34 CIRCULATION OF THE BLOOD. 



^ Describe the heart-sounds. 



The first sound is heard best over the apex of the heart. It is 

 of a dull, prolonged, booming character. The second sound is heard 

 immediately after the first, and is a sharp, quick, almost clicking 

 sound ; it is heard most clearly over the base. The sounds are said 

 to somewhat resemble that expressed by lubb-dilp. 



What is the cause of the heart-sounds ? 



The origin of the first sound of the heart is not fully explained ; 

 but as it is synchronous with the ventricular systole and with the 

 closure of the auriculo-ventricular valves, it is supposed to be due 

 to muscular action and vibration of auriculo-ventricular valves and 

 chordae tendinese. The second sound is synchronous with the 

 closure of the seniilunar valves, and is caused by this action. 



^ What is the normal frequency of the heart's action ? 



At birth, about 130 per minute. 



At three years, " 100 " " 



In adult life, " 75 " " 



In old age, " 65 " " 



This rate is varied from time to time by conditions of bodily 

 health and by environment. The heart in women is somewhat 

 more rapid than in men. After eating, during exercise, in a hot or 

 rarefied atmosphere the heart is more rapid. The relative frequency 

 of heart and respiratory action is about three or four heart-beats 

 to one respiratory act. 



What force does the heart exert in systole? 



The left ventricle exerts more than twice as much power as the 

 right. The exact intraventricular pressure in man has not been as- 

 certained. The expansion of the heart exerts a negative (or suc- 

 tion) pressure, which aids the onflow of the blood, especially from 

 the lungs to the left ventricle. The intra-auricular pressure is very 

 much less than the intraventricular, and there is a negative pres- 

 sure during diastole in the auricles. 



What is the estimated work of the heart in systole ? 



Estimated in foot-pounds, each ventricular contraction repre- 

 sents 3j to 4 foot-pounds. In twenty-four hours this is estimated 

 to equal more than 120 foot-tons! In another light, if the blood is 

 one-twelfth of the body-weight, and if the amount of blood pumped 

 with each ventricular contraction is 6 oz., in an ordinary man an 



