VASCULAR SYSTEM. 673 



(mitral) valve with two membranous flaps, with chordje 

 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 arteries given off near the heart and in 

 the abdominal region 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 oft^ 



the left subclavian artery, with 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 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. 



The Venous System. — The two superior venae cava; bring blood from 

 the head, neck, thorax, and fore limbs. Each is formed from the 

 union of 



a subclavian from the shoulder and fore limb, 

 an external jugular from the face and ear, 

 an internal jugular from the brain, 



43 



