THE HEAET AND ITS VESSELS. 281 



branchial arteries (Fig. 225, /, II}. The venous current, on the 

 other hand, passes from the right ventricle into the third and fourth 

 branchial arteries and thence to the corresponding gills, where it 

 becomes purified ; it reaches the aortic arches by means of the 

 branchial veins (///, IV, 3, 4, RA}. The pulmonary artery 

 (Ap) arises from the fourth branchial vein, so that the blood is thus 



BA 



-Ao 



FIG. 225. DIAGRAM OF THE BRANCHIAL CIRCULATION OF PROTOPTERUS. 



Co, conns arteriosus, which consists of two divisions, a and b : through b pure arterial 

 blood passes to the two anterior branchial arteries (/ and //) ; through a 

 venous blood passes to the two posterior branchial arteries (III and IV); 3 and 

 4 indicate the branchial veins and capillaries of the gills ; Ap, the pulmonary 

 artery, present only on the left side (?) ; RA, arch of the aorta ; Ao, aorta ; 

 Ca, carotid. 



once more purified before it passes by means of the pulmonary 

 vein into the left ventricle. In Ceratodus the lung is supplied 

 with blood from the coeliac artery, and owing to the fact that the 

 longitudinal valve of the conus is incomplete, the blood passing to 

 the two anterior branchial arteries is of a mixed nature (comp 

 Fig. 224,7, II}. 



Amphibia. With the exception of the Gymnophiona, in 

 which it is situated some distance back, the heart in all Amphi- 

 bians lies far forwards, below the anterior vertebrae. As in the 

 Dipnoi, there is a more or less complete septum atriorum (that 

 is, the septum is either entire or fenestrated). There are always 

 two fibrous pocket-like atrio-ventricular valves, which are con- 

 nected with the walls of the ventricle by cords. 



The cavity of the ventricle is unpaired, and neither in Urodela 

 nor Anura shows any trace of a septum, so that the blood passing 

 out from it must have a mixed character (Fig. 226). The 

 ventricle is usually of a short and compressed form, but is more 



