688 MAMMALIA. 



The pulmonary trunk divides into two pulmonary arteries, 

 which branch into capillaries on the walls of the lungs. There 

 the red blood corpuscles gain oxygen, and the blood is freed 

 from much 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 chordse 

 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 

 aiteries arise from the aortic trunk and supply the heart itself. 



The aortic trunk bends over to the left, and passes back- 

 ward under the backbone, dividing near the pelvis into two 

 common iliac arteries, which supply the hind-legs and pos- 

 terior 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 mammary 

 to the ventral wall of the thorax ; 

 (/>) 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 spermatic or ovarian arteries to the reproductive organs, 

 the posterior mesenteric to the rectum, 

 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 the 



