CIRCULATION AND RESPIRATION 



125 



cutaneous artery divides into (1) a great cutaneous artery, which is 

 distributed mainly in the skin of the dorsal and lateral regions, 

 and (2) a pulmonary artery to a lung. One division of the great 

 cutaneous artery, the auricularis, forms anastomoses with other 

 arteries, such as the occipital and carotid (Fig. 81). 



If the truncus arteriosus is opened by a ventral incision from 

 its point of origin to the bifurcation, its bulbar region is found to 

 contain a number of valves, of which the largest is the septum 

 bulbi or longitudinal valve (Fig. 

 77). The septum bulbi is at- 

 tached at its posterior end to 

 the ventral wall of the proximal 

 end of the bulbus, but as one 

 traces the valve forward, its 

 point of attachment spirals 

 sharply to the left through an 

 arc of 180 degrees to the dorsal 

 wall of the bulbus, which it 

 follows in a slight curve from 

 left to right to the beginning of 

 the truncus impar where it is 

 united dorsally and laterally to 

 the truncal walls. Since the 

 ventral edge of the septum is ssection proxir j end of 



free throughout the greater part truncus arteriosus of Rana catesbeiana, 

 r •■ i ,i i i j- ■ with walls spread apart, d, dorsal 



of its length, a complete divi- valve . LVm left valve . R v ( right valve 

 sion of the bulbar channel into 



two can only occur when the free edge of the septum is pressed 

 tightly against the ventral wall of the bulbus. The attached 

 edge of the septum is broad and the free edge is thin and movable. 

 The two channels formed in the bulbus by the septum are known 

 as the cavum aorticum, on the right, and the cavumpulmocutaneum, 

 on the left. 



At the posterior end of the septum bulbi, where it is attached 

 to the ventral and lateral wall of the bulbus, are three valves: (1) 

 a left ventral valve, closely applied to the right lateral face of the 

 septum; (2) a right ventral valve, attached to the right wall 

 of the bulbus; and (3) a dorsal valve, very small and lying between 

 the first two. These valves are connective tissue flaps, whose 

 free edges are attached to the walls of the bulbus by chordae 



