ARTERIAL HYXTEM. 



53 3 



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

 is continuous with the ri///^ fourth arch, and also with the common 



FIG. 805. DEVELOPMENT or THE GREAT ARTERIAL TRVXKS IN THE EMBRYOS OF 

 A. A LIZARD; B. THE COMMON FOWL; C. THE Pi.;. (From Gegenbaur; after llathke.) 



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

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



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

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

 nil. dorsal aorta; a. auricle; v. ventricle; n. nasal pit ; in. 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 (h and /'). The 

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

 side of the oesophagus to give origin to the dorsal aorta ((/}. 



In Birds (fig. 366 C) the left fourth arch (h) 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 (fig. 366 13) 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 (*) is 

 only continued into the rmlit vertebral and right subclavian arteries. 



/ O * * 



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

 365, p) and is continuous with one of the divisions of the truncus 

 arteriosus. In Lizards (fig. 366 A, i), Cheloniaus 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 during 

 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 (Jhelonia. In Ophidia (fig. 366 B, h) and 

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

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

 and the left in Mammalia. 



