunar valves, and (5) the propulsion of blood into the 

 arteries. The sound is succeeded, in about one-thirtieth of a second, by 

 the pulsation of the facial arteries, and in about one-sixth of a second, 

 by the pulsation of the arteries at the wrist. The second sound, in point 

 of time, immediately follows the cessation of the ventricular contraction, 

 and corresponds with (a) the closure of the semilunar valves, (b) the con- 

 tinued dilatation of the auricles, (c) the commencing dilatation of the 

 ventricles, and (d) the opening of the auriculo-ventricular valves. The 

 pause immediately follows the second sound, and corresponds in its first 

 part with the completed distension of the auricles, and in its second 

 with their contraction, and the completed distension of the ventricles; 

 the auriculo-ventricular valves being, all the time of the pause, open, and 

 the arterial valves closed. 



Causes. The chief cause of the first sound of the heart appears to 

 be the vibration of the auriculo-ventricular valves, due to their stretch- 

 ing, 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. The effect 

 may be intensified by the muscular sound produced by contraction of the 

 mass of muscular fibres which form the ventricle. 



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. The influence of the valves in produc- 

 ing the sound is illustrated by the experiment performed on large ani- 

 mals, 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 attach- 

 ment of the semilunar valves, and, after being carried upward 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 composing the walls of the heart to such an extent as are 



