Tin-: CIRCULATORY SYSTEM 597 



gills develop, a vessel forms dorso-laterally to the aortic arch, along the 

 base of the gill to supply it. The vessel opens out of the ventral end 

 of the aortic arch only to join it again at its upper end. The lower end 

 of the aortic arch is then called the afferent branchial artery. 



The Efferent Branchial Arteries (Fig. 345, ef). These are but the 

 dorsal ends of the aortic arches. Loops of tiny capillaries formed in the 

 external gill and connect afferent and efferent vessels. Later, after the ex- 

 ternal gills are lost and the internal ones developed, part of the aortic 

 arches disappear, so that afferent and efferent vessels are connected by 

 vessels of the internal gills. This causes the original aortic arch to be- 

 come almost entirely an efferent branchial artery, while the vessel which 

 connected ventral and dorsal ends of the aortic arch becomes the afferent 

 branchial artery (Fig. 345). 



As the internal gills disappear when the tadpole becomes a frog, the 

 lower end of the efferent branchial artery, which is the original aortic 

 arch, again acquires a direct connection with the afferent branchial artery 

 so that the blood again passes from truncus arteriosus to dorsal aorta. 



The gill capillaries diminish and the connection which has thus been 

 re-acquired becomes larger, and forms the adult persistent vessels of the 

 branchial arches. As the fourth branchial arch has no external gills de- 

 veloped upon it, the part described above regarding external gills does 

 not apply to it. In all other respects, its blood supply is similar to those 

 which do have external gills. 



The above description applies to Rana esculenta, which, although 

 different in detail, is essentially alike in all species. 



The following account of the mandibular and hyoid vascular ar- 

 rangement is for Rana temporaria: 



Hyoidean Vein. This is a small outgrowth of the lateral dorsal 

 aorta which extends toward, but never reaches the vessel which repre- 

 sents the aortic arch of the hyoid arch. It disappears at the time the 

 mouth opens. 



At the time of hatching there is a small outgrowth from the truncus 

 arteriosus extending into the lower end of the hyoid arch, but this, too, 

 disappears shortly, though it is at this time that the vestige of the 

 aortic arch has already become divided into dorsal and ventral portions. 

 The dorsal portion also disappears, and the ventral becomes the hyoidean 

 vein. This disappears with the oral sucker. 



Pharyngeal Artery. Just before hatching, the vessels of the mandib- 

 ular arch appear. Here," too, there is a vestigeal aortic arch in the lower 

 portion of the mandibular arch, which soon unites with an outgrowth 

 from the lateral dorsal aorta. After union, this vessel joins the hyoidal 

 vein. 



After the mouth opens, the outgrowth from the lateral dorsal aorta 

 separates from the other vessels, growing forward. It is then known 



