42 THE MECHANISM OF THE CIRCULATION. 



venes. Thus, there is no extraordinary difference between the heart 

 and the skeletal muscle. A skeletal muscle, like the heart, can 

 rhythmically perform work once a second, without fatigue, for a long 

 period of time. The heart, like the skeletal muscle, executes most work 

 with a certain weight and at a certain rate. Increase of either beyond 

 a certain point hastens the fatigue and lessens the power of the heart. 



More than 150 years ago, Stephen Hales 1 attempted to measure the 

 pressure and work of the heart. He injected melted beeswax into the 

 left ventricle and obtained a cast of the cavity. This cast he covered 

 over with small pieces of paper, pinning the pieces on, in exact apposi- 

 tion to each other. He then removed the pieces of paper, and, 

 placing them once more in apposition on a flat surface, measured the 

 superficial area of the cavity of the ventricle. From this measurement 

 he calculated the output of the ventricle, and, having experimentally 

 obtained the height of the normal arterial pressure, he drew conclusions 

 as to the power and work of the heart. Ingenious as this method is, 

 there are many fallacies. It is impossible to obtain from the dead 

 heart the diastolic capacity of the living heart ; moreover, the ventricle 

 never completely empties itself in systole. 



The energy of the circulation. — The energy of the circulation can 

 be reckoned in the following way by the use of hydrodynamic formulae. 



The work done by a force is proportional to the force, and to the 

 distance through which it moves its point of application. The energy 

 of a body is its capacity for doing work, measured by the work to 

 which it is equivalent. 2 



1. Let Q be the systolic output of the heart in cubic centimetres, 

 v the mean velocity of the issuing stream, t the time of escape, A the 

 area of the aortic orifice. Let A be the area of the ascending aorta, u the 

 mean velocity of the blood in it, t' the time of a complete cardiac cycle — 



Then Avt = A'ut' = Q. 



That is to say, the output of the left ventricle can be reckoned, 

 if the mean velocity of the blood-flow in the ascending aorta be found 

 by experiment, and if the area of the ascending aorta and the time 

 of a cardiac cycle be known. 



The following example is given by Nicolls. 3 Assuming the following 

 data, namely, the time of the cardiac cycle, - 8 sec. ; the time of the systolic 

 output, 0T sec. ; the diameter of the aortic orifice, 2r — 2 M 5 cms. ; of the 

 aorta, 2r' = 2 , 8 cms. ; the mean velocity in the aorta, u = 32 cms. — 



Avt = A'ut' = Q 

 ■nr 2 xux'U 7r(V) 2 x u x OS 

 7ri(2-5) 2 x v x -1 = (2-8) 2 xux OS 

 25% = 28 2 x8« 

 625t> = 6272m 



v = 10m = 320 cms. per sec. 



* "Statical Essays," 1733. vol. i. 



2 Cm. Grm. Sec. Units. 



Dyne. — The force which, acting upon 1 grm., produces per sec. velocity of 1 cm. 



per sec. 

 Erg. — The work done by a dyne when it has moved its point of application 



through 1 cm. 

 Density. — Grm. per c.e. 

 Velocity. — Cm. per sec. 

 Acceleration. — Cm. per sec. per sec. 

 Pressure. — Dyne per sq. cm. 



3 Joum. Physiol., Cambridge and London, 1896, vol. xx. 



