CLASS AMPHIBIA 



511 



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

 distal portion as the synangkim. 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. 



Near the anterior free end of the spiral valve where it is the 

 widest, there is a pair of small synangial valves which, together 



Carotid A 



System.A 



Pulmocuta- 

 neous f\. 



R auricle - 

 Spiral valve 



Conus orte-_ 

 nosus 



Semilunar vaUz 



Truncus arteriosus 



'^-Pulmonary aperburz 



-Sinu-auricular aper- 

 ture 



—Loft auricle 



— Intzrauricular sep- 

 tum 



--Dmtle inpulmo- 



cutaneous A. 



- -/luricufo- ventricular 

 valve 



- -">:7 Ventricle 



Fig. 277.- 



-Heart of frog with the ventral wall removed and bristles shown through 

 the arteries of the truncus arteriosus. 



with the end of the spiral valve, separate the pylangium from the 

 synangium. Just below these valves is an aperture which leads into 

 the trunk formed by the union of the two pulmocutaneous arteries. 

 The synangial chamber is very short and gives off almost imme- 

 diately two large branches, one to the right and the other to the 

 left. In each of these branches originate the three main trunks or 

 arches of the arterial system. They are formed by two longitudinal 

 septa dividing the vessel into three compartments. All three trunks 



