ANATOMY OF THE FROG 



47 



guarded, on its ventricular side, by a pair of valves which have 

 the form of flaps of membrane springing from the walls of the 

 heart. One of these flaps is on the dorsal, the other on the 

 ventral side of the aperture, and each is connected with the 



ven- 



Fig. 7. 



A, The frog's heart dissected from the ventral surface. Z>, 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, aw, auriculo-ventricular valve, with its cordae 

 tendineae ; c.ao, cavum aorticum of the truncus arteriosus ; car, carotid 

 artery ; c.////, cavum pulmonale of the truncus arteriosus ; l.au, left auricle ; 

 J>.ca, opening of the pulmo-cutaneous arches into the cavum pulmonale ; 

 /we-, pulmo-cutaneous artery ; pv, opening of the pulmonary vein into the 

 left auricle ; r.ait, right auricle ; r.sa, opening of the right systemic artery ; 

 in />' a rod is p:i>sed up the opening and projects from the cut end of the 

 middle or systemic channel of the right aortic arch ; s.tti, septum medium of 

 the upper part of the truncus ; s./>, septum principale ; sp.v, spiral valve, 

 in />' 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 ; sj>s, systemic artery ; f 1 , proximal row of semi-lunar 

 valves guarding the passage from the ventricle into the truncus ; E>-, distal 

 row of semi-lunar valves in the truncus ; ven, ventricle. 



muscular ridges of the ventricular wall by about a dozen very 

 fine fibrous cords the cordae tendinese. The length of these 

 cords is such that they allow the membranous flaps to :ise up 

 and meet one another in the middle line so as to close the 

 aperture, but they prevent their rising farther into the cavity 

 of the auricles. 



