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 

 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 rise up 



Fig. 7- 



A, The frog's heart dissected from the ventral surface. B, an enlarged view of 

 the truncus arteriosus. Ji A , right auricle ; LA, left auricle ; V, ventricle ; 

 T, truncus arteriosus, divided into Py, pyiangium, and S^, synangium; 

 ^ V, opening of the sinus venosus into the right auricle ; Pv, opening of the 

 pulmonary vein into the left auricle ; Aw, auriculo-ventricular valve ; s/v, 

 semi-lunar valves guarding the passage from the ventricle to the pylangiiim; 

 C, carotid arch ; S, systemic arch ; P, pulmo-cutaneous arch ; spv, spiral 

 valve in the pyiangium ; ^a, opening of the right pulmo-cutaneous arch ; co, 

 opening of the carotid arches. 



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. 



The left-hand anterior corner of the ventricle is prolonged, 

 on its ventral side, into a stout vessel, which passes forwards 

 and to the left across the auricles, and divides at the upper 

 limits of the latter into two vessels, right and left, each of 



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