THE PROPERTIES OF THE HEART MUSCLE 



195 



a second moment of complete closure of the ventricles, from the time of the 

 closing of the semilunar valves until the ventricular pressure falls below the 

 auricular pressure which permits the tricuspid and mitral valves to open. 



The Force of the Cardiac Action. In estimating the amount of 

 work done by a machine it is usual to express it in terms of work units. A 

 convenient work unit for this purpose is the amount of energy required to 

 lift a unit of weight, i.e., i gram or i kilogram, through a unit of height; i.e., 

 i centimeter or i meter, the work required being i gramcentimeter for 

 small units, and i kilogrammeter for large units, respectively. The 

 average work done by the heart at each contraction can be readily com- 

 puted by multiplying the weight of blood expelled by the ventricles by the 

 height through which it would have to be lifted to overcome the resistance 

 to its discharge from the cavities into the arteries. 



The quantity of blood expelled and the pressure of the arteries can only 

 be estimated for man. But the computations from indirect observations 

 on other mammals indicate that the quantity of blood discharged from 

 each ventricle at a single contraction is from 80 to 100 c.c. The pressure 

 of the aorta, see page 221, is an average of, say, 120 mm. of mercury, or 

 126 cm. of blood. The pressure in the pulmonary artery is much less, say, 

 30 mm. (20 to 40), of mercury or 40 cm. of blood. Collecting these facts, 

 we have the following computation: 



This computation shows that each heart-beat expends 17,640 gramcenti- 

 meters (17.64 grammeters) of work. The amount of energy developed in 

 the contractions of the auricles may be ignored in this calculation, which is 

 at best only of relative value. Calculations based on the determinations of 

 Vierordt, also other earlier determinations, give much higher figures than 

 are presented here. 



The Properties of the Heart Muscle. It is evident that if we are 

 to arrive at any adequate explanation of the action of the heart, one of the 

 first questions that must be considered is, what are the fundamental 

 properties of heart muscle as such? 



