VASCULAR SYSTEM 



309 



mainly venous respectively, pass out from the heart side by side. 

 The former comes from the pulmonary vein, from which it passes 

 into the left atrium, thence into the left side of the ventricle, and 

 so to the two anterior branchial arteries. The venous current, 

 on the other hand, passes from the right side of the ventiicle into 

 the third and fourth afferent branchial arteries and thence to the 

 corresponding gills, where it becomes purified ; it reaches the aortic 

 roots by means of the efferent branchial arteries. The paired pul- 

 monary artery arises from the fourth efferent branchial in Ceratodus 

 (Fig. 248), and from the aortic root in Protopterus and Lepidosiren, 

 that of the right side bifurcating to supply the dorsal surface of the 

 lung or lungs (p. 288), while that of the left side supplies the 

 ventral surface. The two pulmonary veins unite to form a median 

 trunk which becomes closely connected with the sinus venosus, so 

 as to appear sunk within its walls (Fig. 247). Thus the blood 

 becomes once more purified before it passes into the left ventricle. 

 K postcaval vein, present from the Dipnoi onwards, opens into the 

 sinus venosus posteriorly to the precavals, and with it the hepatic 

 veins communicate (Figs. 248 and 267). 



Amphibia. — -With the exception of the Gymnophiona, in which 

 it is situated some distance back, the heart in all Amphibians lies far 

 forwards, below the anterior vertebrae. A septum atriorum is well 

 developed, but in Urodela and Gymnophiona it is more or less fenes- 



A 



Fig. 249.— Diagram showin-<4 the Cocrsk ok the Blood through the Heart 

 IX Urodela (A) a:nd Aiiura (B). 



A, right atrium ; A\ left atrixim ; V, ventricle; fr, conus arteriosus, divided i in 

 Anura (B) into two portions, tr, fr^ : tlirougli tr venous blood passes into the 

 pulmonary arteries, Ap^, Ap'-, while through fr^ mixed blood goes to the 

 carotids, ci— ce, and to the roots of the aorta., RA ; Ir, />; pulmonary veins ;. 

 V, i; pre- and po.st-cavals (only one precaval is indicated). 



trated (Fig. 250). There are always two fibrous, pooket-likc atrio- 

 ventricular valves, which are connected with the walls of the 

 ventricle by cords. The two pulmonary veins unite before opening 

 into the left atrium. 



