118 ANIMAL PHYSIOLOGY. 



but is shut up after birth. In the rare instances in which it 

 continues after birth to allow blood to pass through it, the 

 circulation of dark blood in the system is the result, con- 

 stituting the disease called cyanosis, and destroying life. 



Fig. 65. RIGHT SIDE OF THE HEART, a, b, Superior and inferior 

 vena cava entering the right auricle; c, Eustachian valve; d, 

 annulus ovalis; e, /, anterior and posterior cusps of the tricuspid 

 valve descending from the margin of the auriculo-ventricular 

 opening; g, pulmonary artery with its orifice shut by distension 

 of the three pouches of its semilunar valve; h, aorta. 



The right auricle receives blood in two streams nearly verti- 

 cally opposite one another, one from the vena cava superior, 

 bringing the blood from the head and upper limbs, the other 

 from the vena cava inferior, bringing the blood from the 

 lower limbs and greater part of the trunk; while, in addition, 

 the blood from the walls of the heart enters by oiie consider- 

 able and several smaller orifices. The left auricle receives 

 its blood by streams transversely opposite one another, 

 entering by the pulmonary veins from the right and left 

 lungs. i , 



86. The heart can be seen in action by laying open a frog, 

 but still more satisfactorily in the chest of a mammal. The 



