Following the reorganization of the aortic arches to accommodate blood flow to the lungs, the great vessels which enter and leave the heart were modified.  Subsequent variation in specific lineages led to further changes.  The left subclavian artery forms near the base of the heart while the right subclavian forms as a later branch. The common carotid arteries may branch from the same great vessel or from different ones. 

     In reptiles, the left and right systemic vessels (aortas) leaving the heart fuse with the fourth embryonic aortic arches, left and right, to form a pair of aortic arches which fuse in the abdomen to form the descending aorta.  In reptiles, the right fourth aortic arch larger than the left and the single common carotid originates from the right arch.  In birds, the left fourth aortic arch degenerates during development and the right branch of the fourth aortic arch composes the aorta.  In mammals, the right branch degenerates (its only remnant being the base of the right subclavian artery) and the left branch of the fourth aortic arch composes the aorta (Torrey, 1979).  In birds, there is a left systemic vessel in addition to the aorta as in crocodiles, but it degenerates.

     Some mammals have cartilage or even bone associated with their hearts as in the cartilaginous aortic ring in horses (which may calcify) or the 1 or 2 ossa cordis bones in some cattle, both of which are associated with the aortic semilunar valve (Weichert, 1970).

     Amniotes were the first to possess a completely separate pulmonary circuit, with no ductus arteriosus connecting pulmonary and systemic circuits (which still persists in salamanders and tadpoles) (Kardong).  In sarcopterygian fish,  the pulmonary artery develops as a branch of aortic arch VI which also services gills before emptying into the dorsal aorta through the ductus arteriosus (amphibians have the same) and the pulmonary vein carries oxygenated blood from lungs to left atrium (Kardong p. 465).

TURTLE

OPOSSUM

CAT

CORONARY VESSELS

---Blood vessels servicing the heart appear for the first time in sharks.

RENAL

     All vertebrate embryos possess another portal system in which blood from the posterior portions of the body is routed through kidney capillaries.  This renal portal system is retained in the adults of fish, amphibians, and reptiles but is lost in the adults of birds and mammals (Torrey). 

FROG

OPOSSUM

CAT

MONKEY