96 CIRCULATION 



somewhat in length, but the apex does not leave the chest 

 wall, because the injected arteries at the base of the heart 

 lengthen and push the entire heart forward, thus fully com- 

 pensating for the shortening of the ventricle. The apex of 

 the heart as it contracts hardens, and protrudes the chest 

 wall in the intercostal space of the fourth and fifth ribs, midway 

 between the left margin of the sternum and a vertical line, 

 let fall from the left nipple. This constitutes the impulse or 

 apex beat, and it coincides with the upstroke of the pulse. 

 Around the protrusion caused by the apex beat the soft parts 

 of the chest wall are drawn in slightly, which is due to the 

 fact that the heart becomes smaller at that time, and is, there- 

 fore, in contact with the chest wall over a smaller area. This 

 drawing in is called the negative impulse. 



Heart Sounds. The heart during each cycle produces three 

 sounds. The first, low pitched and muffled, coincides with 

 the systoles of the ventricles, and is therefore heard when the 

 apex beat is felt. The second sound is shorter, higher, and 

 clearer than the other, and follows after a scarcely appreciable 

 interval. It coincides with the early part of the diastole of 

 the ventricle. After the second sound there is a period of 

 silence, which is coincident with the latter portion of the dias- 

 tole of the ventricle and the systole of the auricles. It has 

 been proved that the second sound is due to the closure of 

 the semilunar valves of the pulmonary artery and the aorta. 

 When these are experimentally rendered incompetent, the 

 second sound disappears and is replaced by a murmur. The 

 first sound is believed to be due to the combined action of the 

 closure of the tricuspid and mitral valves, and by the muscle 

 sound produced by the ventricles in contracting. The first 

 sound is heard best over the apex of the heart. The closure of 

 the tricuspid valve can be listened to best at the lower end 

 of the sternum. The second sound is heard best at each side of 

 the sternum, between the first and second ribs, being propa- 

 gated upward along the great vessels to which the semilunar 

 valves are attached. 



The third heart sound, heard in certain individuals, occurs 

 about 0.13 second after the beginning of the second sound. It 

 is best listened Jor over the apex of the heart. It is described 

 as being softer and lower in pitch than the second sound and 



