182 



AMPHIBIA. 



and unite beneath the vertebral column to form the two roots of the 

 descending aorta. With the appearance of gills the three anterior 

 pairs of arches give off vascular loops, which form the system of the 

 branchial capillaries, while the dorsal parts of the arches unite with 

 one another in various ways to form the roots of the descending 

 aorta (fig. 623). 



The fourth vascular arch, which, moreover, is frequently a branch 

 of the third (Batrachians), or arises in a common ostium with the 

 latter on the bulbus (Salamander), has no relation to the branchial 

 respiration, and leads directly into the root of the aorta. It is this* 



posterior vascular 

 arch which sends 

 a branch, one on 

 each side, to the 

 developing lungs 

 (fig. 624, Ap), 

 and so constitutes 

 the first rudiments 

 of the pulmonary 

 arteries, which 

 soon increase in 

 size and import- 

 ance. In the 

 Perennibranchi- 

 ates these ar- 

 rangements per- 

 sist in essentials 

 through life, but 

 in Batrachians 

 and Salamanders 

 the disappearance of the gills is followed by further reductions, which 

 lead to the arrangement of vessels found in the higher Vertebrates. 

 With the atrophy of the branchial capillaries the connection between 

 the bulbus arteriosus and the descending aorta is again represented 

 by simple arches, which are in part reduced to narrow canals or even 

 to solid cords of tissue (ductus Botalli) (fig. 624 and fig. 59). The 

 anterior arch sends off branches to the tongue, and also the carotids, 

 at the origin of which there is a swelling the so-called carotid gland 

 (fig. 624). The two middle arches form the roots of the descending 

 aorta and branches may be also given off from them to the head. The 

 posterior arches, which at their origin are often fused with the preced- 



Fie. 624. Heart and principal arteries of a toad. Ad, Right 

 aortic arch ; As, left aortic arch : Ca, carotid ; Cd, carotid 

 gland; Ap, pulmonary artery; H, cutaneous artery; M, 

 mesenteric artery. 



