1 8 ELEMENTARY BIOLOGY. [CHAP. 



into the right auricle. Into the left auricle, the common 

 pulmonary vein, formed by the junction of the veins from 

 the right and left lungs, opens. 



At its posterior end the atrium opens by the auriculo- 

 ventricular aperture into the ventricle. 



A small valve, prevented from flapping back by fine ten- 

 dinous cords, exists on each side of this aperture, and the 

 septum of the auricles is continued back upon the faces of 

 these valves and ends by a free edge between them, thus 

 dividing the auriculo-ventricular aperture itself into two 

 openings. 



The walls of the sinus and of the atrium are very thin. 

 Those of the ventricle, on the other hand, are thick and 

 spong)', only a comparatively small, transversely elongated, 

 cavity being left at its anterior end or base. At the 

 right-hand extremity of this is the aperture which leads 

 into the truncus arteriosus. Three semilunar valves, which 

 open from the ventricle into the truncus, surround this 

 opening. 



The walls of the iriuicus arteriosus are thick and mus- 

 cular, though not nearly so thick as those of the ventricle. 

 At its anterior end it appears to divide into two trunks, 

 which diverge and immediately leave the pericardium to 

 pass on to the sides of the gullet. The elongated undi- 

 vided part is the pylangium, the terminal part common to 

 the divergent trunks is the synangium. The former is 

 divided throughout its length by a sort of fold which is 

 attached to the dorsal wall while its opposite edge is free. 

 Three semilunar valves separate the pylangium from the 

 synangium, in wliich are the openings, posteriorly, of the 

 pulmonary arteries, anteriorly of the carotid trujiks ; while, 

 at the sides, the cavity of the synangium opens into those 

 of the right and left aortic arches. The apparently simple 



