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The Aortic Arches and their Ventral and Dorsal Roots.—The aortic arches are the vessels which connect the ventral portions of the primitive ventral aortæ with the primitive dorsal aortæ. Six such arches are formed on each side. They spring from the ventral aortæ or from the heart, they pass round the side walls of the fore-gut, in the branchial arches, and they terminate, in relation with the dorsal wall
7th pair of interof the pharynx, in the primitive dorsal
segmental arteries aortæ. All six arches are not present at
1st pair of interthe same time, for as the more caudally
segmental arteries situated arches are formed those situated more cranialwards disappear (Figs. 821, 822). Five pairs of arches, the first four and the sixth, are present in embryos 5 mm. long; but by the time the length of the embryo has increased to 7 mm. the first two arches on each side have begun to disappear, and the very transitory fifth arch has been formed (Fig. 822)
The first aortic arch, on each side, is part of the primitive aorta, and is formed as the head fold is developed and the fore-gut is enclosed. It passes through the mandibular or first branchial arch, and connects the cephalic parts
1st cephalic aortic arch of the ventral and dorsal portions of the
2nd cephalic aortic arch
3rd cephalic aortic arch primitive aortæ together (Fig. 821). When
4th cephalic aortic arch the number of mesodermal somites has in
6th cephalic aortic arch creased to twenty-three pairs and the embryo
Bulbus cordis is about 2.5 mm. long, a second pair of aortic arches has appeared. They spring from the Atrium cephalic end of the heart, dorsal to the ventral sinus venosus roots of the first arches, and pass through the hyoid or second pair of branchial arches to Fig. 821.-SCHEMA OF THE STAGE OF FIVE AORTIC the dorsal aortæ.
ARCHES. The cardinal veins and ducts of Cuvier In embryos 5 mm. long the number of
are not shown. aortic arches has increased to five on each side—the first four and the sixth, the fifth appearing later, between the fourth and the sixth. At the period when five pairs of arches are present only four vessels spring from the cephalic end of the heart, which is now called the bulbus cordis; they are, on each side, an anterior stem which forms the ventral roots of the first and second arches, and a posterior stem common to the third, fourth, and sixth arches (Fig. 821). Up to this period the head lies quite close to the thorax, and a distinct neck can scarcely be said to exist. As the neck appears and the head is moved away from the thorax the third and fourth aortic
arches also move headwards 2nd arches atrophied
and are transposed from the
posterior to the anterior 5th arches
stem, which is simultaneously elongated. When the transposition is completed the condition depicted in Fig. 822 is attained ; the most cephalic, or anterior, part of each dorsal aorta is
connected with the cephalic 1st arches atrophied
Pulmonary arteries end of the heart by the sixth
aortic arch ; and by the External carotids
first, second, third, and Ventral root of 3rd atch
fourth arches it is connected Ventral root of 4th and 5th arches
with a ventral vessel which Truncus arteriosus
springs from the cephalic
The Fig. 822.-SCHEMA OF AORTIC ARCHES OF AN EMBRYO, 9 MM. LONG. (After
end of the heart. Tandler, modified.), The second and third arches have atrophied and portion of each dorsal aorta
which is caudal to any the transitory fifth has appeared.
given arch is called the
dorsal root of the arch, and the portion of the ventral stem which lies caudal to any arch is the ventral root of that arch. The sixth arch has no ventral root, for it arises directly from the heart. Its dorsal root converges towards and meets its fellow of the opposite side about the level of the tenth mesodermal somite, where the two fuse together to form the single dorsal aorta, which becomes the descending aorta of the adult, and which extends caudalwards to the level of the twenty-third somite, where it gives off the umbilical arteries and becomes the middle sacral artery:
By the time the embryo has attained a length of 7 mm. the first two arches have partly disappeared, only their dorsal and ventral extremities being left, and the transitory fifth arch has
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 Arch of aorta
the main stems Right subclavian artery
of the permanent 1 Left subclavian artery
arterial system Internal carotid Internal carotid :
Right subclavian artery
(Figs. 823, 824). Internal carotid;
Union of ductus arteriosus The ventral roots with aorta
of the first and Union of dorsal roots of 6th arches
second arches form
the external caroDorsal aorta tid. The third
arches themselves Left 6th arch
and the dorsal Right pul.
roots of the second monary artery
and first arches Left pulmonary artery
are converted into
Innominate artery External carotid
the internal caro
Right 6th arch
tids, which are Left common carotid
prolonged headRight common carotid
wards into the Arch of aorta, Left 6th arch
cerebral region, coAscending aorta
incidently with Fig. 823.—SCHEMA OF PART OF THE ARTERIAL SYSTEM OF A FETUS SEEN FROM THE LEFT the growth of
SIDE. Parts of the first and second arches, the dorsal roots of the third arches, the the head. dorsal part of the right sixth arch, and the dorsal roots of the right fourth and fifth The ventral arches have atrophied. The position of the fifth arch is not indicated ; see Fig. 822. 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 aortæ. 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 fætal life, the ductus arteriosus, which carries the venous blood from the right ventricle of the heart to the aorta. After birth it is converted 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 arche themselves.
The Descending Aorta. — The greater part of the descending aorta is formed by the fusion of the primitive dorsal aortæ. : In embryos about 2.6 mm. long, possessing twenty-three mesodermal somites, the primitive dorsal aortæ are fused together from the tenth to the sixteenth segment (Fig. 820). At a later period the fusion is continued caudalwards to the twenty-thin body segment—the level of the fourth lumbar vertebra-where the common iliac arteries arise Stiủ later the small terminal portions of the primitive dorsal aortæ 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,
Dorsal root of left 3rd arch
Arch of 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 aortæ 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
External carotids aorta which is formed by the process of the primitive dorsal aortæ must move caudalwards during the developmental period.
The Branches of the Dorsal Aorta.-- Each primitive dorsal aorta gives off a series of dorsal, lateral, and ventral branches. The dorsal
External branches are distributed to the neural tube and body wall; the lateral branches to the primi
External tive excretory organs - the Wolffian bodies ; and the ventral branches pass to the alimentary canal, the yolk-sac, and to the placenta.
Internal carotid The dorsal branches are intersegmental in
Internal carotid arrangement, and when they first appear they
Right common carotid
Arch of aorta pass dorsally, in the intervals between the Dorsal root of 3rd arch
(th arch) mesodermal somites, supplying the walls of the
Left subcla neural tube and the adjacent mesoderm and
Innominate artery nerve ganglia. After a time each primitive Right pulmonary
Ductus dorsal intersegmental artery gives off a ventral
artery branch which passes first laterally and then
Arch of ventrally in the body wall, towards the ventral Pulmonary artery median line. At this time each interseg
ary artery mental artery consists of a stem and a dorsal and a ventral branch. As the ventral part of
Dorsal root of
right 6th arch 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 intersegmental 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 primitive vessel is reduced to the position of a posterior ramus. The dorsal branches of the intersegmental
FIG. 824.-SCHEMA OF PART OF THE VASCULAR SYSTEM
OF A FÆTUS SEEN FROM THE FRONT. The origin arteries become connected together by longi
of the positions of the first and second arches, tudinal anastomosing channels, some of which the dorsal roots of the third arches on both sides, lie ventral and others dorsal to the rudiments of and the dorsal roots of the fourth and fifth arches Che transverse processes of the vertebræ. Each
on the right side are shown in dotted lines. The entral branch of an intersegmental artery, as positions of the fifth arches are not shown. t passes towards the mid-ventral line, gives off - 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 eighbours. 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 eural tube, which divides into dorsal and ventral neural branches; these accompany the posterior nd anterior roots of the spinal nerves. As the dorsal and ventral neural branches approach the nedian plane, those of each side become connected together by a longitudinal plexus of fine essels, and on the ventral surface of the neural tube the longitudinal plexuses of opposite des are connected together at or near the median plane.
The permanent arteries derived partially or wholly from the primitive dorsal intersegmental rteries and their branches and anastomoses are: (1) The intercostal and lumbar arteries and eir posterior or dorsal rami; (2) the subclavian and axillary arteries and their continuations 2 the upper extremities ; (3) the vertebral arteries ; (4) the spinal arteries ; (5) the basilar rtery; (6) the superior intercostal arteries ; (7) the internal mammary and the superior and nferior epigastric arteries.
In the cervical region the stems of the first six intersegmental arteries disappear, but the Eventh persists and forms, on each side, a portion of the stem of the corresponding subclavian tery. The dorsal branch of the seventh segmental artery and the anastomoses, between it and ne first six dorsal branches, which pass ventral to the true transverse processes of the cervical ertebræ, form the vertical part of the vertebral artery of the adult. The neural ramus of the rst cervical segmental artery and its preneural branch form the part of the vertebral artery hich lies on the atlas and passes to the ventral surface of the medulla oblongata. The cranial
upward prolongation of the vertebral, to its union with the basilar, is developed from the twork of vessels which connects the medial ends of the preneural arteries.
The ventral branch of the seventh cervical segmental artery on each side forms the trunk of the subclávian artery, from the origin of the vertebral to the origin of the internal mammary, and that part of the internal mammary which extends from its origin to the upper margin of the first costal cartilage. The remainder of the internal mammary artery, and the superior and inferior epigastric arteries, are derived from a longitudinal anastomosis which forms between the terminal extremities of the ventral branches of the intersegmental arteries.
The lateral ramus of the ventral branch of the seventh cervical intersegmental artery forms the trunk of the subclavian artery beyond the origin of the internal mammary branch, and from its prolongation into the upper extremity are formed the main arterial stems of the upper limb.
The superior intercostal arteries are derived from longitudinal anastomoses which connect together the ventral branches of the intersegmental arteries, on the inner sides of the dorsal parts of the ribs.
The anterior and posterior spinal arteries are derived from plexiform anastomoses which form on the dorsal and ventral aspects of the neural tube between the terminal ends of the dorsal and ventral neural branches of the neural rami of the posterior divisions of the intersegmental arteries
The lateral branches of the primitive dorsal aortæ supply structures derived from the inter. mediate cell tract, and from them are formed the renal, the suprarenal, the inferior phrenic, and the internal spermatic and ovarian arteries.
The ventral branches of the primitive dorsal aortæ are not definitely segmental or intersegmental in arrangement. In the early stages they pass not only to the gut wall but also). beyond it, to the wall of the yolk-sac. They are connected together by longitudinal anastomosing channels which lie in the dorsal mesentery of the gut and also upon the wall of the gut itseli. As the yolk-sac atrophies the prolongations of the ventral branches which pass to its wall: disappear and, simultaneously, the portions of the corresponding vessels of opposite sides, which lie in the mesentery, dorsal to the gut, and the longitudinal anastomoses which connect then, fuse together to form unpaired stem-trunks from which the three great vessels of the abdominal part of the alimentary canal are derived, namely, the cæliac, the superior mesenteric, and the inferior mesenteric arteries ; but the original stem of each of these three important vessels is not that which eventually forms its origin from the abdominal part of the aorta, for the celiar artery, which originalịy arose opposite the seventh cervical segment, wanders caudalwards to the twelfth thoracic segment as the roots of origin of the ventral vessels which are situated nearer the head disappear; and in the same manner the superior mesenteric is transposed from the level of the second thoracic to the level of the first lumbar segment, and the inferior mesenteri wanders from the twelfth thoracic to the third lumbar segment.
The Umbilical and Iliac Arteries.—It was pointed out, in the account of general features of embryology, that the umbilical arteries which carry blood to the placental area of the chorion arise, in a human embryo about 1:38 mm. long, about the level where the fourth cervical mesodermal somite will be developed at a later stage. They spring from plexuses formed, or
the lateral walls of the caudal part of the primitive gut, by the anastomosis of some of the most caudally situated ventral
or vitelline branches of the -Pronephric duct
primitive dorsal aorta. The -Secondary part of
origins of the arteries are
gradually moved caudally a: Primary part of -.
the embryo grow, until umbilical artery
eventually, they spring from
the primitive dorsal aorta. Fig. 825.-- DIAGRAM SHOWING THE FORMATION OF THE SECONDARY
opposite the twenty - third
body somite, that is the fourth PART OF THE PRIMITIVE UMBILICAL ARTERY.
umbilical artery passes from its origin on the ventral wall of the primitive dorsal aorta to the body-stalk it lies to the media. side of the pronephric duct. The ventral origin is, however, but temporary; for, by the time the embryo has attained a length of 5 mm., and the primitive dorsal aortæ have fused to form the permanent descending aorta, a new vessel has arisen, on each side, from the lateral part of the caudal end of the aorta. This new vessel passes ventrally, to the lateral side of the Wolffian daci, and then unites, on a plane ventral to the aorta, with the primitive umbilical artery of the sanside. After the union has taken place the ventral origin of the umbilical artery disappears, and the primitive umbilical artery, then arises from the side of the caudal end of the aorta. Fron the newly formed vessel, which now constitutes the only origin of the umbilical artery, the inferior glutäal artery, which is the primitive main artery of the lower limb, arises. At a later period, and at a more dorsal level, a second branch arises from the dorsal root of origin of that umbilical artery; this is the second main vessel of the lower extremity, which becomes the external iliac and the femoral arteries of the adult. As soon as the external iliac artery is forme that portion of the umbilical stem which lies dorsal to it becomes the common iliac artery, and the more ventral part, which descends into the pelvis minor, becomes the hypogastric artery But that portion of the original umbilical artery which runs along the side of the pelvis minor te the ventral wall of the abdomen, then cephalwards to the umbilicus and through the umbilicuto the placenta, is still called the umbilical artery. After birth, when the placental circulatia ceases, the greater part of the intra-abdominal portion of the umbilical artery atrophies ani
External iliac artery..
becomes converted into the lateral umbilical ligament, but a portion of the part which lies in the pelvis minor reinains pervious and from it springs the superior vesical artery.
The Arteries of the Limbs.—Our knowledge of the development of the arteries of the limbs is still very deficient, but during the last few years some investigations have been made and certain facts have been established. The very earliest stages of development have not yet been seen in the human subject, but it is not probable that they differ, in any essential respects, from those found in other mammals; therefore it may be assumed that the upper limb is supplied, at first, by a number of branches which spring from the sides of the primitive dorsal aortæ. As development proceeds the number of the vessels is reduced until only one remains.
I That loses its direct connexion with the aorta and becomes
Primitive ventral aorta
Primitive ventral aorta attached to the seventh segmental artery forming the ventral branch of that vessel, and the lateral division of the branch; the ventral continua
Truncus arteriosus tion, past the lateral branch,
II being, apparently, a later formation. In the earlier stages the portion of the artery which lies in the free part of the limb
Sinus venosus does not consist of a single stem
Vitelline vein but of a plexiform series of vessels which only gradually become reduced to a stem.
Truncus arteriosus When the stem is definitely established it is divisible into
Bulbus cordis subclavian, axillary, brachial, and volar interosseous portions. Atrio-ventricular
Atrio-ventricular The median, the radial, and the ulnar arteries are of later
Ventricle formation. Vitelline vein
Vitelline vein In the case of the lower limb the earliest known artery is
Truncus arteriosus the primitive inferior
IVA IVB gluteal, which springs from
Bulbus cordis the secondary root of origin of
Bulbus cordis the umbilical artery. It is conAtrium
Atrium verted into the inferior gluteal Position of orificeof atrio
Atrio-ventriand popliteal arteries of the ventricular canal
cular canal adult. The external iliac and Ventricle
Ventricle femoral arteries are parts of a later formation which arises Vitelline vein
-Vitelline vein from the cephalic aspect of the secondary root of the umbilical
Truncus arteriosus artery dorsal to the
origin of the inferior gluteal. This second- Auricle of atrium
-Auricle of atriúin ary vessel anastomoses distally,
Bulbus cordis at the level of the proximal Position of orifice part of the popliteal fossa, with
ventricular canal the primitive inferior gluteal,
Right ventricle and, afterwards, the proximal part of the primitive inferior gluteal undergoes a certain amount of atrophy. The developmental history of the
Fig. 826.- DEVELOPMENT OF THE HEART. arteries of the leg and foot is Diagram showing the changes of form and external appearances at different not definitely known.
stages. Modified froin His's models. III.B and IV.B are side The Heart. — The rudi- views; the other figures represent the heart as seen from the front. ments of the heart are the caudal portions of the primitive ventral aorte, which lie in the ventral wall of the fore-gut and the dorsal wall of the pericardium; therefore the heart may be said to consist, at first, of two tubes ; the two tubes fuse, and the heart is then a single median tube, separated by constrictions into five compartments which, from the caudal to the cephalic end, are the sinus venosus, the atrium, the ventricle, the bulbus cordis, and the truncus arteriosus. The constricted region between the atrium and the ventricle is called the atrio-ventricular canal.
At a later stage the longitudinal tube becomes folded on its long axis and at the same time twisted. The caudal limb of the loop thus produced is formed by the sinus venosus, the atrium, and part of the ventricle ; and the cranial limb by the remainder of the ventricle, the bulbus cordis, and the short truncus arteriosus. Subsequently, for a time, the ventricular and bulbar