THE FROG ; VISCERA AND VASCULAR SYSTEM 55 



Thus the blood from the left auricle (and therefore from the 

 lungs), which is the last to enter the truncus, passes along 



Fig. 28. — A ventral view of the heart of a frog, opened to 

 show the internal structure. The ventral wall of the 

 truncus, ventricle, and auricles has been removed, with 

 part of the spiral valve (at the dotted line). 



au.7<., Auriculo- ventricular valves \c.a., carotid arch ;c.ao., cavum aorticum ; 

 c.pu. t cavum pulmocutaneum ; ch.t., chorda? tendineae ; l.au. , left auricle ; 

 o.p.v., opening of pulmonary vein ; o.^c, opening of dorsal division of 

 synangium, by which blood passes from the cavum pulmocutaneum to the 

 pulmocutaneous arch \pc.a., pulmocutaneous arch ; r.au., right auricle ; 

 s.du., sinu-auricular opening with valves ; j/. ( first row of semilunar 

 valves ; si'., semilunar valves of second row ; j/.i, the semilunar valve 

 from which the spiral valve starts ; sl'.z, small semilunar valve at 

 end of cavum pulmocutaneum ; sF.%, a small part of a large semi- 

 lunar valve, of which the rest extends across that portion of the 

 front wall of the truncus which has been removed; sp.v., spiral 

 valve; sy.a., systemic arch; tr.a. y wall of truncus arteriosus; tr'.t 

 one of the two bundles of arteries into which the truncus divides ; 

 v., ventricle. 



the cavum aorticum into the systemic and carotid arches. 

 The blood in the systemic arch is a mixture of that from 

 the right and left auricles ; the final portion which passes 



