THE CIRCULATION OF THE BLOOD. 109 
blood from the lungs, and propel it 
over the body. The left ventricle 
has more to do than any other cavi- 
ty. The two auricles contract at the 
same instant; so, also, do the ventri- 
cles. The sounds which they respect- 
ively make may be imitated by the 
words lubd, tup. 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” 
The ventricle then contracts, and the 
blood rushes through the pulmonary 
artery past its semilunar valves into 
into the right ventricle. 
Fig. 74.—Plan of Circula- 
tion in Fishes: a, auri- 
cle; b, ventricle; ¢c, pul- 
monary artery; é, pul- 
monary veins, bringing 
blood from the gills, d, 
and uniting in the aorta, 
JF; g, vena cava. 
the lungs, where it is changed from blue to crimson, re- 
turning by the Ste vein to the left auricle. This 
Fries. 75, 76.—A, Plan of Cirenlation in Amphibia 
and Reptiles : B, Plan of Circulation in Birds 
sends it past the tmi- 
tral valves into the left 
ventricle, which drives 
it by the semilunar 
valves into the aorta, 
and thence, by its ram- 
ifying arteries and cap- 
illaries, into all parts 
of the body except the 
lungs. From the cap- 
illaries, the blood, now 
changed from crimson 
to blue, is gathered by 
and Mammals: a, right auricle receiving venous {])\e veins, and conveyed 
blood from the system; }, left auricle receiving 
arterial blood from the lungs: ¢, e’, ventricles; back to the heart. 
d, e, f, systemic artery, vein, and capillaries; g, 
h, k, pulmonary artery, vein, and capillaries. The Rate of the 
