156 



THE CIRCULATION OF THE BLOOD 



The position of the valves relative to the events of the cardiac cycle is 

 shown in Fig. 36. 



The Venous Reservoir of the Mammalian Heart 



In birds and mammals there are no valves between the venae cavae and 

 the auricles such as there are in lower animals at the sino-auricular junc- 

 tion. Coincident with the appearance of the diaphragm, the sinus is 

 merged with the auricles which are left open to a large venous cistern 

 consisting of the venae cavae, the innominate, iliac, hepatic and renal veins. 

 This cistern is shut off from the remainder of the venous system by six 

 pairs of valves at the femoral, the subclavian and the jugular veins. Not 

 only is it capacious, being capable of holding at least 400 c.c. of blood in 

 man, but it is subject to considerable alteration in capacity because of the 

 collapsible nature of its walls. This cistern is moreover in free communi- 

 cation with another large cistern, capable of holding when full about 1000 



Fig. 36. Diagram to show the positions of the cardiac valves: 1, during diastole; 2, during 

 the presphygmic period; 3, during the sphygmic period. 



c.c. of blood, represented by the liver and portal system. Alterations in 

 thoracic and abdominal pressures will greatly affect the emptying of this 

 cistern into the auricles which explains the marked influence of the tho- 

 racic movements and of abdominal compression on the filling of the heart. 

 A most important function of this cistern is to furnish an immediate re- 

 serve of blood to fill the heart during the performance of a great muscular 

 effort. 



THE HEART SOUNDS 



During certain phases of the cycle distinct sounds, the heart sounds, 

 can be heard by applying a stethoscope to the thoracic wall. The first 

 occurs at the beginning of ventricular systole and is best heard over the 

 apex beat; the second occurs at the beginning of diastole and is heard 

 best at the second right costal cartilage or in the second left intercostal 



