48 HUMAN PHYSIOLOGY. 



The Function of the Heart is to propel the blood to all portions of 

 the vascular system; accomplished by successive alternate contractions and 

 relaxations of its muscular walls, constituting the systole and diastole. 



Course of the blood through the Heart. The venous blood returned 

 to the heart by the superior and inferior venae cavae is emptied, during the 

 diastole, into the right auricle, on the contraction of which it is forced 

 through the right auriculo-ventricular opening into the right ventricle and 

 distends it. Upon contraction of the ventricle, the blood is propelled 

 through the pulmonary artery into the lungs, where it undergoes aeration 

 and is changed in color. 



The arterial blood is now collected by the pulmonary veins and poured 

 into the left auricle ; thence it passes into the left ventricle, which becomes 

 fully distended. Upon the contraction of the ventricle, the blood is pro- 

 pelled into the aorta, and by it distributed to the system at large, to be again 

 returned to the heart by the veins. 



Regurgitation from the ventricles into the auricles during the systole is 

 prevented by the closure of the tricuspid and mitral valves ; regurgitation 

 from the pulmonary artery and aorta into the ventricles during the diastole 

 is prevented by the closure of the semilunar valves. 



Movements of the Heart. At each revolution, during the systole, 

 the heart hardens and becomes shortened in its long diameter; its apex is 

 raised up, rotated on its axis from left to right and thrown forward against 

 the walls of the chest. The impulse of the heart, observed about two 

 inches below the nipple, and one inch to the sternal side, between the fifth 

 and sixth ribs, is caused mainly by the apex of the heart striking against 

 the chest walls, assisted by the distention of the great vessels about the 

 base of the heart. 



Sounds of the Heart. If the ear be placed over the cardiac region, 

 two distinct sounds are heard during each revolution of the heart, closely 

 following each other and which differ in character. 



The sound coinciding with the systole in point of time, the first sound, 

 is long and dull, and caused by the closure and vibration of the auriculo- 

 ventricular valves, the contraction of the walls of the ventricles and the 

 apex beat ; the second sound, occurring during the diastole, is short and 

 sharp, and caused by the closure of the semilunar valves. 



The capacity of the left ventricle when fully distended is estimated at 

 from four to seven ounces. 



The frequency of the heart's action varies at different periods of 

 life, but in the adult male it beats about 72 times per minute. It is influ- 

 enced by age, exercise, posture, digestion, etc. 



