AND PHYSIOLOGY. 



203 



than the Ventricles, or lower ones. The auricles also have 

 the thinnest walls, and are capable of considerable dilatation, 

 since by a sudden effort of the body the blood is liable to be 

 sent in great quantities to the heart, and the veins would be 

 in danger of rupture were there no elasticity in the receptacle. 

 The thickened walls of the ventricles give increased power of 



Fig. 205. 



Vena Cava Sup. Art. Tulrn. Aorta. Art. Pulm. 



Pulmonary Veins. ««:: 



Eight Auricle. — 



Tricuspid Valve. — 



Vena Cava Inferior. — 



Eight Ventricle. ' 



,._.«. Pulmonary Veins. 



Left Auricle. 

 Mitral Valve. 



Left Ventricle. 



Septum. Aorta. 

 Theoretical Section of the Heart in Man. 



contraction. This is needed because the ventricles drive the 

 blood from the heart, and the auricles receive it on its return. 

 The right ventricle, however, propels the blood only to the 

 lungs, while the left ventricle sends it to all parts of the body 

 except the lungs. The left auricle receives only the blood 

 from the lungs, while the right auricle receives it from all the 

 other parts of the body. 



368. Valves of the Heart, — Between the auricles and ven- 

 tricles are peculiar forms of muscular and tendinous fibers, 

 resembling cords and pillars, that are termed valves, making 

 a sort of curtain to allow the flow of blood from the auricles 

 to the ventricles, but not in the opposite direction. At the 



367. Give the essential differences between the auricles and ventricles. Why does the 

 left ventricle need the thickest walls ? 368. Describe the valves which lie between the 

 auricles and ventricles. 



