THE HEART 37 



ceases to stream through the narrow crevice. In this way there 

 is no regurgitation, as the valves are closed before the recoil of 

 the aorta. If this explanation be correct, the second sound of 

 the heart must be considered as due to the sudden tension, and 

 not the closure, of the aortic valves at the time of the aortic 

 recoil. 



It is of the utmost importance to bear in mind that the force 

 of the aortic reflex is not wholly expended on the valves, but 

 largely on the muscular tissue of the ventricle, which here, as the 

 result of the orifice contracting, forms a large circular pad. In 

 order to admit of this strain coming on the heart wall itself, the 

 diameter of the aorta is much greater than the opening out of the 

 ventricle. 



Movements of the Heart. — If the exposed mammalian heart be 

 watched at work, a great deal may be learnt of its action. It 

 will be observed that both auricles contract together and both 

 ventricles together ; further, that certain changes in shape occur. 

 The contraction of either auricle or ventricle is spoken of as its 

 systole, while the subsequent relaxation is described as its diastole. 

 The contraction of the ventricles is succeeded by a pause, during 

 which the heart is in a state of relaxation. 



A Cardiac Cycle is the term used to describe the changes which 

 occur in the heart during the time which elapses between one 

 contraction or relaxation of the auricle, and the one which 

 immediately succeeds it. 



We may take the moment when the blood is entering the 

 auricles from the venae cavae and pulmonary veins as the most 

 convenient point to start from. This flow is brought about by 

 the pressure of blood in the veins, which, though low, is yet higher 

 than that in the auricles. The influence of gravity is also a 

 great aid to filling the auricles, for, speaking broadly, excepting 

 in the limbs, the bulk of the veins in the body of quadrupeds 

 are above the heart. The emptying of the anterior and posterior 

 vena cava is largely assisted by gravity ; even in the latter there 

 is a downward incline in the vessel from the last valves met with 

 in the iliac veins to the right auricle. Besides this factor there 

 is the contraction of the abdominal wall and consequent pressure 

 on the viscera, so that at each expiration the blood is spurted 

 along the posterior vena cava towards the auricle. Finally, 

 there is an aspiration in the veins produced by the auricle, and 

 caused by a relaxation of its walls after the previous contraction. 

 There is also an aspiration in the thorax, the result of inspiration, 

 which produces a negative pressure in the veins leading to the 

 heart, and so draws blood towards the heart from the veins lying 

 outside the thorax. 



By a combination of these means the auricles are filled with 



