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meet together when the ventricle contracts, thus closing 

 the opening. 



The blood is driven to the lungs through the pulmonary 

 artery ; it arises from the right ventricle anteriorly, and 

 its opening is guarded by three semilunar valves. The 

 blood (oxygenated) returns from the lungs by two pul- 

 monary veins, which together enter the left auricle dorsally. 

 The large left auriculo -ventricular opening is guarded by 

 the bicuspid or mitral valve ; it consists of two membranous 

 lobes attached by chordae tendineae to musculi papillares 

 on the ventricle wall. The mitral valve is closed when the 

 left ventricle contracts ; and the pure blood is driven to 

 the body through the aorta, which arises anteriorly. The 

 aorta opening is guarded by three semilunar valves ; just 

 beyond these the coronary arteries, which supply the heart, 

 are given off. The aortic trunk bends to the left (aortic 

 arch) and passes backwards, close to the backbone, as the 

 dorsal aorta. 



The following arteries arise from the aortic arch : 



1. The innominate, which divides into the right subclavian 

 and the right common carotid, and which gives off, at its 

 base, a left common carotid. 



2. The left subclavian. 



The following arteries are branches of the dorsal aorta : 



1. The coeliac. 



2. The anterior mesenteric. 



3. Two renals. 



4. Two spermatics, or two ovarians. 



5. The posterior mesenteric. 



6. The median sacral or caudal (the posterior continuation 

 of the dorsal aorta). 



7. Two common iliacs, which give off internal iliacs and 

 are continued into the hind-limbs as femorals. 



The principal veins are : 



1. The right superior vena cava (=duct of Cuvier), formed 

 by union of external jugular and subclavian, and receiving 

 a small internal jugular (=anterior cardinal sinus) and the 

 azygos vein (=front portion of posterior cardinal sinus). 



2. The left superior vena cava is formed as above, but 

 does not receive an azygos vein. 



3. The inferior vena cava receives the hepatics, two 

 renals, two spermatics or two ovarians, two external iliacs 



s 



