SOUNDS OF THE HEART. 649 



valves. The occurrence of changes in, and the occasional cessation 

 of, the second sound, in cases of disease of the semilunar valves, also 

 favor this explanation; so, likewise, does the fact that the second 

 sound is so short, and occurs at the instant of the tightening out of 

 these valves, and not during their subsequent and quiet closure. 

 Lastly, there is no other simultaneous condition, or action, of any 

 part of the heart which could produce a sound; the ventricles and 

 auricles are, at this time, both quietly dilating, and the resilience of 

 the great arterial trunks, forcing the blood back upon the semilunar 

 valves, is the only mechanical action then capable of causing a sound. 



The contraction of the auricles, which occurs in the latter part of 

 the pause in the heart's beat, produces no audible sound, unless the 

 heart be exposed, and the stethoscope be placed upon it. 



It is usually held to be confirmatory of the preceding views, as to 

 the causes of the two sounds of the heart, that the first sound is more 

 distinctly heard opposite the fifth intercostal space of the left side, 

 below the left nipple, i. e., over the region of the apex of the ventricles, 

 where these approach nearest to the walls of the chest, to which they 

 communicate the sound ; whilst the second sound is heard most clearly 

 in the third left intercostal space, close to the sternum, i. e. over the 

 base of the ventricles, and the commencement of the great arteries, 

 where these approach nearest to the thoracic walls. 



The diiference in the pitch and character of the two sounds is partly 

 explicable thus: the deep and dull tone of the first sound may depend 

 on the greater size and deeper position of the auriculo-ventricular 

 valves, on their thicker attachments, and on the quantity of muscular 

 substance which overlies them ; the higher and sharper tone of the 

 second sound may be connected with the thinner structure and attach- 

 ments, and with the less covered position of the semilunar valves. 



The following Table shows the order and duration of the complicated 

 actions of the different parts of the heart during each complete beat, 

 and also their relations, in point of time, to the two sounds of the heart, 

 to the subsequent pause or interval of silence, and to the occurrence 

 of the pulse at the wrist. The duration of the events is given after 

 Laennec. 



It is seen that the first sound occupies J of the entire beat, the 

 second nearly J of the beat, and the period of silence rather more than 

 the remaining J. The Table likewise exhibits the alternation of the 

 systole and diastole of the auricles and ventricles, as well as the rela- 

 tive duration of their respective systolic and diastolic conditions. 

 Thus the rapid systole of the auricles, occupies only Jth of the whole 

 period, and their diastole |ths ; whilst the slower systole of the ven- 

 tricles occupies J, and their diastole the other J of the whole period or 

 beat. According to Vierordt, the period of diastole of the auricle, is 

 really shorter, being only fths or fths of the whole period; the re- 

 maining |ths or fths of that time, usually regarded as the period of 

 diastole, must, in that case, be viewed as representing a distended, or 

 continued dilated, condition of the auricle. In the Table, the systole 

 of the auricles and ventricles together, is seen to occupy fths, and the 

 interval between the contraction of the ventricles and the commence- 



