THE FROG 



59 



The heart, then, may be regarded as the centre of the vascu- 

 lar system which, as we have seen, is a closed series of tubes. It 

 consists of five chambers, a sinus venosus, two auricles, a ventricle 

 and a conus arteriosus. As has already been noticed, it is situated 

 far forwards in the body in front of the main mass of the liver, and 

 ventral to the lungs. Around it is a more or less closely fitting bag, 

 the pericardium, composed of a white, glistening, semi-transparent 

 membrane that is reflected back and covers the heart very 



ven 



FIG. 19. A, The frog's heart dissected from the ventral surface. B, an 

 enlarged semi-diagrammatic view of the truncus arteriosus ; the ventral 

 wall has been cut through somewhat to the observer's right of the middle 

 line, and the walls have been turned back. From Bourne. 



aw., auriculo-ventricular valve, with its cordse tendineae ; c.ao., cavum aorticum of the 

 truncus arteriosus ; car., carotid artery ; c.pm., cavum pulmonale of the truncus arteriosus ; 

 l.au., left auricle ; p.ca., opening of the pulmo-cutaneous arches into the cavum pulmonale ; 

 pmc., pulmo-cutaneous artery ; pv., opening of the pulmonary vein into the left auricle ; r.au., 

 right auricle ; r.sa., opening of the right systemic artery ; in B a rod is passed up the opening and 

 projects from the cut end of the middle or systemic channel of the right aortic arch ; s.m., septum 

 medium of the upper part of the truncus ; s.p., septum principale ; s/>.t>., spiral valve, in B the 

 reference line points to the surface of attachment to the ventral wall of the truncus which has been 

 cut through ; sv., opening of the sinus venosus into the right auricle ; sys., systemic artery ; v. 1 , 

 proximal row of semi-lunar valves guarding the passage from the ventricle into the truncus ; 

 v. z , distal row of semi-lunar valves in the truncus ; ten., ventricle. 



intimately. Thus the heart comes to lie in a specially separated 

 part of the body cavity. 



The sinus venosus is a dark-coloured thin-walled sac lying on 

 the dorsal side of the heart ; it is triangular in shape, with the apex 

 pointing backwards and into its three corners open the three main 

 veins of the body, the Venae cavae or Caval veins. It opens into the 

 right auricle by a fairly large transverse slit whose edges are guarded 

 by valves, the sinu-auricular valves, which allow the blood to pass 

 from sinus to auricle, but not in the reverse direction. 



