XII 



REPTILIA: ARTERIAL SYSTEM 



425 



and left pulmonary arteries {r.p and l.p) while the portion of conus and 

 ventral aorta out of which they open forms the common pulmonary 

 artery {p). The dorsal part of arch VI persists in some cases (Lizards 

 and Chelonians) as a duct of Botallus, or a vestige of it may remain as 

 a solid strandj but more usually it disappears entirely. 



Arch V is greatly reduced in all amniotes^ having either disappeared 



diC. 



ac. 



entirely from develop- 

 ment or being at the 

 most recognizable as a 

 fleeting and transient 



vestige. A clue to the I 



tendency of this arch to 



disappear is given ,by 



the lung - fish (p. 384), ^ ' 



in which arches V and -nr... 



VI are seen both to 



originate from the dorsal 



(pulmonary) cavity of 



the ventral aorta. It is VI' 



clear that the more blood 



passes into arch VI the 



less will be left for arch 



V, and consequently the 



advancing evolution of 



the lung and its taking 



off a greater proportion 



of the available blood Fig. 183. 



will naturally be aCCOm- Diagram illustrating the arterial system of a reptile as seen 



. , , J J.' r from the ventral side. Those portions of the embryonic aortic 



paniea uy a reuUCtlOn 01 arches which are no longer present in the adult are represented in 



arch V outline. ^, Dorsal aorta; Z.c, left (common) carotid ; ^.c, dorsal 



. _-_ . , carotid; i, innominate; l.A, left aortic root; l.p, left pul- 



Arcn i V is trie SyS~ monary ; I.S, left systemic ; p, pulmonary ; r.A , right aortic 



temiC arch which is re- ^°°^ ■ ''"' "^''' (<=°™'"°''' carotid ; r.p, right puhnonary ; 



S, systemic; v.c, ventral carotids, 

 sponsible for the blood- [The darkly shaded vessels receive venous blood from 



supply to the hinder the right side of the ventricle.] 



part of the aortic root and through it to the dorsal aorta. That of the 

 left side is continuous with the left systemic cavity of ventral aorta and 

 conus (Fig. 183, l.S) and therefore like the pulmonary artery receives 

 venous blood from the right side of the ventricle. That of the right 

 side on the other hand receives arterial blood from the left side of the 

 ventricle through the right or main systemic portion of ventral aorta 

 and conus. 



