THE PHENOMENA NOTED DURING EACH CARDIAC CYCLE 299 



change permits of an equalization of the pressures, so that the filling 

 may take place more slowly. This period is indicated in the dia- 

 gram by the letters E and A, the latter marking the beginning of the 

 subsequent auricular systole. 



As the orifices of the venae cavae and pulmonary veins are not 

 guarded by valves, the variations in intra-auricular pressure must 

 necessarily be propagated outward into the adjoining venous trunks. 

 They appear here in the form of the physiological venous pulse. ^ 

 In accordance with the preceding discussion, it must also be evident 

 that an incompetency of either the tricuspid or mitral valve must 

 occasion a much greater second rise in the intra-auricular pressure than 

 is present under normal conditions. This must be so because the auric- 

 uloventricular flaps now not only encroach upon the space of the 

 auricles, but permit a certain quantity of ventricular blood to escape 

 into these cavities. The summit at C is then rendered more conspicu- 

 ous, until, in severe types of insufficiencies of these valves, it com- 

 pletely obliterates the first elevation {A to B). This condition gives 

 rise to a similar modification of the physiological pulse in the central 

 veins, the second elevation increasing in size until it becomes almost 

 confluent with the first. It is then known as the pathological venous 

 pulse. 



The filling of the auricles is accomplished during the intervals be- 

 tween the successive rises in intra-auricular pressure. It has been 

 shown by Burton-Opitz^ that the influx of blood is rapid during early 

 diastole (B to C), but is much diminished during the rise in pressure 

 occasioned by the upward bulging of the auriculoventricular septum 

 (at C). Immediately following this phase, another rapid inrush of 

 blood results (C to D), which, as has been stated above, is responsible 

 for the third summit upon the curve of intra-auricular pressure. Dur- 

 ing the subsequent pause {E to A), the flow becomes slower and slower 

 until it again ceases during the next systole (A to B). It will be seen, 

 therefore, that the venous blood enters the auricles at a time when 

 their musculature is at rest, and hence, it may be inferred that the 

 influx of the blood into the auricles, or the filling of the heart, is 

 occasioned passively by the circumstance that the pressure prevaihng 

 in the central veins, is higher than that existing in the diastolic auricu- 

 lar cavities. 



The auricles serve as storehouses for the ventricles, because they 

 hold a certain quantity of blood in readiness until the very moment 

 when they must deliver it to the ventricles. But the dynamical 

 conditions in the vascular system are subject to considerable variations, 

 and hence, the quantity of blood which must be accommodated by 

 them, is not always the same. Owing to their very distensible append- 

 ages, the auricles are structurally well fitted to adjust themselves to 



^ A more detailed discussion of the venous pulse will be found upon page 388. 

 2 Am. Jour, of Physiol., vi, 1902, 435. 



