146 A TEXTBOOK OF PHYSIOLOGY 



(Fig. 52) teach us the following facts: (1) The auricular contraction 

 is less sudden than the ventricular, and lasts only a very short time, 

 The ventricle, on the other hand, contracts suddenly and forcibly, 

 and remains contracted a considerable time, as shown respectively 

 by the ascent of the curve and the flat top of the curve which succeeds. 



(2) The auricular movement precedes the ventricular, and the latter 

 coincides with the impulse of the apex against the wall of the chest. 



(3) The contraction of the auricle influences the pressure in the ven- 

 tricle, as shown by the small rise a b', and that of the ventricle influ- 

 ences the pressure in the auricle somewhat, as shown by the wave c' d'. 



A tracing of aortic and intra ventricular pressure curves is given in 

 Fig. 54. The beginning of the aortic pulse curve (1) obviously marks 

 the opening of the semilunar valves, the dicrotic notch (4) follows their 

 closure. The moment of closure (3) can be ascertained by listening to 

 the second sound of the heart, which occurs immediately after the 

 closure, and is produced by the sudden tension of the closed valves. It 



FIG. 54. AORTIC AND I NTRAVENTRICULAR PRESSURE CURVES. (Hiirthle.) 



Time trace = T ^ Tr sec.; 0-1, period of rising tension; 1-4, period of output; 4, dicrotic 



notch. 



can also be determined by connecting the two-way sound to a differential 

 manometer, and recording the difference of pressure in the ventricle 

 and aorta. The valves open at the moment when the ventricular 

 pressure exceeds the aortic, and close when the aortic exceeds the 

 ventricular pressure. If the differential manometer be used, with 

 one sound opening in the right auricle and the other in the right 

 ventricle, the tracing indicates the moment when the auriculo-ven- 

 tricular valves close and open. 



The absence of a mechanism for preventing regurgitation of blood 

 from the auricles of birds and mammals is remarkable, for in fishes, 

 amphibia, and reptiles, this is effected by valves guarding the sinu- 

 auricular junction (Fig. 2 B). In the warm-blooded vertebrates, with the 

 appearance of the diaphragm and the fusion of the sinus venosus with 

 the right auricle, the venous cistern formed by the superior and inferior 

 venae cavae, the innominate, iliac, hepatic, and renal veins takes the 

 place of the sinus. Six pairs of valves prevent regurgitation from this. 



