134 



HUMAN PHYSIOLOGY. 



begins. If the blood is not to be returned to the auricles the tri- 

 cuspid and mitral valves must be instantly" and completely closed. 

 This is accomplished by the upward pressure of the blood which 

 brings their free edges in close apposition. Reversal of these valves 

 is prevented by the contraction of the papillary muscles which 

 exert a traction on their under surfaces and edges and hold them 

 steady. 



The blood now confined in the ventricles between the closed 

 auriculoventricular and semilunar valves is subjected to pressure 

 on all sides ; as the pressure rises proportionately to the vigor of 

 the contraction there comes a moment when the intra-ventricular 

 pressure exceeds that in the aorta and pulmonary artery ; at once the 

 semilunar valves are thrown open and the blood discharged. Both 

 contraction and outflow continue until the ventricles are practically 

 empty, when relaxation sets in attended by a rapid fall of pressure, 

 under the influence of the positive pressure of the blood in the 

 aorta and pulmonary artery, the semilunar valves are again closed. 

 The accumulation of blood in the auricles, attended by a rise in 

 pressure, again forces the tricuspid and mitral valves open. With 

 these events the cardiac cycle is again completed. 



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 prolonged and dull, and caused by the closure and vibra- 

 tion of the auriculoventricular 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 frequency of the heart's action varies at different periods 

 of life, but in the adult male it beats about seventy-two times a 

 minute. It is influenced by age, exercise, posture, digestion, etc. 



Age. Before birth, the number of pulsations a minute averages . 140 



During the first year it diminishes to . . . .128 



During the third year it diminishes to 95 



From the eighth to the fourteenth year averages .... 84 

 In adult life the average is 72 



