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GENERAL ZOOLOGY 



edges of the valves are prevented from being pushed back into 

 the atria by small fibers, known as chordae tendineae, extending 

 from the edges of the valves to the wall of the ventricle. 



The truncus arteriosus takes its origin from the anterior margin 

 of the right side of the ventral wall of the ventricle and then 

 follows a diagonal course across the atria from right to left. It is 

 composed of two regions: a proximal segment, the bulbus cordis, 



CAQ 



Fig. 77. — Dissection of truncus arteriosus of Rana catesbeiana from ventral 

 side. a. a., aortic arch; c.a., carotid arch; c.ao., cavum aorticum of bulbus 

 cordis; c.p., cavum pulmocutaneum of bulbus cordis; l.v., longitudinal valve; 

 p.a., pulmonary arch; s.i., septum interaorticum; 3, valve. 



marked off by a transverse constriction from a distal segment, the 

 truncus impar, which almost immediately bifurcates into right 

 and left trunks. Each of these trunks separates into three 

 branches, known as arterial arches, of which the anterior one 

 is the common carotid artery, the next, the aorta, and the posterior 

 one the pulmocutaneous artery. Each common carotid artery 

 divides almost at once into internal and external carotid arteries 

 which supply the head. Each aortic arch curves outward and 

 dorsally to join its fellow of the opposite side at about the level 

 of the sixth vertebra, to form the dorsal aorta. Each pulmo- 



