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 6i 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 auricle, lhis sends it past 
^,n^n g b “ the mitral valves into the left ventri- 
from the gills, d, and c i e 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 systemic cap¬ 
illaries, the 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, 
Vinwpvpr if ia nmnnanQ Fig. 76.—A, Plan of Circulation in Amphibia and 
now ever, It is compara- Reptiles; B> P]an of Circulation in Birds and 
tively slow I but this is Mammals: a, right auricle receiving venous 
. . blood from the system ; 6, left auricle receiving 
because the air is taken arterial blood from the lungs; c, c', ventricles; 
, ,r i I -i ,] •. , d, e,f, systemic artery, vein, and capillaries; g, 
tO tne DlOGQ tUe WllOle pulmouary artery; h, k, vein and capillaries. 
