110 



COMPARATIVE ZOOLOGY. 



so also do the ventricles. The course 

 of the current in Birds and Mammals 

 is as follows : the venous blood 

 brought from the system is discharged 

 by two or three large trunks 61 into 

 the right auricle, which immediately 

 forces it past a valve 62 into the right 

 ventricle. The ventricle then con- 

 tracts, and the blood rushes through 

 the pulmonary artery past its semi- 

 lunar valves into the lungs, where it 

 is changed from venous to arterial, 

 fig. 75. — pian of circnia- returning by the pulmonary veins to 



tion in Fishes: a, auri- - , - . rn , . , 



cie. b, ventricle; c, bran- the lett auricle, lhis sends it past 

 ^ r Sn g b T.ood the mitral valves into the left ventri- 

 from the gills, d, and c ] e w hich drives it past the semilunar 



uniting in the aorta, /; g, L 



vena cava. valves into the aorta, and thence, by 



its ramifying arteries and capillaries, into all parts of the 

 body except the lungs. 

 From the capillaries, 

 the blood, now changed 

 from arterial to venous, 

 is gathered by the veins, 

 and conveyed back to 

 the heart. 



The Rate of the 

 Blood -current gener- 

 ally increases with the 

 activity of the animal, 

 being most rapid in 

 Birds. 63 In Insects, 

 however, it is compara- 

 tively slow; but this is 

 because the air is taken 

 to the blood — the whole 



Fig. 76. — A, Plan of Circulation in Amphibia and 

 Reptiles ; B, Plan of Circulation in Birds and 

 Mammals: a, right auricle receiving venous 

 blood from the system ; 6, left auricle receiving 

 arterial blood from the lungs; c, c\ ventricles ; 

 d, e,f, systemic artery, vein, and capillaries; g, 

 pulmonary artery; h, k, vein and capillaries. 



