218 



VASCULAK SYSTEM 



At the stage when there are five complete arches (fig. 274) the loops arise in a 

 radiating fashion from the truncus arteriosus, the two anterior arches being 

 connected with an ascending, the remaining arches with a descending trunk. As 



FIG. 272. THE AORTIC ARCHES IN A HUMAN EMBRYO OP 5 MM. LONG. (After Tandler.) 



The arches are represented in their relations to the visceral pouches of the pharynx. I to VI aortic 

 arches T.A., truncus arteriosus ; ao, dorsal aorta ; tr, trachea; ce, oesophagus in, island. 



the heart is gradually displaced backwards, the truncus arteriosus comes to lie 

 first opposite the third and then opposite the fourth arch, so that all save the 

 pulmonary now spring from the ascending limb or, as it may be termed, the 



int. car. 



tr <x ao. 

 FIG. 273. THE AORTIC ARCHES IN A HUMAN EMBRYO OF 9 MM. LONG. (After Tandler.) 



I to VI aortic arches; T.A., truncus arteriosus; v.a., ventral aorta; T.P., truncus pulmonalis 

 p.a., pulmonary artery ; int.car., internal carotid artery. Other lettering as in fig. 272. 



ventral aorta. At the stage when the truncus lies opposite the fourth arch the 

 aortic septum begins to develop in its lumen backwards from the interval 

 between the systemic and pulmonary arches ; when it has completed the division 

 of the trunk into the aorta and pulmonary artery the aorta remains connected 



