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 . — Plan of circuia- returning by the pulmonary veins to 
tion in Fishes: a, auri- - , ^ rrn . , 
cle• b, ventricle; c, bran- the left aill’lCle. IhlS Sends it past 
the mitral valves into the left ventri- 
from the gills, d, and c l e which drives it past the semilunar 
uniting in the aorta, f; 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 systemic cap¬ 
illaries, .^e blood, now 
changed from arterial 
to venous, is gathered 
by the veins, and con¬ 
veyed 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 anr'< 
Reptiles; B, Plan of Circulation in Birds ana 
Mammals: a, right auricle receiving venous 
blood from the system ; b , left auricle receiving 
arterial blood from the lungs; c, c\ ventricles. 
d, e,f, systemic artery, vein, and capillaries; < 7 , 
pulmonary artery; h, fc, vein and capillaries. 
