148 A TEXTBOOK OF PHYSIOLOGY 



contents and forces blood from the liver and cistern into the heart. 

 These forces take the place of the sinus venosus, and are far more 

 efficient. The intra-abdominal pressure may be raised considerably 

 on bending or straining. Under such conditions, the pericardium 

 protects the right side of the heart from being over -distended with 

 venous blood. 



With these facts in view, we can now describe the complete course 

 of a cardiac cycle (Fig. 56). We will start at the moment when both 

 chambers of the heart are in diastole. The blood pours from the vena 

 cava and pulmonary veins into the two auricles. These are relaxed, and 

 their cavities open into the ventricles by the funnel-shaped apertures 

 formed by the dependent segments of the tricuspid and mitral valves. 

 The blood passes freely through these apertures into the ventricles. 

 The small positive pressure which is always present in the venous 

 cistern (aided by the respiratory forces) is at this time filling both 

 chambers of the right heart, while the positive pressure in the pul- 

 monary veins is filling those of the left heart. The auricular systole 

 now takes place. Their circular muscle bands compress the blood out 

 of the auricles into the ventricles, while the longitudinal bands aid 

 in this, and also pull up the base of the ventricles to meet the load of 

 blood (Fig. 58). As the contraction starts from the mouths of the 

 vena cava, and sweeps towards the ventricles, little or no regurgita- 

 tion of blood occurs into the venous cistern under normal conditions, 

 but the cessation of flow into the auricle during its systole does produce 

 a slight rise of pressure in the cistern, which is shown by the wave a 

 in tracings taken from the jugular pulse (see Fig. 116). The function 

 of the auricles is to rapidly complete the filling of the ventricles, and 

 thereby slightly distend its walls. Within normal limits, the greater 

 the distension the more forcible the contraction. 



The auriculo-ventricular valves are floated up and brought into 

 apposition by eddies set up in the blood which streams into the ven- 

 tricles, and close without noise or jar at the moment when the intra- 

 ventricular pressure in the least exceeds that in the auricles. The thin, 

 moist, film-like edges of the tricuspid and mitral valves of the heart 

 come into perfect apposition, and prevent all leakage, while the fibrous 

 parts give strength and support. Then follows ventricular systole 

 (A, Fig. 56). The contraction of the ventricular musculature around 

 these orifices limits the size of the auriculo-ventricular orifices, and 

 maintains the competency of the valves. By contracting synchron- 

 ously with the muscular wall of the ventricles, the papillary muscles, 

 with the aid of the chordae tendineae, pull down the diaphragm formed 

 by the closed auriculo-ventricular valves. As these form the floor of 

 the auricles, then: cavities are thereby expanded. At the same time, 

 the papillary muscles shorten the longitudinal diameter of the ventricles, 

 and enable the valvular and muscular parts of the ventricles to approach 

 together and squeeze out the blood from the ventricles. By the 

 attachment of the chordae tendineae to the auriculo-ventricular valves, 

 and the action of the papillary muscles, the membranous diaphragm 

 formed by the valves is made to act as part of the muscular wall of 



