ARTERIAL SYSTEM. 



533 



i.e. that opening furthest to the left side of the ventricle (e and cT), 

 is continuous with the riylit fourth arch, and also with the coinniou 



C 3 B A 



Fig. 365. Devkloiment of the great auterial trunks in the embryos of 

 A. A Lizard; B. the co.mmon Fowl; C. the Pkj. (From Gegenbaur ; after Eathke.) 



The first two arches have disappeared iu all three. In A and B the last three are 

 still complete, but in C the last two are alone complete. 



p. pulmonary artery springing from the fifth arch, but still connected with the sys- 

 tem of the dorsal aorta by a ductus Botalli; c. external carotid; c. internal carotid; 

 ad. dorsal aorta; a. auricle; v. ventricle; /;. nasal pit; m. rudiment of fore limb. 



carotid arteries (c) ; while a second springing from the right side of 

 the ventricle is continuous with the left fourth arch Qi and/). The 

 right and left divisions of the fourth arch meet however on the dorsal 

 side of the oesophagus to give origin to the dorsal aorta {g). 



In Birds (fig. 366 C) the left fourth arch (A) loses its connection 

 with the dorsal aorta, though the ventral part remains as the root of 

 the left subclavian. The truncus arteriosus is moreover only divided 

 into two parts, one of which is continuous with all the systemic 

 arteries. Thus it comes about that in Birds the right fourth arch {e) 

 alone gives rise to the dorsal aorta. 



In Mammals (tig. 366 D) the truncus arteriosus is only divided 

 into two, but the left fourth arch (e), instead of the right, is that 

 continuous with the dorsal aorta, and the right fourth arch (i) is 

 only continued into the right vertebral and right subclavian arteries. 



The fifth arch always gives origin to the pulmonary artery (fig. 

 365, j!)) and is continuous with one of the divisions of the t. uncus 

 arteriosus. In Lizards (fig. 366 A, i), Chelonians and Birds (Hg. 366 

 C, i) and probably in Crocodilia, the right and left pulmonary arteries 

 spring respectively from the right and left fifth arches, and durii.g 

 the greater part of embryonic life the parts of the fifth arches be- 

 tween the origins of the pulmonary arteries and the system of the 

 dorsal aorta are preserved as ductus Botalli. These ductus Botalli 

 persist for life in the Chelonia. In Ophidia (fig. 366 B, h) and 

 Mammalia (fig. 366 D, m) only one of the fifth arches gives origin to 

 the two pulmonary arteries, viz. that on the right side in Opiiidia, 

 and the left in Mammalia. 



