714 MAMMALIA. 
of the carbonic acid gas which it has borne away from the 
tissues. The purified blood returns to the heart by two 
pulmonary veins, which unite as they enter the left auricle. 
From the left auricle the pure blood passes into the left 
ventricle through a funnel-like opening, bordered by a 
(mitral) valve with two membranous flaps, with chordz 
tendineze and musculi papillares as on the right side, but 
the muscles here are larger. 
The left ventricle receives the pure blood and drives it 
to the body. During contraction the mitral valve is closed, 
so that no blood can flow back into the auricle. The 
blood leaves the left ventricle by an aortic trunk, whose 
base is guarded by three semilunar valves, just above 
which coronary arteries arise from the aortic trunk and 
supply the heart itself. 
The aortic trunk bends over to the left, and passes 
backward under the backbone, dividing near the pelvis 
into two common iliac arteries, which supply the hind- 
legs and posterior parts. The chief blood vessels may be 
grouped as follows :— 
The aortic trunk 
gives off the innominate artery, 
which divides into (a) the right subclavian, continued as the 
brachial to the fore-limb, but giving 
off the vertebral to the spinal cord 
and brain, and the internal mam- 
mary to the ventral wall of the 
thorax ; 
(4) the right carotid, running along the 
trachea, dividing into the right 
internal carotid to the brain, and 
the right external carotid to the 
head and face ; 
(c) the left carotid, with a similar course ; 
thereafter the aorta gives off— 
the left subclavian artery, which branches like the right ; 
the cceliac artery to the liver, stomach, and spleen ; 
the anterior mesenteric to the pancreas and intestine ; 
the renal arteries to the kidneys ; 
the single posterior mesenteric to the rectum ; 
the paired spermatic or ovarian arteries to the reproductive 
organs ; 
the lumbar arteries to the posterior body walls. 
The aorta is continued terminally in the median sacral artery to the 
tail, and laterally in the common iliacs, which form the femorals of 
