THE FROG : VISCERA AND VASCULAR SYSTEM 53 



the ventricle, into which each opens. On the dorsal surface 

 of the heart, opening into the right auricle, lies the still 

 thinner walled, triangular sinus venosus. 



The openings between these chambers are guarded by 

 certain valves or folds of the lining of the heart. Two 

 simple lips of the opening between the sinus and right auricle 

 are the sinu-auricular valves ; these allow blood to flow into 

 the auricle, but fold over to prevent its reflux. The edge of 

 the auricular septum is cleft and projects into the ventricle 

 as two flaps, the auriculo-ventricular valves. Each of these 

 is connected with the 

 walls of the ventricle 

 by fine cords, the 

 chorda tendinem, so 

 that, while blood can 

 pass from auricles to 

 ventricle, its reflux is 

 prevented by its rais- 

 ing the valves, which 

 are kept from turning 

 back into the auricle 

 by the chordae ten- 

 dineae. The opening 

 from ventricle to 

 truncus is guarded by 

 three semilunar valves, 

 shaped like watch- 

 pockets, which are 

 spread out by the 

 blood when it tends to 

 flow backwards from 

 truncus to ventricle. 



The truncus arteriosus is divided internally by a second row 

 of semilunar valves into two unequal parts, a long conus 

 arteriosus or pylangium next the ventricle, and a short 

 ventral aorta or synangium. The conus is incompletely 

 divided longitudinally by a spiral valve into a cavum aorti- 

 cum, which begins dorsal and curves round by the right to 

 become ventral, and a cavum pulmocutaneum, which begins 

 ventral and curves round by the left to become dorsal. The 

 synangium is completely divided into a dorsal and a ventral 



FIG. 27. — The heart of a frog, removed 

 from the pericardium and seen from the 

 back with the sinus venosus opened. 



z.u.c, Inferior vena cava; p.v., pulmonary veins; 

 r.au.i Law., right and left auricles; s.au., 

 opening from sinus to right auricle ; s.v.c., 

 superior vena cava ; s.v., sinus venosus ; rV'., 

 branches of truncus cut across ; v., ventricle. 



