126 



GENERAL ZOOLOGY 



C.A 



tendineae. Another set of three valves is found at the anterior 

 end of the septum, as follows: (1) a large one, valve 3, on the left 

 side of the septum, beyond which is an opening leading to the 

 pulmocutaneous arteries; (2) another one, called valve 2, lying 

 ventral and to the right of valve 3; and (3) a cup-shaped valve 

 formed by the anterior end of the septum bulbi and known as 

 valve 1 (Figs. 78 and 79). 



The truncus impar is divided into dorsal and ventral chan- 

 nels by the septum principale, which, 

 taking its origin from the anterior 

 edge of the septum bulbi, divides the 

 cup-shaped valve of the latter into 

 dorsal and ventral portions, valves la 

 and 16. The dorsal channel, lead- 

 ing from the cavum pulmocutaneum 

 of the bulbus, divides with the trun- 

 cal arms of the truncus impar into 

 right and left pulmocutaneous arches. 

 The ventral channel of the truncus 

 impar is divided into right and left 

 portions by the septum inter aorticum, 

 Fig. 79.— Dissection of distal which is at right angles to the septum 



end of truncus arteriosus of Rana pr i nc i pa l e . The two channels thllS 

 catesbeiana. a. a., aortic arch; 



c.a., carotid arch; c.p., cavum formed are the beginnings of the 

 pulmocutaneum; L.v.iongitu- aortic arc hes. The carotid arches 



dinal valve of cordis bulbus; p. a., 



pulmonary arch; s.i., septum originate as two small openings in the 

 interaorticum; s.p. septum prm- septum interaorticum, leading from 



cipale; la, lb, 3, valves. (Valve *■ . ~. 



2, removed with the ventral wall the right aortic branch of the 



of the truncus, is not shown.) truncus. Blood leaving the cavum 

 aorticum of the bulbus enters the aortic and carotid arches. 



The sinus venosus, situated on the dorsal' side of the heart, 

 receives a right and left anterior vena cava (precava) in front, and 

 an unpaired postcava behind. The blood brought to the sinus 

 venosus is for the most part deoxygenated or venous blood, 

 coming from the capillaries of all parts of the body except the 

 lungs. Since the sinus venosus empties into the right atrium, 

 the blood of the right atrium is venous in character. On the other 

 hand, blood returned from the lungs to the heart by the pul- 

 monary veins is usually oxygenated blood, and this blood enters 



