58 



EVENTS DURING A CARDIAC CYCLE. 



(a) A slight stagnation of the blood in the large venous trunks, as can be 

 observed in a rabbit after division of the pectoral muscles so as to expose the 

 junction of the jugular with the subclavian vein. There is no actual regurgitation 

 of the blood, but only a partial interruption of the inflow into the auricles, because 

 the mouths of the veins are contracted, and the pressure in the superior vena cava 

 and pulmonary veins soon holds in equilibrium any reflux of blood ; and lastly, 

 because any reflux into the cardiac veins is prevented by valves. The movement 



B 



Fig. 32. 

 Cast of the ventricles of the human heart viewed from behind and above ; the walls have been 

 removed, and only the fibrous rings and the auriculo-ventricular valves are retained. L, 

 left, R, right ventricle ; S, septum ; F, left fibrous ring, with mitral valve closed ; D, 

 right fibrous ring, with tricuspid closed ; A, aorta, with the left (C L ) and right (C) coronary 

 arteries ; S, sinus of Valsalva ; P, pulmonary artery. 



of the heart causes a regular pulsatile phenomenon in the blood of the venae cavse, 

 which under abnormal circumstances may produce a venous pulse (see 99). 



(6) The chief motor effect of the contraction of the auricles is the dilatation of 

 tlie relaxed ventricle, which has already been dilated to a slight extent by the 

 elastic traction of the lungs. 



Aspiration of the Ventricles. The dilatation of the ventricles has been ascribed to the 

 elasticity of the muscular walls the strongly contracted ventricular walls (like a compressed 

 india-rubber bag), in virtue of their elasticity, are supposed, in returning to their normal resting 

 form, to suck in or aspirate the blood under a negative pressure ; this power on the part of the 

 ventricle is not great (p. 59). 



