THE HEART. 323 



The cause of the second sound is universally admitted to be the sudden 

 closure and tension of the aortic and pulmonary valves. This fact is 

 established by the following proofs : 1st. The sound is heard with per- 

 fect distinctness, as mentioned above, directly over the situation of these 

 valves at the base of the heart ; 2d. The further we recede in any direc- 

 tion from this point, the fainter becomes the sound ; and 3d, in experi- 

 ments upon the living animal, b}' different observers, it has been found 

 that if a curved needle be introduced into the base of the large vessels, 

 so as to hook back the semilunar valves, the second sound disappears, 

 and remains absent until the valve is again liberated. The valves con- 

 sist of fibrous sheets, covered with a layer of endocardial epithelium. 

 They have the form of semilunar festoons, the free edge of which is 

 directed from the cavity of the ventricle, while the attached edge is 

 fastened to the inner surface of the base of the artery. While the blood 

 is passing from the ventricle to the artery, the valves are thrown for- 

 ward and relaxed ; but when the artery reacts upon its contents they 

 shut back, and their fibres, becoming suddenly tense, yield a clear, 

 characteristic, snapping sound. The character of this valvular sound 

 may be closely imitated by snapping a piece of tape or ribbon (Fig. 105), 



Fig. 105. 



alternately loosening and extending it, while firmly held between the 

 fingers of the two hands. A short piece of ribbon by this sudden tension 

 will give out a sharp and distinct sound ; a longer one will yield a sound 

 which is more dull and prolonged. 



The first sound of the heart contains two elements, which are mingled 

 in different proportions according to the point at which it is heard. One 

 of these elements is comparatively dull in tone, and when heard over 

 the apex or front of the heart communicates its character to the whole 

 of the first sound. It is variously attributed to the muscular contrac- 

 tion of the cardiac fibres and to the movement of the surface of the 

 heart against the inner walls of the chest. The remaining element of 

 the first sound is valvular in character, and is caused by the tension of 

 the auriclo-ventricular valves at the time of the ventricular pulsation. 

 It gradually predominates over the other, at points further removed 

 from the apex of the heart, toward the left border of the organ and the 



