VASCULAR SYSTEM 



369 



VASCULAR SYSTEM : HEART 



The heart of the frog (Figs. 286, 287), is a hollow, conical, 

 muscular organ, which lies, with the apex pointing backwards, in 

 the body cavity, between the breast-bone and the gullet. It is 

 enclosed in a thin sac, the pericardium, whose cavity is a part of 

 the body cavity (coelom) separated from the rest during develop- 

 ment, the heart having the same relation to it that the alimentary 

 canal has to the pleuroperitoneal or general body cavity — that is, 



r.av 



tr.a. 



ty. 



(>.v. 



s.v. 



.1 pm. 



r.au 

 s.v.c 



-s.au. 



Fig. 



286. — The heart of a frog, seen 

 from the ventral side. 



c.a., Carotid arch ; eg/., carotid gland ; i.e., 

 internal carotid artery ; La., lingual artery ; 

 pea., pulmocutaneous arch ; pm., peri- 

 cardium ; r.au., l.au., right and left auricles; 

 s.v.c, superior vena cava ; sy.a., systemic 

 arch ; tr.a., truncus arteriosus ; v., ventricle. 



Fig. 287. — The heart of a frog, 

 removed from the pericardium 

 and seen from the back with the 

 sinus venosus opened. 



i.7'.c., Inferior vena cava ; p.v., pulmonary 

 veins ; r.au., l.au., right and left auricles ; 

 s.au., opening from sinus to right auricle; 

 s.v.c, superior vena cava ; s.v., sinus 

 venosus ; tr'., branches of truncus cut 

 across : v.. ventricle. 



being covered by a continuation of the pericardial membrane 

 as the gut is by the peritoneum. The heart contains five chambers. 

 Of these the most conspicuous is the ventricle, a large, conical 

 structure, with thick, muscular walls, with projections called 

 columnse carnese ; from it arises in front, on the right side of 

 the ventral surface, the much smaller tubular truncus arteriosus. 

 The right and left auricles or atria are relatively thin-walled 

 chambers, the right larger than the left, separated by a septum 

 and lying in front of 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 ; into the 

 left auricle opens the pulmonary vein (p. 374) 



