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" into 

 the right auricle, which immediately 

 forces it past a valve " 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. Plan of circuia- returning by the pulmonary veins to 



tion in Fishes: o, auri- . , ,, . , mi . ", 



cle : b, ventricle; e, bran- the left auricle. IhlS SCIlds it past 



veinir^ringin^Tiood tne m ^ 1 '^ valves into the left veutri- 

 from the gills, d, and c ] 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 capillaries, 

 the blood, now changed 

 from arterial to venous, 

 is gathered by the veins, 

 and conveyed back to 

 the heart. 



The Bate of the 

 Blood -current gener- 

 ally increases with the 

 activity of the animal, 

 being most rapid in 

 Birds." In Insects, 

 however, it is compara- 

 tively slow hllf tin's ifi Mammals: a, right auricle receiving venous 



lively slow , put tins is b]ood from the gystem . ^ ]eft anric]e receivil)2 



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

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

 tO the blOOd the Whole pulmonary artery ; h, h, vein and capillaries. 



