190 HANDBOOK OF PHYSIOLOGY. 



ular walls. The valvular element is probably the more important of 

 the two factors. 



The cause of the second sound is more simple than that of the first. 

 It is entirely due to the vibration consequent on the sudden closure of 

 the semilunar valves when they are pressed down across the orifices of 

 the aorta and pulmonary artery. The influence of these valves in pro- 

 ducing the sound was first demonstrated by Hope who experimented 

 with the hearts of calves. In these experiments two delicate curved 

 needles were inserted, one into the aorta, and another into the pulmo- 

 nary artery, below the line of attachment 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 ap- 

 plying the stethoscope to the vessels, after such an operation, the second 

 sound had ceased to be audible. Disease of these valves, when sufficient 

 to interfere with their efficient action, also demonstrates the same fact 

 by modifying the valvular cause of the second sound or destroying its 

 distinctness. 



One reason that the second sound is clearer and sharper 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-veutricular. It might be expected therefore that their vibra- 

 tion would be more easily heard by means of 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 is said to be heard, when the 

 heart is exposed and the stethoscope placed on it, as a slight sound pre- 

 ceding and coutiuued into the louder sound of the ventricular contrac- 

 tion. 



The Impulse of the Heart. 



With each contraction the heart may be felt to beat with a slight 

 shock or impulse against the walls of the chest. The force of the im- 

 pulse and the extent to which it may be perceived beyond this point 

 vary considerably in different individuals, and in the same individual 

 under different circumstances. It is felt more distinctly, and over a 

 larger extent of surface, in emaciated than in fat and robust persons, 

 and more during a forced expiration than in a deep inspiration; for, in 

 the one case, the intervention of a thick layer of fat or muscle between 

 the heart and the surface of the chest, and in the other the inflation of 

 the portion of lung which overlaps the heart, prevents the impulse from 

 being fully transmitted to the surface. An excited action of the heart, 



