ARTERIES 



219 



with the four anterior pairs of arches and the pulmonary artery with the pulmonary 

 pair alone. 



The first and second arches early become interrupted in their course, but the 

 dorsal part of the second, and possibly also of the first, persist and take part in the 

 formation of an embryonic vessel called the stapedial artery (see below). The third 

 arch remains complete, and is for a time the largest of the series. The dorsal aorta 

 between the third and fourth arches 

 next becomes obliterated, and the 

 carotid system of vessels begins to 

 take shape. The ventral aorta in 

 front of the third arch, now discon- 

 nected from the dorsal aorta, becomes 

 the external carotid ; the third arch 

 and the dorsal aorta constitute the 

 internal carotid ; and the ventral aorta 

 behind the third arch is the common 

 carotid. This is at first very short, but 

 when the heart is displaced back- 

 wards, and the neck is formed it 

 becomes much drawn out, and the 

 internal carotid assumes a directly 

 ascending course by the straightening 

 out of the third arch. 



The fourth arches persist on both 

 sides, but the left early assumes larger 

 proportions. When, somewhat later, 

 the dorsal aorta on the right side is 

 obliterated between the fourth arch 

 and the point of fusion of the two 

 dorsal aortsR into the descending aorta, 

 the left fourth arch alone remains in 

 connexion with that vessel and forms 

 the aortic arch (in the strict morpho- 

 logical sense of that term) ; while the 

 right fourth arch becomes the first part 

 of the subclavian artery. At an early 

 stage, when the truncus arteriosus has 

 been displaced backwards to the level 

 of the fourth arches, the proximal 

 segment of the ventral aorta forms a 

 common stem for the fourth and third 

 arches. On the right side this becomes 

 the innominate artery ; but on the" left 

 side, though the arrangement is at first 

 symmetrical, the common stem con- 

 tracts as the fourth arch enlarges, 

 and is only represented by the portion 



of the adult arch between the origins of the innominate and left common 

 carotid arteries. The pulmonary arch on each side at an early stage gives off 

 a branch which runs along the developing lung-rudiment of its own side. On 

 the right side, the portion of the arch distal to this disappears at the same 

 time as the section of the dorsal aortse with which it was at first connected. On 

 the left side, however, it persists as the ductus arteriosus connecting the pulmonary 



FIG. 274. PROFILE VIEW OF A HUMAN EMBBYO OF 



ABOUT THREE WEEKS, SHOWING ALL THE 

 CEPHALIC VISCERAL ARCHES AND CLEFTS. 



mx, maxillary process ; mn, mandibular arch ; 

 d.C., duct of Cuvier; j.v., jugular vein; c.v., car- 

 dinal vein ; v .v. } vitelline vein ; u.v., umbilical vein ; 

 u.a., umbilical artery ; all, allantois ; pi, placenta! 

 attachment of allantoic stalk ; olf, olfactory depres- 

 sion ; ot, otic vesicle. 



