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with one another, through the portions of the intersegmental vessels which they connect together, and thus form longitudinal trunks. The longitudinal trunks are mainly, though not exclusively, segmental. From them the main stem vessels of the individual are formed, and from or to these latter the intersegmental vessels appear to proceed as branches or tributaries.

So.SA. 1.

CAA.I

LB

D.D

V.D

D.D.

D.Sp.

L.B.

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Po.T.

3.

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Po.C.

4.
Pr.C.

Co.

IV

V

CCC

C.A.A. I, II, III, IV, V. The cephalic

aortic arches.

Co.

ticulum. P.D.A. Primitive dorsal aorta.

Anastomosing vessel between the
primitive ventral aorta and the
ventral somatic anastomosis.
Dorsal division of a somatic inter-
segmental artery.

5.

Dorsal splanchnic anastomosis.
Lateral branch of ventral division
of somatic intersegmental

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artery.

L.E.D. Branch to lateral enteric diver

THE SEGMENTAL
ARTERIES AND
THEIR ANAS-
TOMOSES.

The main longitudinal trunks are the primitive aorta. The descending aorta is formed, in the greater part of its extent, by the fusion of the dorsal parts of the primitive aorta. and from it the intersegmental, lateral and ventral arterie arise in pairs.

In a typical portion of the body of the embryo there are three arteries on each side. One arises from the dorsal surface of the primitive dorsal aorta, i.e. from the dorsal longitudinal trunk, and runs laterally and ventrally in the tissues developed from the somatic mesoderm; it is distributed to the body wall, including the vertebral column and its contents, and is termed a somatic intersegmental artery. A second vessel arises from the side of the primitive dorsal aorta; it is distributed to the structures developed from the intermediate cell mass, viz., the suprarenal gland, the kidney, and the ovary or the testis, and it is accordingly termed a lateral or an intermediate visceral artery. The third artery, which is known as the splanchnic artery, Cd.A.A. springs from the ventral surface of D.Sp. the aorta. It runs in the tissues developed from the splanchnic M.S. mesoderm, and supplies the wall Pa.C. of the alimentary canal.

FIG. 840.-DIAGRAM OF THE CEPHALIC AORTIC ARCHES, AND OF THE SEGMENTAL
AND INTERSEGMENTAL ARTERIES IN THE REGION IN FRONT OF THE UMBILICUS.

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Sp.SA.
D.Sp.

V.SP.

Vitelline vessels.

Ventral somatic anastomosis.
Ventral splanchnic anastomosis.

SOSA

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P.D.A.

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In the course of development the longitudinal trunks become the most important trunks in the individual, and they are formed before the branches and tributaries make their appearance.

MS.

Cd AA.

P.V.A.

Caudal aortic arch.
Dorsal splanchnic ana-
stomosis.

Middle sacral artery.
Parietal branch from
caudal arch.
Primitive dorsal aorta.
Primitive ventral aorta.

VED (Hy

V.V.

FIG. 841.-DIAGRAM OF THE ARTERIES IN THE REGION CAUDAL
TO THE UMBILICUS.

V.V. Vitelline vessels.

PaC.

So.S.A. Somatic intersegmentai
arteries.
Sp.S.A. Splanchnic arteries.
V.E.D. (Hy). Branch to a ventral
enteric diverticulum.
Visceral branch from the
caudal arch.

Vi.C.

The somatic intersegmental P.D.A. arteries form, in the early embryo, P.V.A. a regular series of paired vessels throughout the cervical, thoracic,

lumbar, and sacral regions. It is, however only in the thoracic and lumbar regions that their

original characters are retained. The paired vessels pass dorsally, by the sides of the vertebræ, and divide into dorsal and ventral branches which accompany the corresponding anterior and posterior divisions of the spinal nerves.

The ventral branches run ventro-laterally, between the ribs, in the thoracic region, and in corresponding positions in the lumbar region, and together with the stems they form the main parts or trunks of the vessels in the thoracic and lumbar regions. They are connected together, near their commencements, by a series of pre-costal anastomoses which pass in front of the necks of the ribs, and they are also connected together, near their terminations, by ventral anastomosing channels which run, in the thoracic region, behind the costal cartilages, and in the lumbar region behind or in the substance of the rectus abdominis muscle. Each ventral branch gives off a lateral offset which is distributed like the lateral cutaneous branch of a spinal nerve, and the ventral branch together with the stem of the intersegmental artery forms the trunk of an intercostal or lumbar artery in the adult.

The dorsal branches, which are present before the ventral branches, run dorsally between the transverse processes of the vertebræ, and form the posterior branches of the intercostal arteries and the dorsal branches of the lumbar arteries of the adult; they are connected, behind the necks of the ribs, by post-costal anastomoses, and again, behind the transverse processes of the vertebræ, by

Pre-laminar anastomosis

Pre-neural anastomosis

Post-neural anastomosis

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Ventral somatic anastomosis

FIG. 842.-DIAGRAM SHOWING THE ARRANGEMENT AND COMMUNICATIONS OF THE SEGMENTAL AND
INTERSEGMENTAL ARTERIES AT AN EARLY STAGE OF DEVELOPMENT.
C, Colom; IN, Intestine.

The

post-transverse anastomosing channels. Moreover, each dorsal branch, as it passes by the cor-
responding intervertebral foramen, gives off a spinal offset which enters the spinal canal, along
the corresponding nerve-root, and divides into a dorsal, a ventral, and a neural branch.
dorsal branches of these spinal arteries are connected together along the ventral surfaces of the
laminæ by pre-laminar anastomoses, and the ventral branches are united on the dorsal surfaces
of the vertebral bodies (or centra) with their fellows above and below by post-central anastomoses;
they are also united with their fellows of the opposite side by transverse communicating channels.
The neural branches of the spinal arteries divide similarly into dorsal and ventral branches;
the dorsal branches of each side are connected together by post-neural anastomoses, which form
the posterior spinal arteries; and the ventral branches unite, in the median line, both with their
fellows above and below and with those of the opposite side, forming a single longitudinal
pre-neural trunk, the anterior spinal artery.

In the thoracic and lumbar regions of the body the somatic intersegmental arteries persist, and form the intercostal and lumbar arteries. These vessels spring from the dorsal aspect of the descending aorta, usually in pairs. The corresponding vessels of opposite sides, however, occasionally fuse together at their origins, simultaneously with the fusion of the dorsal longitudinal trunks to form the descending aorta, and then the arteries of opposite sides arise by common

stems.

The pre-costal anastomoses between the ventral branches of the somatic intersegmental arteries are only represented in the thoracic region by the superior intercostal arteries; in the lumbar region they disappear entirely. The anastomoses between the anterior ends of the ventral

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branches of the somatic intersegmental arteries persist as the internal mammary and superior and inferior epigastric arteries.

The lateral offsets of the ventral branches are represented by the cutaneous arteries which accompany the lateral cutaneous branches of the spinal nerves, and the lateral branch of the seventh somatic intersegmental artery forms the greater part of the arterial stem of the upper limb.

The post-costal and post-transverse anastomoses usually disappear in the thoracic and lumbar regions, but the post-costal anastomoses occasionally persist in the upper thoracic region, and take part in the formation of the vertebral artery, which in such cases arises from the first or second intercostal artery. In some carnivora the post-costal longitudinal vessels persist in the upper thoracic region, and form, on each side, a trunk which is connected with the first aortic intercostal, and which supplies the first five intercostal spaces.

DSP

IN

VSP

C

VED

FIG. 843.-DIAGRAM OF THE SEGMENTAL AND INTERSEGMENTAL of somatic intersegmental arteries
ARTERIES AT A LATER PERIOD OF DEVELOPMENT THAN IN lose their connexions with the
FIG. 842.
dorsal roots of the aortic arches, ie,

Post-neural anastomosis
Post-central anastomosis

C, Cœlom; D.A, Dorsal aorta; D.Sp, Dorsal splanchnic anastomosis; in other words, with the longi-
IN, Intestine; V.E.D, Branch to ventral enteric diverticulum; tudinal anastomosing channels in
V.Sp, Ventral splanchnic anastomosis.
that region. The seventh pair,
however, persist in their entirety:
and from them are formed, on the right side, a portion of the subclavian trunk, and on the
left side the whole of the subclavian stem from its commencement up to the origin of the
vertebral artery. On each side the ventral branch of the seventh intersegmental artery forms
that portion of the
subclavian artery
which lies between
the origins of the
vertebral and internal
mammary arteries,
and also the trunk of
the internal mam-
mary artery as far as
the upper border of
the first costal cartil-
age. The remainder
of the internal mam-
mary artery repre-
sents the ventral
longitudinal anasto-
moses between the
ventral branches of
the seventh and the
following somatic in-
tersegmental arteries.
The continuation of
the subclavian artery,

Post-transverse anastomosis
Post-costal anastomosis

Pre-costal anastomosis

Somatic intersegmental_
artery

Primitive dorsal aorta
Lateral branch of a somatic)
intersegmental artery
Cephalic aortic arch

The pre-laminar, the postcentral, and the pre- and postneural anastomoses persist, the latter two aiding in the formation of the thoracic and lumbar portions of the anterior and posterior spinal arteries respectively.

It is in the cervical region, however, that the most interesting changes occur. The first six pairs

Pre-laminar anastomosis
Pre-neural anastomosis

6

IN

Branch to a ventral enteric diverticulum

SEGMENTAL ARTERIES IN THE REGION OF THE CEPHALIC AORTIC ARCHES.
IN, Intestine.

beyond the inner FIG. 844.-DIAGRAM SHOWING THE ARRANGEMENT AND COMMUNICATIONS OF THE
margin of the first
rib, is the persistent
and enlarged lateral offset of the ventral branch of the seventh somatic intersegmental artery,
which is continued into the upper limb, caudal or postaxial to the shoulder girdle. The thyreo-
cervical trunk and the superior intercostal artery, both branches of the subclavian artery, are

Branch to a lateral enteric ·
diverticulum

Primitive ventral aortat

Ventral somatic anastomosis

persistent pre-costal anastomoses, and the ascending cervical artery belongs to the same series of vessels. The vertebral artery, which appears as a branch of the subclavian in the adult, is, morphologically, somewhat complex. The first part represents the dorsal branch of the seventh somatic intersegmental artery; the second part, that passing through the cervical transverse processes, consists of the persistent post-costal anastomoses between the dorsal branches of the first seven intersegmental arteries; a third part, that lying on the arch of the atlas, is the spinal branch of the first somatic intersegmental artery and its neural continuation; whilst finally the upper part of the vertebral artery, the part in the cranial cavity, appears to represent a prolongation of the pre-neural anastomoses, which still farther upwards are probably represented by the basilar artery. As already stated, the post-costal anastomoses below the seventh intersegmental artery occasionally persist, and in such cases the vertebral may lose its connexion with the subclavian, and spring from one or other of the posterior branches of the upper intercostal arteries.

The profunda cervicis artery is to be regarded as a remnant of the post-transverse longitudinal anastomoses.

The origin of the seventh somatic intersegmental artery from the dorsal longitudinal trunk is, at first, some distance caudal to the sixth aortic arch, but, simultaneously with the elongation of the neck and the retraction of the heart into the thoracic region, it is shifted cranialwards until it is opposite the dorsal end of the fourth aortic arch.

The middle sacral artery is formed by the fusion of two vessels, each of which springs from the dorsal surface of the aorta. It is regarded as the direct continuation of the descending aorta.

The lateral or intermediate visceral arteries supply the organs derived from the intermediate cell mass. They form a somewhat irregular series of vessels in the adult, but presumably in the primitive condition there was a pair in each segment of the body; many of these disappear, however, and the series is only represented in the adult by the suprarenal, the right renal, part of the left renal, and the testicular or ovarian arteries-possibly, also, by some of the branches of the hypogastric arteries.

The splanchnic arteries arise in the embryo from the ventral aspects of the primitive dorsal aortæ, and are not strictly either segmental or intersegmental in arrangement. They are distributed to the walls of the alimentary canal. Each anastomoses with its immediate neighbours on the dorsal wall and the ventral walls of the gut.

After the fusion of the dorsal longitudinal trunks to form the descending aorta, the roots of each pair of the splanchnic arteries fuse into a common stem, or either the right or left artery altogether disappears, whilst at a later period the majority of the splanchnic arteries lose their direct connexion with the descending aorta; those which retain their connexion are the cœliac artery and the superior and inferior mesenteric arteries.

The bronchial and oesophageal arteries are later formations. They appear to correspond morphologically with the more primitive splanchnic arteries, but the developmental history is not known.

The left gastric branch of the coeliac artery, as it passes from its origin to the small curvature of the stomach, represents a right splanchnic artery; the remainder of the left gastric artery and the right gastric branch of the hepatic are remnants of the ventral anastomoses between the splanchnic arteries cephalwards of the umbilicus.

The splenic artery is a branch given off from a splanchnic artery to an organ developed in the mesogastrium, and the hepatic is a branch from the ventral splanchnic anastomoses to the hepatic diverticulum from the wall of the duodenal portion of the fore-gut.

The superior and inferior mesenteric arteries represent at their origins splanchnic branches, and in the remainder of their extent they represent the dorsal anastomoses on the gut wall.

THE AORTA, PULMONARY ARTERY, AND OTHER CHIEF STEM VESSELS.

The heart and the majority of the great arterial trunks of the body, including the aorta, the innominate, part of the right subclavian, the common, external, and greater parts of the internal carotids, and the pulmonary arteries, are all modified portions either of the primitive aortæ or of the aortic arches. The developmental changes, which result in the formation of the vessels named, are described in the preceding chapter, and the morphology of these vessels is obviously the same as that of the trunks from which they are derived."

It will be sufficient, therefore, to point out that the primitive aorta may be regarded as the greatly enlarged pre-central or pre-vertebral longitudinal anastomoses between the successive intersegmental arteries of each side; obviously, therefore, each primitive aorta, like the rest of the longitudinal anastomoses, consists chiefly of segmental elements. The origins of the intersegmental vessels enter into its formation only in so far as they connect the segmental vessels together, and so complete the longitudinal anastomoses.

The first cephalic aortic arches are simply portions of the primitive aorta. The other aortic arches have possibly a different morphological significance, but their exact nature is not definitely settled.

The second, third, fourth, fifth, and sixth cephalic aortic arches of each side are developed in the undivided mesoderm of the head region, caudal to the first arch. They spring from the part of the primitive aorta which, after the head fold is formed, lies on the ventral aspect of the foregut, and they extend, at the side of the pharyngeal part of the fore-gut, to the dorsal aorta. Thus, in some respects they may be looked upon as segmental vessels. In addition to the vessels already mentioned, there are given off from the ventral aorta and the aortic arches a series of branches which supply ventral and lateral diverticula from the alimentary canal; these are represented in the adult by the superior thyreoid, the thyreoidea ima.

Iliac Arteries and their Branches.-The common iliac arteries are formed from the secondary roots of the umbilical arteries, and their exact morphological position is uncertain. The true morphological position of the hypogastric arteries is not yet defined. They also are parts of the secondary roots of the umbilical arteries, and they give off both somatic and splanchnic branches; therefore they do not correspond either with somatic intersegmental or with splanchnic arteries. The branches of the hypogastric artery are arranged in two groups(1) a visceral set which supplies the walls of the hind-gut and the genital organs, and (2) a parietal set which is distributed to the body wall and to the hind-limbs. The branches distributed to the gut probably represent the splanchnic vessels, more or less homologous with ordinary splanchnic branches of the primitive aorta, and the parietal branches are possibly the homologues of intersegmental arteries.

THE LIMB ARTERIES.

In all probability the vessels of both the upper and the lower limbs are derived originally from several somatic intersegmental arteries, the majority of which, however, have atrophied. The upper limb is supplied in man by the lateral offset from the ventral branch of the seventh somatic intersegmental artery. It passes into the extremity caudal to the shoulder girdle, courses through the arm, enters the cubital fossa, and is continued through the forearm, in the early stages, as the volar interosseous artery, which terminates in the deep part of the palm, in the deep volar arch. At a later period, ontogenetically, a median artery appears as a branch of the parent stem, and it terminates in a superficial volar arch; still later the radial and ulnar branches are formed. The latter grow rapidly, soon exceeding in size the parent stem, and they terminate in the superficial and deep volar arches. The interosseous and median arteries decrease, and generally lose their direct connexions with the volar arches. The dorsal interosseous artery is also a secondary branch from the parent stem, and the digital arteries are offsets from the volar arterial arches.

The chief arteries of the lower extremities spring directly from the secondary roots of the umbilical arteries, and may be looked upon as being essentially intersegmental; whether they represent the whole or only parts of typical somatic intersegmental arteries, however, is not clear.

The arteries of the lower limbs certainly show no very obvious indications of division into dorsal and ventral branches, though such indications are not entirely wanting. In their comparative absence it is supposed that the dorsal branches have been either suppressed or incorporated with the common stems; that similarly the ventral branches and their lateral offsets are indistinguishably fused, and that probably both are represented in a limb artery.

The original stem vessel of the lower limb is the inferior gluteal artery, which is continued distally, caudal to the pelvic girdle, into the popliteal and peroneal arteries, and so to the plantar arch. Subsequently the external iliac artery is given off from the secondary root of the umbilical artery, dorsal to the origin of the inferior gluteal, and, passing into the limb cranialward of the pelvic girdle, it becomes the femoral artery. This vessel ultimately unites with the proximal part of the popliteal artery, and after this communication is established the distal part of the inferior gluteal atrophies and loses its connexion with the popliteal, which henceforth appears to be the direct continuation of the femoral trunk; therefore, whilst the main artery of the upper limb is formed by the prolongation of the lateral branch of one segmental artery, the corresponding vessel of the lower extremity is developed from representatives of, probably, two somatic segmental arteries, the external iliac and femoral trunks being the representatives of one, whilst the popliteal and its continuation, the peroneal, are parts of another.

The first main artery of the leg, ontogenetically, is the peroneal, which is continued into the plantar arch; after a time, however, the posterior and anterior tibial branches are given off from the stem, over which, as a rule, they soon preponderate in size, and they terminate in the plantar arch, whilst the parent trunk diminishes and loses its direct connexion with the arch.

The peroneal artery corresponds in position and development with the common interosseous trunk and the volar interosseous artery in the forearm. The posterior tibial apparently corre sponds with the median artery; it develops in a similar way, and has similar relations to homologous nerves, the tibial nerve representing the combined median and ulnar nerves of the upper extremity.

The anterior tibial artery represents the dorsal interosseous, whilst the radial and ulna: arteries of the upper extremity are not represented in the lower limb.

MORPHOLOGY OF THE VEINS.

Two dorsal longitudinal vessels, one on each side, connect the successive intersegmenta. veins together. They do not, however, in any part of their course, fuse together to form a single vessel comparable to the descending aorta.

Of these dorsal longitudinal vessels, that on the right side greatly enlarges, and from it the main stem vessels which return blood from the body walls, the head and neck, and the limbs, are almost entirely formed. The left dorsal longitudinal vessel remains relatively small-in parts, indeed, it altogether disappears-and the blood conveyed to it by the corresponding intersegmental veins is transmitted, across the median plane, to the chief functional stem by later

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