VASCULAR SYSTEM OF THE RABBIT. 559 



the thoracic cavity between the lungs. It is surrounded by 

 a thin pericardium, and immediately in front of it there lies 

 the soft thymus, which is larger in the young than in the 

 adult animal. 



By two superior venee cavse, and by the inferior vena cava, 

 the venous blood collected from the body enters the right 

 auricle. Thence the blood passes into the right ventricle 

 through a crescentic opening, bordered by a threefold (tricus- 

 pid) membranous valve (worked by chordae tendineae attached 

 to papillary muscles projecting from the wall of the ventricle). 



The right ventricle is not so muscular as the left, which 

 it partly surrounds. By its contraction the blood is driven 

 into the pulmonary trunk, whose orifice is guarded by three 

 semilunar valves. During contraction, the tricuspid valves 

 are pressed together, so that no regurgitation into the right 

 auricle can take place. 



The pulmonary trunk divides into two pulmonary arteries, 

 which divide into capillaries on the walls of the lungs. 

 There the red blood-corpuscles gain oxygen, and the blood is 

 freed from much of the carbonic acid gas which it has borne 

 away from the tissues. The purified blood returns to the 

 heart by two pulmonary veins, which unite as they enter the 

 left auricle. 



From the left auricle, the pure blood passes into the left 

 ventricle through a funnel-like opening, bordered by a 

 (mitral) valve with two membranous flaps, with chordae 

 tendineae and musculi papiUares as on the right side, but 

 the muscles here are larger. 



The left ventricle receives the pure blood and drives it to 

 the body. During contraction, the mitral valve is closed, so 

 that no blood can flow back into the auricle. The blood 

 leaves the left ventricle by an aortic trunk, whose base is 

 guarded by three semilunar valves, just above which coronary 

 arteries arise from the aortic trunk and supply the heart 

 itself. 



The aortic trunk bends over to the left, and passes back- 

 ward under the backbone, dividing near the pelvis into two 

 common iliac arteries, which supply the hind-legs and pos- 

 terior parts. The arteries given off near the heart and in 

 the abdominal region may be grouped as follows : — 



