THE CIRCULATORY SYSTEM 



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cavities of the heart are lined with a glistening membrane, the 

 endocardium, which is continuous with the lining of the arteries 

 and veins. 



Arteries are the tubes that carry the blood from the heart to 

 the tissues. Their walls are thick, strong, and contain much 

 yellow elastic tissue which renders them extensible. When 

 empty they do not collapse. Most arteries occupy protected 

 situations and are straight in their course. When a vessel has 

 to accommodate itself to the movements or contour of a part it 

 makes a bend. Arteries communicate freely with one another, 

 thus promoting equality of distribution and pressure and making 



Fig. 42. — Diagram of the circulation: L, Pulmonary circulation; H. and F.L., 

 circulation through the head and fore limb; M, mesenteric circulation; P, portal 

 circulation; K, renal circulation; B.M., circulation through the body muscles; 

 P.C. and HXi., pelvic cavity and hind limb; A.y.c, anterior vena cava; p.v.c. 

 posterior vena cava; P.v., pulmonary vein; A., common aorta; T.A., thoracic aorta ; 

 A.A., abdominal aorta; b.a., right auricle; l.a., left auricle; b.v., right ventricle; 

 L.v., left ventricle. The shaded vessels carry arterial blood, the others venous 

 blood. 



good circulation possible even after a large vessel becomes 

 obliterated. A single larger vessel, the pulmonary artery, takes 

 origin from the right ventricle, and another, the aorta, from the 

 left ventricle. 



The pulmonary artery carries dark or venous blood from the 

 right ventricle to the lungs, where it divides into numerous small 

 vessels that ramify in the lung tissue. 



The aorta gives passage to the red blood from the left ventricle 

 (Pig. 42) that supplies all the organs of the body except the lungs. 

 It is a short vessel that soon divides into thoracic and abdominal 

 parts. Its walls are much thicker than those of the pulmonary 

 artery, as the blood is under greater pressure. 



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