764 



THE VASCULAR SYSTEMS 



Further Development of the Arteries. It has been seen (p. 759) that each primitive aorta 

 consists of a ventral and a dorsal stem which are continuous through the first aortic arch. The 

 dorsal aortse at first run backward separately on either side of the notochord, but about the 

 third week they fuse from about the level of the fourth thoracic to that of the fourth lumbar 

 segment to form a single trunk, the descending aorta. The first aortic arches pass through the 

 mandibular arches, and caudad of them five additional pairs are developed within the visceral 

 arches; so that, in all, six pairs of aortic arches are formed (Fig. 544); The first and second 

 arches pass between the ventral and dorsal aortse, while the others arise at first by a common 

 trunk from the aortic bulb, but terminate separately in the dorsal aortse. As the neck elongates, 

 the ventral aortse are lengthened, and the third and fourth arches arise directly from these 

 vessels. 



Second aortic arch. 

 Third aortic arch. 



Auditory vesicl 



Primitive 

 jugular vein." 



Fourth aortic arch'." 



Fifth aortic-, 

 arch. 



Dorsal aorta. 



Cardinal vein. 



Mid-gut. 



' t First aortic arch. 



,' Olfactory pit. 



Maxillary process. 



Hyomandibular cleft. 



'Mandibular arch. 



Aortic bulb. 



Auricle. 



Duct of Cuvier. 



Ventricle. 



Hind-gut. 



Umbilical vein 



"Allantois. 

 ' Umbilical (allantoic artery). 



FIG. 543. Profile view of a human embryo estimated at twenty or twenty-one days old, (After His.) 



s persist and give off branches to the gills, in which the blood is oxygenated, 

 them remain as permanent structures, while others disappear or become 



In fishes these arches 

 In mammals some of 

 impervious (Fig. 544). 



The Ventral Aortse. These persist on both sides. The right forms (a) the innominate 

 artery, (b) the right common and external carotid arteries. The left gives rise to (a) the short 

 portion of the aortic arch, which reaches from the origin of the innominate artery to that 

 of the left common carotid artery; (b) the left common and external carotid trunks. 



The Aortic Arches. The first and second disappear; the third (carotid arch) constitutes 

 the commencement of the internal carotid artery. The fourth right arch forms the right sub- 

 clavian as far as the origin of its internal mammary branch; while the fourth left arch con- 

 stitutes the arch of the aorta between the origin of the left carotid artery and the termination of 

 the ductus arteriosus. The fifth arch disappears on both sides. The sixth right arch disappears ; 



