HEART 403 



(Fig. 305), and from the aortic root in Protopterus and Lepidosiren, 

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

 lung, 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 in the form of two valve-like 

 projections l (Fig. 304). Thus the blood once more becomes 

 purified before it passes into the left side of the ventricle. A post- 

 caved vein, present from the Dipnoi onwards, opens into the sinus 

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

 communicate (Figs. 305 and 328). 



Amphibians. With the exception of the Gymnophiona, 

 the heart in all Amphibians lies far forwards, below the anterior 

 vertebras. A septum atriorum is present,' 2 but in Urodela 

 and Gymnophiona it is more or less fenestrated (Fig. 307) : 

 it never completely separates the cavities of the two atria, 

 and forms an arch over the atrioventricular aperture. There 



A 



FIG. 306. DIAGRAM SHOWING THE COURSE OF THE BLOOD THROUGH THE HEART 



ix UrodfJa (A) AND Anura (B). 



A, right atrium ; A 1 , left atrium ; ir, fi; pulmonary veins; tr, conus arteriosus, 

 divided in Anura (B) into two portions, tr, tr 1 : through tr -venous blood 

 passes into the pulmonary arteries, Aj> } , Aj> 1 , while through tr 1 mixed blood 

 goes to the carotids, ci <'e, and to the roots of the aorta, Ft A ; V, ventricle ; 

 11, v, pre- and postcavals (only one precaval is indicated). 



are always two sinu-atrial valves and two atrio- ventricular 

 valves, the latter of which are connected with the walls of 

 the ventricle by cords. The two pulmonary veins unite before 

 opening obliquely, as in all the higher Vertebrates, into the left 

 atrium, at which point there are no special valves : the two 

 precavals and the postcaval open independently into the sinus 

 venosus. 



1 The largest of these, as well as the fibrous cushion described above, take 

 a greater part in separating the two auricles than does the septum proper. 



- In the lungless Urodeles (cf. p. 378) the septum atriorum and pulmonary 

 vein are wanting, though the pulmonary artery can still be recognised. 



D D* 



