CIRCULATION OF THE BLOOD. 135 



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



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

 standing, the average is 81 ; sitting, 71 ; lying, 66 independent, for 

 the most part, of muscular effort. 



The force exerted by the left ventricle at each contraction has 

 been estimated at fifty-two 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 (four 

 pounds) by the area of the internal surface of the left ventricle 

 (about thirteen inches), each inch of the ventricle being capable of 

 supporting a downward pressure of four pounds. 



Work Done by the Heart. The work done by the heart is esti- 

 mated by multiplying the amount of blood sent out from the right 

 and left ventricles at each contraction by the pressure of the pul- 

 monary artery and aorta, respectively e. g., when the right ventricle 

 contracts, it forces out l /4 of a 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 *4 of a pound 3 feet, or ^ X 3, or ^ 

 of a pound i foot. When the left ventricle contracts, it sends out ^4 

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

 a pressure in the aorta sufficient to support a column of blood 9 feet 

 in height ; that is, must exert energy sufficient to raise *4 of a pound 

 9 feet, or *4 X 9, or 2^4 pounds i foot. Work done is estimated 

 by the amount of energy required to raise a definite weight a definite 

 height ; the unit, the foot-pound, being that required to raise i 

 pound i foot. 



The heart, therefore, at each systole exerts energy sufficient to 

 raise 3 foot-pounds, and as it contracts 72 times a minute, it would 

 raise in that time 3 X 72, or 216 foot-pounds; and in one hour 

 216X60, or i2;96o foot-pounds; and in twenty-four hours 12,960X24, 

 or 311,040 foot-pounds, or 138.5 foot-tons. 



The Causation of the Heart Beat. From the fact that the heart 

 will continue to beat for a variable length of time after removal 

 from the body (the time varying with the species of animal from 

 which it has been obtained) it is evident that the beat is independent 

 of the central nerve system. 



