MOVEMENTS OF THE HEART 



39 



presents no material difference from the tricuspid valve, except that it is 

 divided into two curtains instead of three. 



Aortic Valves. The three aortic semilunar valves are the 'same in 

 their arrangement as the pulmonic valves. They prevent regurgitation 

 from the aorta into the left ventricle. 



MOVEMENTS OF THE HEART 



Dilatation of the cavities of the heart is called diastole, and con- 

 traction, systole. A complete revolution, or cycle of the heart consists 

 in the filling and emptying of all its cavities, during which they pre- 

 sent alternations of rest and contraction. As these occupy less than 

 one second, it is evident that their exact relations to each other require 

 careful study. 



Except during the short time occupied in contraction of the auricles, 

 these cavities are constantly receiving blood, 6n the right side from the 

 venae cavae and on the left side from the pulmonary veins. When the 

 auricles have become fully distended, they contract and send the blood 

 on the right side into the right ventricle and on the left side into the 

 left ventricle. During this contraction, the blood ceases to flow from 

 the veins into the auricles, and a small quantity is regurgitated, as 

 the openings are not provided with valves ; but the arrangement of the 

 muscular fibres of the auricles around the openings of the veins limits 

 this regurgitation, and the greater part of the blood is forced into the 

 ventricles. 



Immediately following contraction of the auricles, there is contraction 

 of the ventricles. This is the chief active operation of the heart, and it 

 usually is spoken of as the systole. Regurgitation of blood, during con- 

 traction of the ventricles, is prevented by closure of the auriculo-ventric- 

 ular valves. This act accomplished, the heart has a period of rest, the 

 blood flowing into the auricles and from them very slowly into the ven- 

 tricles, until the auricles are again completely filled, when another cycle 

 of the heart begins. 



The position of the heart in the thoracic cavity is with its base 

 directed slightly to the right and its apex to the left. The movement 

 of the apex from left to right is a necessary consequence of the direc- 

 tion of the superficial fibres from right to left. The fibres on the ante- 

 rior surface are longer than the posterior fibres, and therefore the point 

 of the heart is moved upward and to the right during their contraction. 

 In the ventricular systole, the heart itself is propelled forward by the 

 sudden distention of the great vessels at the base, aided by the recoil of 

 the ventricles. By reason of the spiral course of the superficial fibres of 



