400 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



thus, however, they are of moment. When we think of the vast number of 

 beats executed by the heart every clay, the great amount of energy rendered 

 manifest in maintaining the circulation becomes apparent, and our interest is 

 heightened in the' fact that all of this large sum of energy is liberated in the 

 muscular tissue of the heart itself. Thus, too, the physiological significance 

 of the diastole is accentuated as a time of rest for the cardiac muscle, as well 

 as a necessary pause for the admission of blood into the ventricle. To disre- 

 gard minor considerations, the work done at a systole will evidently depend 

 upon the amount of the pulse-volume, of the arterial pressure overcome, and 

 of the velocity imparted to the ejected blood. All these are variable. The 

 work of the ventricles therefore is eminently variable. 



The Heart's Contraction as a Source of Heat.- In dealing with the 

 movement of the blood in the vessels we have seen that the energy of visible 

 motion liberated by the cardiac contractions is progressively changed into heat 

 by the friction encountered by the blood ; and that this change is nearly com- 

 plete by the time the blood has returned to the heart, the kinetic energy of 

 each systole sufficing to drive the blood from the heart back to the heart again, 

 but probably not being much more than is required for this purpose. Practi- 

 cally, therefore, all the energy of the heart's contraction becomes heat within 

 the body itself, and leaves the body under this form. As the heart liberates 

 during every day an amount of energy whicli is always large but very variable, 

 its contractions evidently make no mean contribution to the heat produced in 

 the body and parted with at its surface. 



H. THE MECHANISM OF THE VALVES OP THE HEART. 



Use and Importance of the Valves. The discussion just concluded 

 shows the work of the heart to be the forcible pumping of a variable pulse- 

 volume out of veins where the pressure is low into arteries where the pressure 

 is high. It is owing to the valves that this is possible, and so dependent is 

 the normal movement of the blood upon the valves at the four ventricular 

 apertures that the crippling of a single valve by disease may suffice to destroy 

 life after a longer or shorter period of impaired circulation. 



The Auriculo- ventricular Valves. The working of the auriculo-ven- 

 tricular valves (see Fig. 103) is not hard to grasp. When the pressure within 

 the ventricle in its diastole is low, the curtains hang free in the ventricle, 

 although probably never in close contact with its wall. As the blood pours 

 into the ventricle, the pressure within it rises, currents flow into the space be- 

 tween the wall and the valve, and probably bring near together the edges of 

 the curtains and also their surfaces for some distance from the edges. Thus, 

 upon the cessation of the auricular systole, the supervening of a superior pres- 

 sure within the ventricle probably applies the already approximated edges 

 and surfaces of the curtains to one another so promptly that the commencing 

 contraction of the ventricle is not attended by regurgitation into the auricle. 

 The principle of closure is the same for the tricuspid valve as for the mi- 

 tral. As the forces are exactly equal and opposite which press together the 



