170 THE CIRCULATION OF THE BLOOD 



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 192, is an average of say 150 mm. of mercury, or 200 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: 



Total 90 c.c. 240 cm. 21,600 



This computation shows that each heart- beat expends 21,600 gramcenti- 

 meters (21.6 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? 



It has already been shown, page 61, that the muscular fibers of the 

 heart differ in structure from skeletal muscle fibers on the one hand, and 

 from unstriped muscle on the other, occupying an intermediate position 

 between the two varieties. The heart muscle, however, possesses a property 

 which is not possessed by skeletal muscle, or by unstriped muscle to such a 

 degree, namely, the property of contracting rhythmically. 



Rhythmicity. The property of rhythmic contraction is shown by the 

 action of the heart within the body; its systole is followed by its diastole in 

 regular sequence throughout the life of the individual. The force and fre- 

 quency of the systole may vary from time to time as occasion requires, but 

 there is no interruption to the action of the normal heart or any interference 

 with its rhythmical contractions. Further, in an animal rapidly bled to 

 death, the heart continues to beat for a time, varying in duration with the 

 kind of animal experimentally dealt with and depending on whether or not 

 there is entire absence of blood within the heart chambers. Furthermore, 

 if the heart of an animal be removed from the body, it still continues, for a 

 varying time, its alternate systolic and diastolic movements. Thus we see 

 that the power of rhythmic contraction depends neither upon connection 

 with the central nervous system nor yet upon the stimulation produced by 



