THE VASCULAR SYSTEM 



375 



in the aorta and three in the pulmonary artery. As a result of the spiral 

 splitting of the bulbus, the pulmonary artery is made to connect with the 

 right ventricle, and the aorta with the left. 



Development of the Aortic Arches. In very early stages of ontogenesis, 

 in 1.5 mm. to 2.00 mm. embryos, before the two halves of the heart are 



arterial circle 

 (of Willis; 



hypophysis 



aortic root ophth. a. 

 ventral to 

 pharynx 



ant. cerebral a. 

 mid. cer. a. 



post. cer. a. 



int. carotid 

 artery 



subclavian a. 

 (left) 

 thoracic 

 intersegmental 

 arteries 



vertebral a. 



subclavian a 



internal 

 mammary 



dorsal aorta 



Fig. 314. — Diagrams illustrating the changes which occur in the aortic arches of 

 mammalian embryos. A, ground plan of complete set of aortic arches; B, early stage in 

 modification of arches; C, derivatives of aortic arches. Abbreviations: br. ceph., 

 brachiocephalic a,TteTy; cer. a., cerebral artery; lin., lingual artery; max., maxillary 

 artery; ophth. a., ophthalmic artery; stap. a., stapedial artery; thy., thyroid artery. 

 (From Patten's "Embryology of the Pig," adapted from several sources.) 



completely united in the mid-ventral line, connexion between ventral and 

 dorsal aortae is established around the pharynx by means of a single aortic 

 arch, the first. In a 2.6 mm. embryo, a second aortic arch, the hyoid, is 

 added. Others are added in succession until in a 4.2 embryo there are five 

 aortic arches. But the fifth or last of these is really the sixth arch, the 

 true fifth aortic arch being a rudimentary vessel which appears only 

 transiently in the 7.0 mm. embryo. By this stage, however, the first two 



