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 M 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, 

 PIG. 75. _ plan of circuia- returning by the pulmonary veins to 



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



cle- b, ventricle- e bran- the left auricle. llllS Sends it past 



: rt S g ';,r^ the mitral valves into the left ventri- 

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



uniting in the aorta, /; g, ' 



veuacava. valves into the aorta, and thence, by 



its ramifying arteries and capillaries, into all parts of the 

 body except the lungs. 

 From the systemic cap- 

 illaries, the blood, now 

 changed from arterial 

 to venous, is gathered 

 by the veins, and con- 

 veyed back to the heart. 

 The Bate of the 

 Blood - current gener- 

 ally increases with the 

 activity of the animal, 

 being most rapid in 

 Birds. 83 In Insects, 



hoWPVPr it i<* pntnnara ^i- 76. A, Plan of Circulation in Amphibia and 



er, 11 mpara- Reptileg . B| Plan of circulation in Birds and 



tively Slow: but this is Mammals: a, right auricle receiving venous 



blood from the system : b, left auricle receiving 



because the air IS taken arterial blood from the lungs; c, c', ventricles, 



1, "Ul J i T_ i d, c,/, systemic artery, vein, and capillaries; g t 



tO tne WOOd the Whole pulmonary artery ; h, *, vein and capillaries. 



