54 HUMAN PHYSIOLOGY. 



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 o'f 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 influenced 



by age, exercise, posture, digestion, etc. 





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



During the first year it diminishes to, . * 128 



During the third year diminishes to, 95 



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

 In adult life the average is, 72 



Exercise and digestion increase the frequency of the heart's action. 



Posture influences the number of pulsations per minute ; in the male, 

 standing, the average is 8l ; sitting, 71; lying, 66 ; independent, for the 

 most part, of muscular effort. 



The Rhythmical movements of the heart are dependent upon i. 

 An inherent irritability of the muscular fibre, which manifests itself as long 

 as the nutrition is maintained. 2. The continuous flow of blood through 

 its cavities, distending them and stimulating the endocardium. 



The force exerted by the left ventricle at each contraction has been 

 estimated at 52 pounds. If a tube be inserted into the aorta, the pressure 

 there will be sufficient to support a column of blood nine feet or a column 

 of mercury six inches in height, the weight in either case being about four 

 pounds. The estimation of the force which the heart is required to exert 

 to support this column of blood, is arrived at by multiplying the pressure 

 in the aorta (4 pounds) by the area of the internal surface of the left 

 ventricle (about 13 inches). Each inch of the ventricle being capable of 

 supporting a downward pressure of 4 pounds. 



Work done by the Heart. The work done by the heart is estimated 

 by multiplying the amount of blood sent out from the right and left ventricles 

 at each contraction, by the pressure in the pulmonary artery and aorta 

 respectively, e. g:, when the right ventricle contracts, it forces out one- 

 quarter pound of blood, and in so doing must overcome a pressure in the 

 pulmonary artery sufficient to support a column of blood three feet in 

 height ; that is, must exert energy sufficient to raise } Ib. 3 feet, or j^ X 3 

 or % It), one foot. When the left ventricle contracts, it sends out ^ Ib. of 

 blood, and in so doing, the left ventricle must overcome a pressure in the 



