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PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



and elastic tubes coming off from the heart, which gradually subdi- 

 vide, like the trunk of a tree, into branches, and which serve to carry 

 oxygenated blood to the tissues; (3) the veins, another sj-stem of branch- 

 ing tubes, also elastic and muscular, but less so than the arteries, which 

 conduct to the heart the blood collected from (4) the capillaries a 

 system of fine tubes situated between the arteries and the veins. 



In man, as in other mammals, we have to deal with a double circu- 

 lation — the systemic circulation and the pulmonic. Their general out- 

 line may be given as follows : Starting from the heart, the arterial 

 blood leaves the left ventricle through the aorta, which immediately 

 gives off two vessels, the coronary arteries, for nourishing the tissues 



Fig. 192.— Diagram op Mammalian Heart. (Biclard.) 



a, left ventricle ; b, right ventricle : c, left auricle; d, right auricle ; f, aorta ; g g, pulmonary 

 arteries ; h, inferior vena cava ; i. superior vena cava : /.-. orifice of the superior vena cava ; I, orifice of 

 the inferior vena cava ; m, orifice of the coronary vein ; o, left pulmonary veins ; p, right pulmonary 

 veins ; r, orifices of the right pulmonary veins ; »,' orifice of the left pulmonary veins. 



of the heart itself; the aorta then divides into branches, which them- 

 selves become successively subdivided to supply arterial blood to the 

 head, trunk, limbs, and all the organs of the body, the vessels 

 becoming finally so minute as to allow merely the passage of a single 

 blood-corpuscle, forming then the so-called capillary vessels. From the 

 capillaries the blood is again collected by converging venous radicals, 

 which finally are united to form two main venous trunks— the vena 

 cava superior, bringing the blood from the head and upper extremities, 

 and the vena cava inferior, collecting the blood from the trunk and 

 lower limbs. Both of these large veins empty into the right auricle, com- 



