8o6 PHYLUM CHORDATA : CLASS MAMMALIA 



the venous blood collected from the body enters the right 

 auricle. Thence the blood passes into the right ven- 

 tricle through a crescentic opening, bordered by a three- 

 fold (tricuspid) 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 regurgita- 

 tion into the right auricle can take place. 



The pulmonary trunk divides into two pulmonary 

 arteries, which branch 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 papillares 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 

 backward under the backbone, dividing near the pelvis 

 into two common iliac arteries, which supply the hind- 

 legs and posterior parts. The chief blood vessels may be 

 grouped as follows : — 



The aortic trunk gives off the iuiioniinate artery, 



wiiicli divides into (a) the right subclavian, continued as the 



brachial to the fore-limb, but giving 

 off the vertebral to the spinal cord and 

 brain, and the internal mammary to 

 the ventral wall of the thorax ; 



