CIRCULATION AND RESPIRATION 



123 



sinus venosus, which communicates with the right atrium. The 

 right and left atria are completely separated by an interatrial 

 septum. The right atrium is larger than the left (Fig. 76). 



The cavity of the ventricle is a single chamber but its thick 

 muscular wall is indented by deep depressions which form 

 numerous alcoves. The atria, on the other hand, have smooth 

 thin walls. The blood which enters the right atrium from the 

 sinus venosus is deoxygenated blood from the systemic circulation. 

 Backflow of blood into the sinus venosus when the heart con- 



A.V.C. 



A B 



Fig. 76. — Heart of Rana catesbeiana. A, dissection from ventral side; B, 

 dorsal view, a.v.c, anterior vena cava; a.v.v., atrioventricular valve (dorsal); 

 B.C., bulbus cordis (proximal segment of truncus arteriosus); i.a.s., interatrial 

 septum; l.a., left atrium; l.v., longitudinal valve; P.C., postcava; p.v., pulmonary 

 veins; r.a., right atrium; s.v., sinus venosus; T.I., truncus impar (distal segment 

 of truncus arteriosus) ; v, ventricle. 



tracts is prevented by a pair of transverse lips or valves guarding 

 the sinu-atrial aperture. The left atrium receives oxygenated 

 blood from the lungs brought to it by the 'pulmonary veins, which 

 enter the atrium at an acute angle so that when the atrium con- 

 tracts, the mouth of the venous aperture is closed. Both atria 

 open into the ventricle through the atrioventricular aperture, 

 which, however, is divided into right and left portions by the 

 interatrial septum (Fig. 76). The atrioventricular aperture is 

 guarded by four valves, viz., a large flap on the dorsal edge, a 

 similar one on the ventral edge, and two smaller valves, one at 

 the right and one at the left edge of the aperture. The free 



