DEVELOPMENT OF THE AETEEIES. 



1029 



External carotids 

 i 

 I 

 A 



1st arch \ 



2nd arch\ 



Internal carotid 



Internal carotid 



Internal carotid 

 Right common carotid_"J^| 



Right sub _ 



clavian artery ~ j_ 



Innominate artery 



Right pulmonary"', 

 artery ^ 

 Ascending aorta~ 



Pulmonary artery 



Dorsal root of- 

 right 6th arch 



Internal carotid 



.External 



carotid 



External 

 carotid 



s Dorsal root of 

 /' left 3rd arch 

 _. Left common 

 carotid 

 Arch of aorta 

 th arch) 

 Left subcla- 

 /vian artery 

 ~" Arch of aorta 

 "" \ s Ductus 

 arteriosus 



\ Arch of 

 s aorta 



N Left pulmon- 

 ary artery 

 Dorsal aorta 



are cephalic somites, then the point of commencement of the median aorta would be situated at 

 the level of the seventh body somite, that is, at the situation of the future seventh cervical 

 vertebra. The position of the anterior point of fusion of the primitive dorsal aortae is indicated 

 in the adult by the origin of the abnormal right subclavian artery, and is situated at the 

 level of the fifth thoracic vertebra ; therefore 

 the anterior end of that part of the descending 

 aorta which is formed by the process of the 

 primitive dorsal aortae must move caudalwards 

 during the developmental period. 



The Branches of the Dorsal Aortae. Each 

 primitive dorsal aorta gives off a series of dorsal, 

 lateral, and ventral branches. The dorsal 

 branches are distributed to the neural tube and 

 body wall ; the lateral branches to the primi- 

 tive excretory organs the Wolffian bodies ; 

 and the ventral branches pass to the alimentary 

 canal, the yolk-sac, and to the placenta. 



The dorsal branches are intersegmental in 

 arrangement, and when they first appear they 

 pass dorsally, in the intervals between the DorsafrootofSrdarci 

 mesodermal somites, supplying the walls of the 

 neural tube and the adjacent mesoderm and 

 nerve ganglia. After a time each primitive 

 dorsal intersegmental artery gives off a ventral 

 branch which passes first laterally and then 

 ventrally in the body wall, towards the ventral 

 median line. At this time each interseg- 

 mental artery consists of a stem and a dorsal 

 and a ventral branch. As the ventral part of 

 the body increases in size more rapidly than 

 the neural tube and the vertebral region, the 

 ventral branch of each primitive interseg- 

 mental artery soon exceeds in size the dorsal 

 continuation, and simultaneously the stem of 

 each primitive iiitersegmental vessel enlarges. 

 Thus it is that the stems of the intersegmental 

 arteries and their lateral branches become the 

 trunks of the intercostal and lumbar arteries, 

 whilst the dorsal continuation of each primi- 

 tive vessel is reduced to the position of" a 

 posterior ramus. 



The dorsal branches of the intersegmental 

 arteries become connected together by longi- 

 tudinal anastomosing channels, some of which 

 lie ventral and others dorsal to the rudiments of 

 the transverse processes of the vertebras. Each 

 ventral branch of an intersegmental artery, as 

 it passes towards the mid-ventral line, gives off 

 a lateral branch to the tissues of the lateral part of the body wall, and at its termination it 

 becomes connected, by longitudinal anastomosing channels, with its more cranialward and caudal 

 neighbours. The lateral branches also become connected by longitudinal anastomoses. 



The dorsal branch of each intersegmental artery gives off a neural ramus to the walls of the 

 neural tube, which divides into dorsal and ventral neural branches ; these accompany the posterior 

 and anterior roots of the spinal nerves. As the dorsal and ventral neural branches approach the 

 median plane, those of each side become connected together by a longitudinal plexus of fine 

 vessels, and on the ventral surface of the neural tube the longitudinal plexuses of opposite 



i sides are connected together at or near the median plane. 



The permanent arteries derived partially or wholly from the primitive dorsal intersegmental 



arteries and their branches and anastomoses are : (1) The intercostal and lumbar arteries and 

 their posterior or dorsal rami ; (2) the subclavian and axillary arteries and their continuations 



i in the upper extremities ; (3) the vertebral arteries ; (4) the spinal arteries ; (5) the basilar 

 artery ; (6) the superior intercostal arteries ; (7) the internal mammary and the superior and 

 inferior epigastric arteries. 



In the cervical region the stems of the first six intersegmental arteries disappear, but the 

 seventh persists and forms, on each side, a portion of the stem of the corresponding subclavian 

 artery. The dorsal branch of the seventh segmental artery and the anastomoses, between it and 

 the first six dorsal branches, which pass ventral to the true transverse processes of the cervical 

 vertebrae, form the vertical part of the vertebral artery of the adult. The neural ramus of the 

 first cervical segmental artery and its preneural branch form the part of the vertebral artery 

 which lies on the atlas and passes to the ventral surface of the medulla oblongata. The cranial 

 or upward prolongation of the vertebral, to its union with the basilar, is developed from the 

 network of vessels which connects the medial ends of the preneural arteries. 



66 & 



Dorsal aorta 



FIG. 824. SCHEMA OF PART OF THE VASCULAR SYSTEM 



OF A F(ETUS SEEN FROM THE FRONT. The Origin 



of the positions of the first and second arches, 

 the dorsal roots of the third arches on both sides, 

 and the dorsal roots of the fourth and fifth arches 

 on the right side are shown in dotted lines. The 

 positions of the fifth arches are not shown. 



