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 chordae 

 tendineae 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 ; 



(b) 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 coeliac 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 



