RESPIRATORY ORGANS 



447 



diffused from the alveoli into the capillaries combines with the 

 haemoglobin of the red cells and is carried to the tissues 

 (For respiratory regulation, see p. i8.) 



BLOOD vessels: heart 



The blood of the rabbit, like that of all vertebrates, is contained 

 in a closed system of vessels. The heart is the pumping organ, 

 vessels which take blood away from it are arteries, and those 



^s.-irc 



cto. CU. 



Fig. 354. — The heart of a rabbit, seen from the right side, after the removal ot 

 the outer wall of the right auricle and ventricle. 



ao.a., aortic arch ; ch.t., chordae tendine« ; col.c, columnae camcce if.ov., fossa ovalis, the site of an opening 

 through which in the embryo there passed into the left auricle blood returning to the heart by the 

 inferior vena cava, much of which came from the placenta and was therefore arterial (p. 668):' this 

 blood was directed by a fold of the auricular wall known as the Eustachian valve, Iving between the 

 openings of the left superior vena cava and the inferior vena cava, traces of which fold remain in the 

 adult heart ; i.v.c, inferior vena cava ; i.v.c'., internal opening of the same ; l.s.v.c'., internal opening of 

 left superior vena cava ; m.p., musculi papillares ; pul.a., pulmonary artery, cut open ; r.au., wall of 

 right auricle ; r.s.v.c, right superior vena cava ; r.s.v.d ., internal opening of the same ; si., semilunar 

 valve ; tr.v., tricuspid valve. 



which bring blood back to the heart are veins. Arteries break up 

 into smaller vessels called arterioles, and these in turn into very 

 fine capillaries, which form a network in nearly every organ. 

 Capillaries join together to give venules, and these in turn unite 

 to form veins. A vein which drains blood from one set of capillaries 

 and takes it to another set is, together with this second set, called 

 a portal system. 



The heart of the rabbit lies in the front part of the chest, 

 enclosed in the thin pericardium immediately behind the soft, pink 



