1028 



THE VASCULAK SYSTEM. 



been formed. It runs from the ventral root of the fourth arch to the dorsal part of the sixth 



arch (Fig. 822), persists for a short time, and then disappears. 



At a still later period the dorsal roots of the third arches on both sides and the dorsal roots 



of the fourth and sixth arches on the right side disappear. The vessels which remain form 



the main stems 

 of the permanent 

 arterial system 



Right subclavian artery (Fig 8 - 823 > 824 )' 



Union of ductus arteriosus 

 f with aorta 



Union of dorsal roots of 

 i 6th arches 



Internal carotid 

 Internal carotid i 

 Internal carotid; 



Arch of aorta 



Right subclavian artery 



I Left subclavian artery 



Dorsal aorta 



Jfc. 

 Left 6th arch 



Right pul- 

 monary artery 

 Left pulmonary artery 



External carotid/ / /^^/^l ' Innoniinate artery 



External carotid / ' /^ m: R ^t 6th arch 

 Left common carotid 



Right common carotid j / 

 Arch of aorta / 

 Left 6th arch / 



Ascending aorta 



FIG. 823. SCHEMA OF PART OF THE ARTERIAL SYSTEM OF A FCETUS SEEN FROM THE LEFT 

 SIDE. Parts of the first and second arches, the dorsal roots of the third arches, the 

 dorsal part of the right sixth arch, and the dorsal roots of the right fourth and fifth 

 arches have atrophied. The position of the fifth arch is not indicated ; see Fig. 822. 



The ventral roots 

 of the first and 

 second arches form 

 the external caro- 

 tid. The third 

 arches themselves 

 and the -dorsal 

 roots of the second 

 and first arches 

 are converted into 

 the internal caro- 

 tids, which are 

 prolonged head- 

 wards into the 

 cerebral region, co- 

 incidently with 

 t;he growth of 

 the head. 



The ventral 

 root of the third 



arch on each side 



becomes the common carotid. The ventral root of the fourth arch on the right side is converted 

 into the innominate artery, and the fourth arch of the right side forms part of the right 

 subclavian artery, i.e. a portion of that part of the right subclavian artery which lies medial to 

 the vertebral artery. On the left side the ventral root of the fourth arch, and the fourth arch 

 itself and its dorsal root, take part in the formation of the arch of the aorta, and the dorsal root 

 of the sixth arch is converted into the most cephalward or anterior portion of the thoracic part 

 of the descending aorta; the remainder of the descending aorta being formed by the earlier 

 fusion of the primitive dorsal aortae. Occasionally the dorsal roots of the fourth and sixth 

 arches on the right side persist (see p. 1051), and in such cases the right subclavian artery, of 

 which they form a part, springs from the descending aorta at the level of the fifth thoracic 

 vertebra. It is probable, therefore, that the portion of the descending aorta formed from the 

 dorsal roots of the left fourth and sixth arches is a comparatively small part. The left subclavian 

 artery, which springs from the aortic arch, in the adult, is an intersegmental artery which sprang 

 originally from the primitive dorsal aorta. It may be presumed, therefore, that that portion of 

 the aortic arch which lies dorsal to the origin of the left subclavian artery is formed from the 

 dorsal roots of the fourth and sixth arches of the left side a presumption which is strengthened 

 by the fact that the ligamentum arteriosum, which is a remnant of the left sixth arch, is attached 

 to the opposite border of the aortic arch immediately dorsal to the origin of the left subclavian 

 artery. 



The sixth arch on the right side forms part of the extra-pulmonary portion of the right 

 pulmonary artery, the remainder of the extra-pulmonary part of the artery being derived from a 

 branch given off from the right sixth arch to the lung bud. 



The ventral part of the left sixth arch becomes absorbed into the stem of the pulmonary artery ; 

 therefore the left pulmonary artery is merely the branch from the left sixth arch to the lung bud. 

 The dorsal part of the left sixth arch forms, during foetal life, the ductus arteriosus, which 

 carries the venous blood from the right ventricle of the heart to the aorta. After birth it is con- 

 verted into the ligamentum arteriosum. 



The Branches of the External Carotid Artery. All the typical branches of the external carotid 

 artery are present in embryos 15-5 mm. long ; little is known, however, regarding the details 

 of their development. It is probable that the internal maxillary artery and its branches are 

 evolved partly from the ventral part of the first aortic arch and partly from an anastomosis with 

 the branches of a temporary stapedial artery, which develops from the dorsal end of the second 

 arch ; but it is not known whether the other branches of the external carotid spring as offsets 

 from the ventral roots of the first or second arches or from the ventral parts of the arches 

 themselves. 



The Descending Aorta. The greater part of the descending aorta is formed by the fusion 

 of the primitive dorsal aortae. In embryos about 2-6 mm. long, possessing twenty-three mesoder- 

 mal somites, the primitive dorsal aortae are fused together ' from the tenth to the sixteenth 

 segment (Fig. 820). At a later period the fusion is continued caudalwards to the twenty-third 

 body segment the level of the fourth lumbar vertebra where the common iliac arteries arise. 

 Still later the small terminal portions of the primitive dorsal aortae fuse together to form the 

 unpaired middle sacral artery, which terminates at the extremity of the coccygeal region. 



If the three somites which lie nearest the head end, in embryos possessing twenty-three somites, 



