THE BRANCHES OF THE PRIMITIVE DORSAL AORTZ. | 883 
together with the dorsal roots of the second and first arches, and is continued forwards to 
the cerebrum by an outgrowth from the anterior end of the dorsal root of the first arch. 
The ventral root of the third arch becomes the common carotid artery. The dorsal root 
of the third arch disappears. On the right side the ventral root of the fourth arch forms the 
innominate artery, and the arch itself is converted into part of the subclavian artery, whilst 
the dorsal root disappears. On the left side the ventral root of the fourth arch forms the 
small portion of aorta which lies between the innominate and left common carotid arteries. 
The left fourth arch itself and its dorsal root form the arch of the aorta from the origin 
of the left carotid artery to the attachment of the ductus arteriosus. i 
The ventral portion of the fifth arch on each side remains as the first part of the cor- 
responding pulmonary artery. The dorsal part disappears early on the right side, but on 
the left side it persists and remains patent up to birth as the ductus arteriosus. After 
birth it is obliterated and transformed into a fibrous cord. 
The fate of the primary caudal arches, their secondary successors, and that of the 
posterior ventral portions of the primitive aortze, has already been fully considered (p. 878). 
THE PRIMITIVE DoRSAL AORTA—FORMATION OF THE DESCENDING AORTA 
Coincidently with the development of the cephalic aortic arches the anterior portions 
of the dorsal sections of the primitive aortee are converted into the dorsal roots of the 
aortic arches. As already pointed out, certain parts of these disappear entirely, whilst 
other parts are utilised in the formation of the permanent vessels. 
Behind the fifth arches the two primitive dorsal aortee remain separate as far back as 
the root of the fore-limb. To this extent the right vessel disappears, whilst the left 
remains and forms a portion of the descending aorta. From the roots of the fore-limbs 
backwards to the caudal arches the dorsal aortz fuse together, beneath the vertebral 
column, to form the remainder of the descending aorta. 
THE BRANCHES OF THE PRIMITIVE DoRSAL AORT. 
Each primitive dorsal aorta gives off from its dorsal surface a series of somatic seg- 
mental arteries, from its sides an irregular series of intermediate (visceral) branches, and 
from its ventral surface a group of segmental splanchnic branches to the walls of the 
alimentary canal see (Figs. 609 and 610). 
The somatic segmental arteries divide into ventral and dorsal branches which accom- 
pany the posterior and anterior primary branches of the spinal nerves respectively, and 
the ventral branches give off lateral offsets. The various branches of the somatic seg- 
mental vessels anastomose freely together. 
The splanchnic segmental arteries also anastomose freely together. Ultimately, from 
the somatic vessels and their branches and anastomoses are developed the vertebral, the 
basilar, and the spinal arteries, part of the right subclavian artery, the whole of the left 
subclavian artery, and their continuations in the fore-limbs, the intercostal and lumbar 
arteries, and the internal mammary and deep epigastric arteries ; whilst from the splanchic 
segmental arteries the majority of the blood-vessels which supply the alimentary canal are 
developed. The intermediate visceral arteries supply the organs derived from the inter- 
mediate cell mass, viz. the suprarenal capsules, the kidney, and the cvaries or testicles ; 
but for a full account of the transformations which the various vessels and anastomoses 
undergo, reference must be made to the account of the morphology of the arteries. 
THE ARTERIES OF THE LIMBS. 
Little is known of the precise details of the development of the arteries of the limbs, 
but there is little doubt that they are formed almost entirely by prolongations of or from 
somatic segmental arteries or their branches. 
The chief arterial stem of each upper extremity is represented by the subclavian, the 
axillary, the brachial, and the anterior interosseous arteries ; these vessels form a con- 
tinuous trunk which is developed, on the left side entirely and on the right side mainly, 
from the corresponding seventh somatic segmental branch of the primitive aorta, from its 
ventral branch, and from the lateral offset of the latter. It is indeed the later ‘al offset 
of the ventral branch, growing outwards into the developing limb, which forms the 
prolongation of the stem. 
The root of the right stem vessel, constituted by the right subclavian artery from its 
