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7th pair of inter-
1st pair of intersegmental arteries
The Aortic Arches and their Ventral and Dorsal Roots.-The aortic arches are the vessels which connect the ventral portions of the primitive ventral aorta with the primitive dorsal aorta. 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 of the pharynx, in the primitive dorsal aorta. All six arches are not present at the same time, for as the more caudally 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 of the ventral and dorsal portions of the primitive aorta together (Fig. 821). When the number of mesodermal somites has increased to twenty-three pairs and the embryo is about 2.5 mm. long, a second pair of aortic arches has appeared. They spring from the cephalic end of the heart, dorsal to the ventral roots of the first arches, and pass through the hyoid or second pair of branchial arches to the dorsal aortæ.
FIG. 821.-SCHEMA OF THE STAGE OF FIVE AORTIC ARCHES. The cardinal veins and ducts of Cuvier are not shown.
In embryos 5 mm. long the number of 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
FIG. 822. SCHEMA OF AORTIC ARCHES OF AN EMBRYO, 9 MM. LONG.
arches also move headwards and are transposed from the posterior to the anterior 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 end of the heart by the sixth aortic arch; and by the first, second, third, and fourth arches it is connected with a ventral vessel which springs from the cephalic end of the heart. The portion of each dorsal aorta which is caudal to any 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. 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
Right subclavian artery
Left subclavian artery
Left common carotid
Right common carotid
Left 6th arch
FIG. 823.—SCHEMA OF PART OF THE ARTERIAL SYSTEM OF A FETUS SEEN FROM THE LEFT 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 main stems of the permanent arterial system (Figs. 823, 824) The ventral roots of the first and second arches form the external caro
arches themselves and the dorsal roots of the second and first arches are converted into the internal carotids, which are prolonged headwards into the cerebral region, coincidently with the 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 aorta. 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 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
The Descending Aorta.-The greater part of the descending aorta is formed by the fusion of the primitive dorsal aorta. In embryos about 2-6 mm. long, possessing twenty-three mesoder mal somites, the primitive dorsal aorta 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 aorta 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.
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 aorta which is formed by the process of the primitive dorsal aortæ must move caudalwards during the developmental period.
The Branches of the Dorsal Aortæ. 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 primitive excretory organs-the Wolffian bodies; and the ventral branches pass to the alimentary canal, the yolk-sac, and to the placenta.
Internal carotid Right common carotidi root of 3rd arch
Right sub--clavian artery, Innominate artery
The dorsal branches are intersegmental in arrangement, and when they first appear they pass dorsally, in the intervals between the Dorsal 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 intersegmental 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 intersegmental 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.
Dorsal root of
Arch of aorta (4th arch)
Left subclavian artery Arch of aorta Ductus arteriosus
Arch of aorta
Left pulmonary artery
The dorsal branches of the intersegmental
FIG. 824.-SCHEMA OF PART OF THE VASCULAR SYSTEM
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 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 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 vertebræ, 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.
The ventral branch of the seventh cervical segmental artery on each side forms the trunk of the subclavian 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 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 intermediate 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 itself. As the yolk-sac atrophies the prolongations of the ventral branches which pass to its walls 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 them, 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 coeliac, 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 cœliac artery, which originally 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 mesenteric 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, on
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 primitive dorsal aorta. The origins of the arteries are gradually moved caudally as the embryo grows, until eventually, they spring from the primitive dorsal aorta. opposite the twenty-third body somite, that is the fourth lumbar segment. As each 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 duct. and then unites, on a plane ventral to the aorta, with the primitive umbilical artery of the same side. 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. From the newly formed vessel, which now constitutes the only origin of the umbilical artery, the inferior gluteal 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 the 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 formes 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 t the ventral wall of the abdomen, then cephalwards to the umbilicus and through the umbilic to the placenta, is still called the umbilical artery. After birth, when the placental circulation ceases, the greater part of the intra-abdominal portion of the umbilical artery atrophies and
becomes converted into the lateral umbilical ligament, but a portion of the part which lies in the pelvis minor remains pervious and from it springs the superior vesical artery.
Primitive ventral aorta
Primitive ventral aorta
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 aorta. development number of the vessels is reduced until only one remains. That loses its direct connexion with the aorta and becomes attached to the seventh segmental artery forming the ventral branch of that vessel, and the lateral division of the branch; the ventral continuation, past the lateral branch, being, apparently, a later formation. In the earlier stages the portion of the artery which lies in the free part of the limb does not consist of a single stem but of a plexiform series of vessels which only gradually become reduced to a stem. When the stem is definitely established it is divisible into subclavian, axillary, brachial, and volar interosseous portions. The median, the radial, and the ulnar arteries are of later formation.
In the case of the lower limb the earliest known artery the primitive inferior gluteal, which springs from the secondary root of origin of the umbilical artery. It is con
of atrioventricular canal Ventricle
Auricle of atrium
verted into the inferior gluteal Position of orifice-
The Heart. The rudiments of the heart are the
Position of orifice
FIG. 826.-DEVELOPMENT OF THE HEART. Diagram showing the changes of form and external appearances at different stages. Modified froin His's models. III.B and IV.B are side views; the other figures represent the heart as seen from the front.
caudal portions of the primitive ventral aortæ, 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