68 HUMAN PHYSIOLOGY 



In the auricle the variations in pressure are much less than 

 in the ventricles. During the auricular systole the highest 

 pressure is 20 mm Hg. 



5. The cardiac sounds, produced by the contraction of 

 the heart, are heard when the ear is applied to the chest- 

 wall. 



The first sound, produced during the ventricular systole, 

 is dull, lasts as long as the systole, and can be best heard 

 over the ventricle. It depends upon the muscle tone (see 

 Chapter XIV) and upon the vibration of the auriculo-ven- 

 tricular valves produced by the sudden systolic contraction. 

 It is still audible in the bloodless heart. 



The second sound is short, clear, and most distinct over the 

 aorta ; it is caused by the vibration of the semi-lunar valves 

 produced by their sudden closure. 



6. The cardiac impulse, or apex beat, is synchronous with 

 the contraction of the heart and is felt at the fourth or fifth 

 intercostal space, about one and one-half inches to the left of 

 the sternum. It is produced mainly as follows: the tensely 

 contracted cardiac muscle at this point pushes forward the 

 soft part of the intercostal space; during the relaxation of 

 the heart, this part is pushed inward by the atmospheric 

 pressure. 



Other factors, supposed to play a part in the formation of the 

 cardiac impulse, are the following: 



During the systole, the apex of the heart is raised upward; 

 during the discharging of the blood, upward and backward, the 

 heart is pushed forward and downward; the arterial trunks, from 

 which the heart is suspended, when filling are slightly twisted and 

 when emptying untwisted from their spiral-like turning. 



If a button [pelotte] is fixed upon the place of cardiac impulse, 

 so that it is moved by the beat of the heart, and if this movement 

 is transferred to a writing-lever, this lever will describe a curve 

 called the cardiogram. This cardiogram is similar to the pressure 

 curve of the heart, as it is, in reality, produced by the contraction 

 of the cardiac muscle; the two curves are, however, not identical, 

 since the cardiogram represents a pressure and volume curve. 



7. The work of the heart. The work which the heart 

 does during one contraction is equal to the product of the 



