138 ADVANCED PHYSIOLOGY 



lower part; Fig. 74. If we begin our description at the rest 

 period of the heart, i.e. the period between any two beats, we 

 shall find that the blood flowing in these veins passes in a large 



Superior Vena, 

 - Coy a 

 Inferior Vena J * & 



Pulmonary Arfenf 



_. . . 

 Tntuspid 



Tricuspid Valve 



SemiLunar 

 to/re- Closed 



B 

 FIG. 74. DIAGRAM 



Showing the mechanism of the heart. At A is shown the right side of the heart 

 at the period of rest, and at B the arrangement of the valves when the heart 

 contracts. The arrows show the direction of the blood flow. 



stream directly into the right auricle (Fig. 74), whence it 

 flows freely through the wide opening from the auricle into the 

 ventricle. Thus the auricle and ventricle are both filling at 

 the same time. 



The opening between the right auricle and right ventricle 

 is guarded by three flap-like membranes, attached at the top 

 of the ventricle; when the ventricle is empty, they hang down 

 loosely (Fig. 74 A], but as the entering blood collects in the 

 bottom, these flaps float on its surface. This condition lasts 

 only for a fraction of a second, when the muscles in the walls 

 of the auricle contract, forcing all the blood into the ventricle 

 with a rush. Carrying the flaps on its surface, the blood rises 

 rapidly until the ventricle is completely filled. At this time 

 the valves are lifted up directly across the opening from the 

 auricle; Fig. 74 B. They are of such size and shape that 

 when in this position they exactly fill the opening, completely 



