300 THE MECHANICS OF THE HEART 



varying quantities of blood. The ventricles, on the other hand, are 

 much more compact and cannot be made to yield so readily. It should 

 also be emphasized that the auricles do not simply store the blood in 

 a passive way, but also develop a driving force suflScientlyhigh to fill 

 the ventricles to their utmost capacity. The pressure values cited 

 previously, however, prove that the power developed by them is 

 relatively slight, but inasmuch as they discharge their contents into 

 the ventricles at a time when the latter are at rest, practically no re- 

 sistance need be overcome by them. We have seen that the inflowing 

 venous blood opens the auriculoventricular valves sometime before 

 the systolic movement of the auricles actually begins. This enables a 

 moderately krge quantity of blood to escape into the ventricles even 

 before the onset of the next auricular systole. Consequently, all the 

 latter needs to accomphsh is to force in an additional amount so that 

 the ventricle becomes fully distended. Their duty is, so to speak, 

 to ram the charge home. 



The Intraventricular Pressure and the Function of the Ventricles. — 

 The determinations with the maximal-minimal manometer have 

 proved that the pressure in the ventricles is subject to much greater 

 variations than the pressure in the auricles. In the second place, 

 it has been found that by far the greatest power is developed by the 

 left ventricle, which fact is in perfect agreement with the extraordinary 

 thickness of its walls. Obviously, an unusually high driving force 

 is necessary to propel the blood through the channels of the systemic 

 circuit. Thus, while the systoUc pressure in the left ventricle of the 

 human heart amounts to about 125-140 mm. Hg., the right ventricle 

 develops a pressure of scarcely more than 50 mm. Hg. Much higher 

 values, however, are obtained whenever the circulatory mechanism 

 is called upon to perform an extra amount of work. For example, one 

 of the most efficient means of raising the intraventricular pressure, 

 as well as the general blood pressure, is muscular exercise. 



During diastole, the pressure falls to within a few millimeters of 

 zero. In fact, a slight negative pressure has been encountered at 

 times in certain hearts, but as this result is not constant, its cause must 

 be sought in certain accidental conditions, rather than in an active 

 relaxation of the cardiac musculature. This conclusion finds confirma- 

 tion in the fact that an elastic recoil, such as is possessed by a rubber- 

 bulb, has not been observed in the case of the heart. Moreover, it 

 has been shown that a normally beating organ is unable to derive it& 

 supply of blood from a U-shaped tube adjusted at its own level. ^ 

 In addition. Porter^ has proved that the negative pressure in the 

 ventricles is not associated with a corresponding fall in the intra- 

 auricular pressure, and hence, it may be inferred that the venous 

 column without is not subjected to an actual suction action. Several 

 explanations have been offered for this occasional negative pressure. 



1 Von den Velden, Zeitschr. fur exp. Path, und Ther., iii, 1906. 



2 Jour, of Physiol., xiii, 1892, 513. 



