312 COMPARATIVE ANATOMY 



of the adult, which probably functions as an accessory heart. Direct 

 connections exist between the second and third afferent and 

 efferent arteries. 



Towards the end of the larval period, the second efferent bran- 

 chial artery increases considerably in relative size, and the fourth 

 ' arterial arch also becomes larger. By a reduction of the anasto- 

 mosis with the' third arch, the fourth carries most of the blood for 

 the pulmonary artery, and the latter thus now contains more venous 

 than arterial blood. When branchial respiration ceases, the anasto- 

 moses between the afferent and etferent branchial arteries no longer 

 consist of capillaries, but a direct connection between them be- 

 comes established (Fig. 252). Finally, the connection between the 



FIG. 252. ARTERIAL ARCHES OF AN ADULT Salamandra maculosa, SHOWN 

 SPREAD OUT. (After J. E. V. Boas. ) 



co, tr, truncus arteriosus ; 1 to 4, the four arterial arches ; ce, external carotid ; 

 cd, " carotid gland " ; ci, internal carotid. The fourth arterial arch, which gives 

 rise to the pulmonary artery (Ap), has increased considerably in size relatively, 

 and is only connected by a delicate ductus Botalli (f) with the second* and 

 third arches ; RA , root of the aorta ; ce, cesophageal vessels. 



first and second branchial arches disappears, the former giving 

 rise to the carotid and the latter forming the large aortic root ; 

 an anastomosis remains throughout' life, however, between the 

 fourth arch, which forms the pulmonary artery, and the second and 

 third arches. This is usually spoken of as the ductus Botalli. 



The third arch varies greatly in its development ; it may be 

 present on one side only, or may even be entirely wanting. 



In the larva) of Anura there are also four afferent branchial 

 arteries present on either side, but these are connected with the 

 corresponding efferent vessels by capillaries only, there being no 

 direct anastomoses (compare Fig. 251). The consequence of this 

 is that all the blood becomes oxygenated. 



In the adult Frog the third arterial arch becomes entirely 



