344 THE CIRCULATION OF THE BLOOD. CHAP, xxvin.] 



denoting that that phenomenon cannot be due to any recoil con- 

 sequent upon the resistance to the passage of the blood through 

 the great vessels, and that its true cause is the contraction of the 

 fibres of the ventricle. 



Certain sounds accompany the heart's action, the accurate inter- 

 pretation of which has shed a flood of light upon the diagnosis of 

 diseases of that organ. On placing the ear over the region of the 

 heart in a healthy individual, the following phenomena may be 

 perceived; first, a heavy, somewhat prolonged sound, which is 

 synchronous with the impulse, and is best heard over the heart's 

 apex; this is the systolic Q? first sound; secondly, a short clicking 

 sound immediately succeeding this ; it is synchronous with (but not 

 caused by) the diastole of the ventricles, and is called the diastolic, 

 or second sound ; it is best heard over the base of the heart near 

 the root of the aorta. After this the heart seems to pause, as it 

 were to take rest, and then follows the first sound again, followed 

 instantly by the second sound, and then the pause. The duration 

 of the first sound is about double that of the second, while that 

 of the second is equal to the pause. Thus if the whole period of 

 the heart's action be divided into four parts, the first two would 

 be occupied by the first sound, the third by the second sound, and 

 the fourth by the pause. 



Numerous experiments and observations have been made with 

 reference to the question of the signification of these sounds. We 

 must here content ourselves with stating the conclusions which we 

 think may be safely drawn from them. The first sound is composed 

 mainly of the muscular sound generated by the contraction of the 

 ventricles, strengthened by that due to the sudden tension of the 

 auriculo-ventricular valves over the blood contained in the ventri- 

 cles, this tension being effected by the contraction of the carnese 

 columnee, which is synchronous with that of the rest of the ventri- 

 cular wall. To these causes of sound may be added the impulse 

 of the heart against the wall of the chest, and, perhaps, also the 

 collision of the blood against the orifices of the great vessels. 



The second sound is due to the sudden tension of the semilunar 

 valves of the two great vessels by the recoil of the columns of 

 blood injected into them by their respective ventricles. An ex- 

 periment, originally suggested by the late Dr. Hope, and repeated 

 by several observers, proves this unequivocally. If in an animal 

 whose respiration is maintained by artificial insufflation, the heart's 

 action being thereby prolonged, a hook be introduced into the 

 aorta so as to hold back one of its valves, and leave a passage for 



