VI 



AETERIES OF REPTILES 



397 



detail there can be seen general agreement in the fate of various 

 aortic arches and of other parts of the primitive arterial scheme. 

 Thus the ventral aorta is continued forwards to form the paired 

 ventrUl (" external ") carotid arteries (v.c) while the aortic roots 

 similarly extend forwards as the dorsal ("internal") carotids (d.c). 

 Aortic arches I and II disappear. Arch III persists as the root of 

 the dorsal carotid while the portion of ventral aorta behind it, when 



Fig. 187a.— Illustrating modification of the carotid arteries, correlated with elongation 



of the neck region. 



C, Crocodile ; D, Bird ; c, coeliac artery ; s*, secondary subclavian. 

 (Other letters as in Fig. 186.) 



paired, is the common carotid (e.c). Arch IV is the Systemic 

 arch which sends the blood to the hinder portions of the aortic 

 roots (a.r) and thence to the dorsal aorta (A). Arch V is reduced, 

 appearing only as inconspicuous and transient vestiges during 

 development. Of arch VI the proximal portion becomes the root 

 of the pulmonary artery (r.p and' l.p) while its dorsal portion 



diSa The a chief differences in detail are as follows: those affecting the 

 carotids will be given more fully later on 



In Lizards and Chelonians the dorsal part of arch VI persists as 



