XII 



REPTILIA: ARTERIAL SYSTEM 



425 



v.c. 



and left pulmonary arteries (r.p and l.p) while the portion oi 

 ventral aorta out of which they open forms the common pulmonary 

 artery (/>). 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 strand, but more usually it disappears entirely. 



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

 entirely from develop- 

 ment or being at the 

 most recognizable as a 

 fleeting and transient 

 vestige. A clue to the 

 tendency of this arch to 

 disappear is given by 

 the lung -fish (p. 384), 

 in which arches V and 

 VI are seen both to 

 originate from the dorsal 

 (pulmonary) cavity of 

 the ventral aorta. It is 

 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 

 will naturally be accom- 

 panied by a reduction of 

 arch V. 



Arch IV is the sys- 

 temic arch which is re- 

 sponsible for the blood- 

 supply to the hinder 

 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. 



A 



FIG. 183. 



Diagram illustrating the arterial system of a reptile as seen 

 from the ventral side. Those portions of the embryonic aortic 

 arches which are no longer present in the adult are represented in 

 outline. A, Dorsal aorta ; l.c, left (common) carotid ; d.c, dorsal 

 carotid; f, innominate; LA, left aortic root; l.p, left pul- 

 monary ; l.S, left systemic ; p, pulmonary ; r.A , right aortic 

 root ; r.c, right (common) carotid ; r.p, right pulmonary ; 

 S, systemic ; v.c, ventral carotids. 



[The darkly shaded vessels receive venous blood from 

 the right side of the ventricle.] 



