28o TEXT-BOOK OF PHYSIOLOGY 



duration of the first sound is almost equal to the duration of the systole viz., 

 0.3 second; the duration of the second sound is not more than o.i second. 

 The systolic sound is heard most distinctly over the body of the heart; the 

 diastolic sound is heard most distinctly in the neighborhood of the third rib 

 to the right of the sternum. 



The causes of the heart-sounds have enlisted the attention of clinicians 

 and physiologists for years, and many factors have been assigned for their 

 production. At present it is generally believed that the first sound is the 

 product of at least two, possibly three, factors: viz., the contraction of the 

 muscle walls of the ventricles, the simultaneous closure and subsequent 

 vibration of the tricuspid and mitral valves, and the sudden increase of 

 pressure of the apex of the heart against the chest wall. 



That the contraction of the ventricular muscle gives rise to a sound is 

 certain from the fact that it is perceptible in an excised heart when the 

 cavities are free from blood and when the valves are prevented from closing. 

 The explanation of this sound is extremely difficult, as the contraction, 

 though prolonged, is not of the nature of a tetanus and therefore not charac- 

 terized by rapid variations of tension. The apex element may be eliminated 

 by placing the individual in the recumbent position. 



The second sound is the product of the simultaneous closure and subse- 

 quent vibration of the aortic and pulmonic valves which occur at the 

 beginning of the ventricular diastole as the blood surges back against the 

 closed valves. This has been definitely proved by the fact that the sound 

 disappears when the valves are destroyed or held back by hooks introduced 

 into the aorta and pulmonary artery. It is also possible that the vibration 

 of the column of blood produces an additional tone which adds itself to that 

 produced by the valves. 



The foregoing events are shown in their order in the following diagram. 



FIG. 122. DIAGRAM SHOWING THE EVENT IN THE CARDIAC CYCLE. 



Relative Functions of Auricles and Ventricles. Though both 

 auricles and ventricles are essential to the continuous movement of blood, 

 they possess unequal values in this respect. The passage of the- blood 

 through the pulmonic and systemic vessels is accomplished by the driving 

 power of the right and left ventricles respectively, aided, however, by minor 

 extra-cardiac forces. They may be regarded therefore SLS force-pumps. 



If the heart consisted of ventricles only, the flow of blood from the vena? 

 cavae and pulmonic veins would be temporarily arrested during their systole 

 and their subsequent refilling delayed. This is obviated, however, by the 

 addition of the auricles; for during the ventricular systole the blood continues 

 to flow into the auricles, in which it is temporarily stored until the ventricular 

 relaxation sets in. With this event the accumulated blood passes into the 

 ventricles, which are thus practically filled before the auricular systole occurs 

 by which the filling is completed. By this means there is no delay in the 



