BULLFROG A!S TYPICAL VERTEBRATE ANIMAL 



77 



Both auricles pass blood into the ventricle through a common open- 

 ing, the auriculo-ventricular aperture, which is divided by the inter- 

 auricular septum separating the two auricles. This aperture has two 

 large valves on each side and two small valves at each end which 

 regulate the discharge of blood into the ventricle and prevent its 

 backflow. 



Blood leaves the ventricle and enters the arterial system through 

 the conus arteriosus. The opening into the conus is protected by 

 three poeketlike semilunar valves which open inwardly into the conus 



Carotid A. 



System.A. 



Pulmocata- 

 neous f\. 



R. auricle - 

 ipirril yaVje. 



Conus Qrt(i-_ 

 rio5Uj 



SzvnWuDQ'c mqWz 



-Jruncus at-tenosas 



-Pulmonary aperture 

 -T-Sinu-auriculor oper- 



—Left auricle 



— Irttzraurlcular sep- 

 tum 



--Briitle inpulmo- 



cutaneoas A. 



- -/lariculo-ventricular 

 valve 



- -s^^:^/ Ventricle 



Fig. 22. — Heart of the frog with the ventral wall removed and bristles shown In 

 the arteries branching from the truncus arteriosus. 



when blood is passing out but are tightly closed at other times. The 

 proximal portion of the conus is known as the pylangium, and the 

 distal portion as the synangium. Running through the length of the 

 pylangium is a longitudinal spiral valve, one edge attached to the 

 dorsal wall of the pylangium and the other edge lying free in the 

 vessel. Upon contraction of the conus this structure is brought into 

 contact with the ventral wall and helps direct the flow of blood into 

 the arches. 



