110 
COMPARATIVE ZOOLOGY. 
so also do the ventricles. The course 
of the current in Bipds 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. — Plan of circuia- returning by the pulmonary veins to 
cie• b, ventricle • c, bran- the left auncle. 1 his sends it past 
the mitral valves into the left ventri- 
from the gills, d, and c le, which drives it past the semilunar 
uniting in the aorta, /; g, 1 
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 CircnTation in Amphibia and 
Reptiles; B, Plan of Circulation in Birds and 
Mammals: a, right auricle receiving venons 
blood from the system; ft, left auricle receiving 
arterial blood from the lnngs; e, c', ventricles; 
d,e,/, systemic artery, vein, and capillaries; g, 
pulmonary artery; ft, k, vein and capillaries. 
