THE HEART. 271 



a pulsation. It is followed by an equal interval of silence ; after 

 which the first sound again recurs. The whole time of a cardiac 

 pulsation may then be divided into four quarters, of which the first 

 two are occupied by the first sound, the third by the second sound, 

 and the fourth by an interval of silence, as follows : 



f 1st quarter 



ist quarter | 



Time of pulsation, j 3d gecond 



^ 4th " Interval of silence. 



The cause of the second sound is universally acknowledged to be 

 the sudden closure and tension of the aortic and pulmonary valves. 

 This fact is established by the following proofs: 1st, this sound is 

 heard with perfect distinctness, as we have already mentioned, 

 direct* 1 - over tho situation of the above-mentioned valves; 2d, the 

 farther we recede in any direction from this point, the fainter be- 

 comes the sound ; and 3d, in experiments upon the living animal, 

 often repeated by 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 at once dis- 

 appears, and remains absent until the valve is again liberated. These 

 valves consist of fibrous sheets, covered with a layer of endocardial 

 epithelium. They have the form of semilunar festoons, the free 

 edge of which is directed away 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, these valves are thrown forward and relaxed ; but when the 

 artery reacts upon its contents they shut back, and their fibres, be- 

 coming suddenly tense, yield a clear, characteristic, snapping sound. 



The production of the first sound has been attributed by some 

 writers to a combination of various causes; such as the rush of 

 blood through the cardiac orifices, the muscular contraction of the 

 parietes of the heart, the tension of the auriculo- ventricular valves, 

 the collision of the particles of blood with each other and with the 

 surface of the ventricle, &c. &c. We believe, however, with Andry 1 

 and some others, that the first sound of the heart has a similar 

 origin with the second ; and that it is dependent altogether on t1>e 

 closure of the auricula-ventricular valves. The reasons for this con- 

 clusion are the following : 



1st. The second sound is undoubtedly caused by the closure of 



1 Diseases of the Heart, Kneeland's translation, Boston, 1846. 



