THE CIRCULATORY SYSTEM 



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cavities of the heart are lined with a glistening, transparent mem- 

 brane, the endocardium. 



The arteries have walls with considerable elastic tissue in them 

 to allow expansion. They begin at the base of the left ventricle, 

 as the aorta, the large vessel through which all the blood for 'the 

 body passes (Fig. 39). For convenience the aorta is divided into 

 thoracic and abdominal parts. From the former the common 

 brachiocephalic artery arises close to the heart and is directed for- 

 ward and upward. Branches from it supply the fore limbs, neck, 

 and head. The abdominal part is termed the posterior aorta. 

 It supplies branches to the walls and viscera of the abdominal 



Fig. 39. — Diagram of the circulation: L, Pulmonary circulation; H. and 

 F.L., circulation through the head and fore hmb; M, mesenteric circulation; P, 

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

 muscles; P.O. and H. L., pelvic cavity and hind limb; a.v.c, anterior vena 

 cava; p.v.c, posterior vena cava; p.v., puhnonary vein; a, common aorta; 

 T.A., thoracic aorta; a. a., abdominal aorta; r.a., right auricle; l.a., left auricle; 

 K.V., right ventricle; l.v., left ventricle. The shaded vessels carry arterial 

 blood, the others venous blood. 



cavity, the body muscles, and the pelvic organs and limbs. The 

 work of the arteries is that of a pipe-line, to convey blood from the 

 heart to the various body organs. 



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 com- 

 municate freely with one another, thus promoting equality of 

 distribution and pressure and making good circulation possible 

 even after a large vessel becomes obliterated. 



The veins differ from the arteries in having much thinner walls. 

 This causes them to collapse when not filled with blood. In many 

 veins the internal coat is folded up to form valves, the free edges 



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