VASCULAR SYSTEM 317 



valve) to which are attached tendinous cords, 1 arising from the 

 papillary muscles. 



In Birds the left atrio-ventricular aperture is provided with a 

 valve consisting of three membranous folds : in Mammals there are 

 only two folds, and the valve is therefore known as the bicuspid or 

 mitral ; three semilunar pocket-like valves are also present? at the 

 origins of the pulmonary artery and aorta in both Birds and 

 Mammals. 



As regards the origin of the great vessels, Birds are distinguished 

 from Mammals by the fact that in them the right, while in Mammals 



Y.C.S.S. 



FIG. 258. FIG. 259. 



FIG. 258. TRANSVERSE SECTION THROUGH THE VENTRICLES OF Grits cinerea. 

 Vd, right, and Vy, left ventricle ; S, septum veiitriculorum. 



FIG. 259. HEART or Ornithorhynchm paradox u*. From the dorsal side. 



(After Rose. ) 



V.c.s.8, V.c.x.d, precaval veins ; V.c.i, postcaval ; V.c.c, coronary vein ; V.c.s.s, 

 coronary sinus ; L.v, pulmonary veins ; Ao, aorta ; /*.*, P.d, pulmonary 

 arteries ; 1?. V.L, right atrium ; S.p.i, Spatium intersepto-valvulare. 



the left aortic arch persists (Fig. 243, E, F) ; the corresponding arch 

 of the other side in both cases gives rise to part of the subclavian 

 artery. Thus in both Birds and Mammals there is only a single 

 aortic arch. As in Amphibians, the posterior arterial arch gives 

 rise to the pulmonary artery. The pulmonary veins, two from 

 each lung, open close together into the left atrium (Fig. 259). 



Amongst the more important points in the development of the 

 heart may be mentioned the fact that in the embryo the two atria 

 communicate with one another secondarily by means of the foramen 

 ovale, through which the blood from the postcaval passes into the 

 left ventricle (Fig. 260). This foramen closes up when the lungs 



1 There are no chordae tendinese in Monotremes, the heart of which in many 

 respects resembles that of the Sauropsida. 



