VASCULAR SYSTEM. 653 



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 

 tendinese 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 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 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 ; 

 (r) 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 

 hind-legs, and give off in the abdomen several branches to the abdominal 

 walls, the pelvic cavity, the bladder, and the uterus. 



