248 THE MESODERMAL DERIVATIVES 



each other by interatrial septum. Right atrium receives systemic veins 

 while left atrium receives pulmonary vein from lung buds. 

 Ventricle — thick-walled, muscular, and generally ventral to atria. Leads 



into truncus and bulbus arteriosus. 

 Bulbus arteriosus — tubular extension of early embryonic heart which leads 



directly into aortic arches. 

 Truncus arteriosus — short, ventrally directed vessel leading into bulbus 

 arteriosus. (Syn., ventral aorta.) 

 Arteries — major arteries only. 

 Afferent branchial arteries. 



Lingual (external carotid) — extensions of ventral aorta into lower jaw. 



First branchial within third visceral arch. 

 Second branchial — forked portion of first branchial found within fourth 



visceral arch. 

 Third branchial — temporarily found within fifth visceral arch; degen- 

 erates. 

 Fourth branchial — within sixth visceral arch. 

 Efferent branchial arteries. 



Anterior cerebral (internal carotid) — gives rise to basilar artery and com- 

 missural artery passing beneath infundibulum to other side of head (see 

 diagram). Extensions of first branchial artery from within first visceral 

 arch. 

 Second branchial — within fourth visceral arch, enlarges and grows pos- 

 teriorly as descending aorta to join corresponding vessel of other side 

 to form systemic trunk. Junction about level of liver. 

 Third branchial — within fifth visceral arch; degenerates. 

 Fourth branchial — within sixth visceral arch, loses its connection (ductus 

 arteriosus) with descending aorta and grows posteriorly to form pul- 

 monary and cutaneous arteries (see diagram). 

 Dorsal aorta — single large artery formed by junction of two descending 

 aortae (radices aortae) and extending posteriorly into tail, giving off 

 various smaller arteries en route. 

 Intersegmental arteries — paired and metameric arteries which grow from 



dorsal aorta dorso-laterally between myotomes. 

 Glomi — short branches from radices aortae which grow toward pronephric 



chambers. Really undeveloped glomeruli. 

 Mesenteric artery — single large vessel growing ventrad from dorsal aorta 



to supply viscera. Later becomes coeliac artery. 

 Caudal artery — extension of dorsal aorta into tail. 

 Veins — major veins only. Generally walls are thinner than arterial walls. 

 Cardinal system. 



Anterior cardinals (superior jugulars) — irregular in cross section, found 



in head, closely associated with internal carotid arteries. 

 Inferior jugulars — bring blood from lower jaw, accompanying external 

 carotid arteries, and join common cardinals. 



