1192 



EMBRYOLOGY. 



descending aorta, become elongated and assume an arched form, curving backward 

 on each side, from the front of the body toward the vertebral column (Fig. 751, A). 

 These are the first or primitive aortic arches. As the heart recedes into the thorax, 

 arid these arches, which correspond in position to the mandibular arch, become 

 elongated, four additional pairs of arches are formed behind them around the 

 pharynx, one in each branchial arch (Fig. 750). The arches, five in number, 

 remain permanent in fishes, giving off from their convex borders the branchial 

 arteries to supply the gills. In many animals the five pairs do not exist together, 

 for the first two have disappeared before the others are formed ; but this is not so 

 in man, where all five arches are present and pervious during a certain period of 

 embryonic existence (Fig. 750). Only some of the arches in mammalia remain as 



Second aortic arch. 

 Third aortic arch. >*** 



Auditory vesicle 



Primitive 

 jugular vein. 



Fourth aortic arch. 



Fifth aortic. ~, 

 arch. 



Dorsal aorta. 



Cardinal vein. --ft- 



Mid-gut. ^Bj 



Hind-gut. 



First aortic arch. 



. Olfactory pit. 



Maxillary process. 



Hyo-mandibular cleft. 



Mandibular arch. 



Aortic bulb. 



Auricle. 



Duct of Cuvier. 



Ventricle. 



~ Allantois. 



-- Umbilical (allantoic artery). 



FIG. 750. Profile view of a human embryo, estimated at twenty or twenty-one days old. (After His.) 



permanent structures ; the others, or portions of them, become obliterated or dis- 

 appear. The first two arches entirely disappear. The third remains as a part of 

 the internal carotid artery, the remainder being formed by the upper part of the 

 posterior aortic root i. e., the descending part of the original vessel which pro- 

 ceeded from the rudimentary tubular heart. The common and external carotid 

 arteries are formed from the anterior aortic root that is, the ascending portion of 

 the same primitive vessel. The fourth arch on the left side becomes developed into 

 the permanent arch of the aorta in mammals ; but in birds it is the fourth arch on 

 the right side which forms the aortic arch ; in reptiles the fourth arch on both sides 

 persists, so that these animals possess a permanent double aortic arch. The fourth 

 arch on the right side forms the subclavian artery, and by the junction of its com- 



