110 
Elementary Manual of Zoology. 
of the body. These large veins can be made out without 
much difficulty, passing down along the inner side of each 
lung. The right anterior vena cava passes directly into 
the right auricle in front, while the left anterior vena cava 
has to cross the heart behind and enters the right auricle 
on the left side. 
(2) The posterior vena cava. This large vein returns the blood 
from all the posterior parts of the body. It can readily 
be seen, on turning up the base of the heart, where it en- 
ters the posterior end of the right auricle. It traverses 
the liver and is again visible in the posterior part of the 
abdominal cavity on the right side. Its chief branches are 
the hepatic veins, which return the blood from the liver, 
the renal veins, which return the blood from the kidneys 
and the iliac and lumbar veins, which return the blood from 
the posterior extremities, 
(3) The pulmonary artery. This isa somewhat smaller thick-wall- 
ed vessel. It arises from the anterior end of the right 
ventricle and is the channel through which the blood is 
driven into the lungs. On pulling back the heart with 
the finger and thumb it can easily be seen where it leaves 
the right ventricle, between the two auricles, and arches 
over behind the left auricle before branching to the lungs. 
(4) The pulmonary veins. These veins return the blood from the 
lungs to the left auricle. Turn up the heart from behind 
and make out the two branches from the lungs, which - 
unite before passing with the dorsal portion of the left 
auricle, 
(5) The aorta. This thick-walled vessel arises from the left ven- 
tricle just behind the origin of the pulmonary artery. It 
is the channel through which the blood is pumped by the 
left ventricle. Trace it forwards as far as possible and see 
where it arches over to the left, after giving off the large 
vessels (innominate and left sub-clavian arteries) which 
supply the head and fore-limb. By pulling the heart back 
with the finger and thumb the three branches (left carotid, 
right sub-clavian and right carotid) into which the innomi- 
nate artery breaks up can easily be made out. Notice the’ 
ligamentous band (ductus arteriosus) which connects the 
arch of the aorta with the pulmonary artery, and which 
represents a channel of communication between these two 
vessels through which blood passes during: embryonic life, 
On pulling back the left lung the continuation of the 
aorta can be seen where it passes down the dorsal portion 
