Chapter XXI (2)

The fourth arch always continues to give rise, as in the Anura, to the system of the dorsal aorta.

[FIG. 365. DEVELOPMENT OF THE GREAT ARTERIAL TRUNKS IN THE EMBRYOS
OF A. A LIZARD; B. THE COMMON FOWL; C. THE PIG. (From Gegenbaur;
after Rathke.)

The first two arches have disappeared in all three. In A and B the
last three are still complete, but in C the last two are alone
complete.

_p._ pulmonary artery springing from the fifth arch, but still
connected with the system of the dorsal aorta by a ductus Botalli;
_c._ external carotid; _c´._ internal carotid; _ad._ dorsal aorta;
_a._ auricle; _v._ ventricle; _n._ nasal pit; _m._ rudiment of
fore-limb.]

In all Reptiles it persists on both sides (fig. 366 A and B), but with the division of the truncus arteriosus into three vessels one of these, _i.e._ that opening furthest to the left side of the ventricle (_e_ and _d_), is continuous with the _right_ fourth arch, and also with the common carotid arteries (_c_); while a second springing from the right side of the ventricle is continuous with the _left_ fourth arch (_h_ and _f_). The right and left divisions of the fourth arch meet however on the dorsal side of the oesophagus to give origin to the dorsal aorta (_g_).

In Birds (fig. 366 C) the _left_ fourth arch (_h_) loses its connection with the dorsal aorta, though the ventral part remains as the root of the left subclavian. The truncus arteriosus is moreover only divided into two parts, one of which is continuous with all the systemic arteries. Thus it comes about that in Birds the right fourth arch (_e_) alone gives rise to the dorsal aorta.

In Mammals (fig. 366 D) the truncus arteriosus is only divided into two, but the _left fourth arch_ (_e_), instead of the right, is that continuous with the dorsal aorta, and the right fourth arch (_i_) is only continued into the right vertebral and right subclavian arteries.

The fifth arch always gives origin to the pulmonary artery (fig. 365, _p_) and is continuous with one of the divisions of the truncus arteriosus. In Lizards (fig. 366 A, _i_), Chelonians and Birds (fig. 366 C, _i_) and probably in Crocodilia, the right and left pulmonary arteries spring respectively from the right and left fifth arches, and during the greater part of embryonic life the parts of the fifth arches between the origins of the pulmonary arteries and the system of the dorsal aorta are preserved as ductus Botalli. These ductus Botalli persist for life in the Chelonia. In Ophidia (fig. 366 B, _h_) and Mammalia (fig. 366 D, _m_) only one of the fifth arches gives origin to the two pulmonary arteries, viz. that on the right side in Ophidia, and the left in Mammalia.

The ductus Botalli of the fifth arch (known in Man as the ductus arteriosus) of the side on which the pulmonary arteries are formed, may remain (_e.g._ in Man) as a solid cord connecting the common stern of the pulmonary aorta with the systemic aorta.

The main history of the arterial arches in the Amniota has been sufficiently dealt with, and the diagram, fig. 366, copied from Rathke, shews at a glance the character of the metamorphosis these arches undergo in the different types. It merely remains for me to say a few words about the subclavian and vertebral arteries.

The subclavian arteries in Fishes usually spring from the trunks connecting the branchial veins with the dorsal aorta. This origin, which is also found in Amphibia, is typically found in the embryos of the Amniota. In the Lizards this origin persists through life, but both subclavians spring from the right side. In most other types the origin of the subclavians is carried upwards, so that they usually spring from a trunk common to them and the carotids (arteria anonyma) (Birds and some Mammals); or the left one, as in Man and some other Mammals, arises from the systemic aorta just beyond the carotids. Various further modifications in the origin of the subclavians of the same general nature are found in Mammalia, but they need not be specified in detail. The vertebral arteries usually arise in close connection with the subclavians, but in Birds they arise from the common carotids.

[FIG. 366. DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OF THE ARTERIAL
ARCHES IN A LIZARD A, A SNAKE B, A BIRD C AND A MAMMAL D. (From
Mivart; after Rathke.)

A. _a._ internal carotid; _b._ external carotid; _c._ common
carotid; _d._ ductus Botalli between the third and fourth arches;
_e._ right aortic trunk; _f._ subclavian; _g._ dorsal aorta; _h._
left aortic trunk; _i._ pulmonary artery; _k._ rudiment of ductus
Botalli between the pulmonary artery and the system of the dorsal
aorta.

B. _a._ internal carotid; _b._ external carotid; _c._ common
carotid; _d._ right aortic trunk; _e._ vertebral artery; _f._ left
aortic trunk of dorsal aorta; _h._ pulmonary artery; _i._ ductus
Botalli of pulmonary artery.

C. _a._ internal carotid; _b._ external carotid; _c._ common
carotid; _d._ systemic aorta; _e._ fourth arch of right side (root
of dorsal aorta); _f._ right subclavian; _g._ dorsal aorta; _h._
left subclavian (fourth arch of left side); _i._ pulmonary artery;
_k._ and _l._ right and left ductus Botalli of pulmonary arteries.

D. _a._ internal carotid; _b._ external carotid; _c._ common carotid; _d._ systemic aorta; _e._ fourth arch of left side (root of dorsal aorta); _f._ dorsal aorta; _g._ left vertebral artery; _h._ left subclavian artery; _i._ right subclavian (fourth arch of right side); _k._ right vertebral; _l._ continuation of right subclavian; _m._ pulmonary artery; _n._ ductus Botalli of pulmonary artery.]

BIBLIOGRAPHY _of the Arterial System_.

(496) H. Rathke. "Ueb. d. Entwick. d. Arterien w. bei d. Säugethiere von d. Bogen d. Aorta ausgehen." Müller's _Archiv_, 1843.

(497) H. Rathke. "Untersuchungen üb. d. Aortenwurzeln d. Saurier." _Denkschriften d. k. Akad. Wien_, Vol. XIII. 1857.

_Vide_ also His (No. 232) and general works on Vertebrate Embryology.

_The Venous System._

The venous system, as it is found in the embryos of Fishes, consists in its earliest condition of a single large trunk, which traverses the splanchnic mesoblast investing the part of the alimentary tract behind the heart. This trunk is directly continuous in front with the heart, and underlies the alimentary canal through both its præanal and postanal sections. It is shown in section in fig. 367, _v_, and may be called the subintestinal vein. This vein has been found in the embryos of Teleostei, Ganoidei, Elasmobranchii and Cyclostomata, and runs parallel to the dorsal aorta above, into which it is sometimes continued behind (Teleostei, Ganoidei, etc.).

In Elasmobranch embryos the subintestinal vein terminates, as may be gathered from sections (fig. 368, _v.cau_), shortly before the end of the tail. The same series of sections also shews that at the cloaca, where the gut enlarges and comes in contact with the skin, this vein bifurcates, the two branches uniting into a single vein both in front of and behind the cloaca.

In most Fishes the anterior part of this vein atrophies, the caudal section alone remaining, but the anterior section of it persists in the fold of the intestine in Petromyzon, and also remains in the spiral valve of some Elasmobranchii. In Amphioxus, moreover, it forms, as in the embryos of higher types, the main venous trunk, though even here it is usually broken up into two or three parallel vessels.

It no doubt represents one of the primitive longitudinal trunks of the vermiform ancestors of the Chordata. The heart and the branchial artery constitute a specially modified anterior continuation of this vein. The dilated portal sinus of Myxine is probably also part of it; and if this is really rhythmically contractile[231] the fact would be interesting as shewing that this quality, which is now localised in the heart, was once probably common to the subintestinal vessel for its whole length.

[231] J. Müller holds that this sack is not rhythmically
contractile.

[FIG. 367. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY
YOUNGER THAN 28 F.

_sp.c._ spinal canal; _W._ white matter of spinal cord; _pr._
posterior nerve-roots; _ch._ notochord; _x._ subnotochordal rod;
_ao._ aorta; _mp._ muscle plate; _mp´._ inner layer of muscle-plate
already converted into muscles; _Vr._ rudiment of vertebral body;
_st._ segmental tube; _sd._ segmental duct; _sp.v._ spiral valve;
_v._ subintestinal vein; _p.o._ primitive generative cells.]

On the development of the cardinal veins (to be described below) considerable changes are effected in the subintestinal vein. Its postanal section, which is known in the adult as the caudal vein, unites with the cardinal veins. On this junction being effected retrogressive changes take place in the præanal section of the original subintestinal vessel. It breaks up in front into a number of smaller vessels, the most important of which is a special vein, which lies in the fold of the spiral valve, and which is more conspicuous in some Elasmobranchii than in Scyllium, in which the development of the vessel has been mainly studied. The lesser of the two branches connecting it round the cloaca with the caudal vein first vanishes, and then the larger; and the two posterior cardinals are left as the sole forward continuations of the caudal vein. The latter then becomes prolonged forwards, so that the two cardinals open into it some little distance in front of the hind end of the kidneys. By these changes, and by the disappearance of the postanal section of the gut, the caudal vein is made to appear as a supraintestinal and not, as it really is, a _subintestinal vessel_.

From the subintestinal vein there is given off a branch which supplies the yolk-sack. This leaves the subintestinal vein close to the liver. The liver, on its development, embraces the subintestinal vein, which then breaks up into a capillary system in the liver, the main part of its blood coming at this period from the yolk-sack.

The portal system is thus established from the subintestinal vein; but is eventually joined by the various visceral, and sometimes by the genital, veins as they become successively developed.

The blood from the liver is brought back to the sinus venosus by veins known as the hepatic veins, which, like the hepatic capillary system, are derivatives of the subintestinal vessel.

There join the portal system in Myxinoids and many Teleostei a number of veins from the anterior abdominal walls, representing a commencement of the anterior abdominal or epigastric vein of higher types[232].

[232] Stannius, _Vergleich. Anat._, p. 251.

[FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN
EMBRYO OF THE SAME AGE AS FIG. 28 F.

A. is the posterior section.

_nc._ neural canal; _al._ postanal gut; _alv._ caudal vesicle of
postanal gut; _x._ subnotochordal rod; _mp._ muscle-plate; _ch._
notochord; _cl.al._ cloaca; _ao._ aorta; _v.cau._ caudal vein.]

In the higher Vertebrates the original subintestinal vessel never attains a full development, even in the embryo. It is represented by (1) the ductus venosus, which, like the true subintestinal vein, gives origin (in the Amniota) to the vitelline veins to the yolk-sack, and (2) by the caudal vein. Whether the partial atrophy of the subintestinal vessel was primitively caused by the development of the cardinal veins, or for some other reason, it is at any rate a fact that in all existing Fishes the cardinal veins form the main venous channels of the trunk.

Their later development than the subintestinal vessel as well as their absence in Amphioxus, probably indicate that they became evolved, at any rate in their present form, within the Vertebrate phylum.

The embryonic condition of the venous system, with a single large subintestinal vein is, as has been stated, always modified by the development of a paired system of vessels, known as the cardinal veins, which bring to the heart the greater part of the blood from the trunk.

[FIG. 369. DIAGRAM OF THE PAIRED VENOUS SYSTEM OF A FISH. (From
Gegenbaur.)

_j._ jugular vein (anterior cardinal vein); _c._ posterior cardinal
vein; _h._ hepatic veins; _sv._ sinus venosus; _dc._ ductus
Cuvieri.]

The cardinal veins appear in Fishes as four paired longitudinal trunks (figs. 363 and 369), two anterior (_j_) and two posterior (_c_). They unite into two transverse trunks on either side, known as the ductus Cuvieri (_dc_), which fall into the sinus venosus, passing from the body wall to the sinus by a lateral mesentery of the heart already spoken of (p. 627, fig. 352). The anterior pair, known as the anterior cardinal or jugular veins, bring to the heart the blood from the head and neck. They are placed one on each side above the level of the branchial arches (fig. 299, _a.cv_). The posterior cardinal veins lie immediately dorsal to the mesonephros (Wolffian body), and are mainly supplied by the blood from this organ and from the walls of the body (fig. 275, _c.a.v_). In many forms (Cyclostomata, Elasmobranchii and many Teleostei) they unite posteriorly with the caudal veins in the manner already described, and in a large number of instances the connecting branch between the two systems, in its passage through the mesonephros, breaks up into a capillary network, and so gives rise to a renal portal system.

The vein from the anterior pair of fins (subclavian) usually unites with the anterior jugular vein.

The venous system of the Amphibia and Amniota always differs from that of Fishes in the presence of a new vessel, the vena cava inferior, which replaces the posterior cardinal veins; the latter only being present, in their piscine form, during embryonic life. It further differs from that of all Fishes, except the Dipnoi, in the presence of pulmonary veins bringing back the blood directly from the lungs.

In the embryos of all the higher forms the general characters of the venous system are at first the same as in Fishes, but with the development of the vena cava inferior the front sections of the posterior cardinal veins atrophy, and the ductus Cuvieri, remaining solely connected with the anterior cardinals and their derivatives, constitute the superior venæ cavæ. The inferior cava receives the hepatic veins.

Apart from the non-development of the subintestinal vein the visceral section of the venous system is very similar to that in Fishes.

The further changes in the venous system must be dealt with separately for each group.

Amphibia. In Amphibia (Götte, No. 296) the anterior and posterior cardinal veins arise as in Pisces. From the former the internal jugular vein arises as a branch; the external jugular constituting the main stem. The subclavian with its large cutaneous branch also springs from the system of the anterior cardinal. The common trunk formed by the junction of these three veins falls into the ductus Cuvieri.

The posterior cardinal veins occupy the same position as in Pisces, and unite behind with the caudal veins, which Götte has shewn to be originally situated below the postanal gut. The iliac veins unite with the posterior cardinal veins, where the latter fall into the caudal vein. The original piscine condition of the veins is not long retained. It is first of all disturbed by the development of the _anterior_ part of the important unpaired venous trunk which forms in the adult the vena cava inferior. This is developed independently, but unites behind with the right posterior cardinal. From this point backwards the two cardinal veins coalesce for some distance, to give rise to the _posterior_ section of the vena cava inferior, situated between the kidneys[233]. The anterior sections of the cardinal veins subsequently atrophy. The posterior part of the cardinal veins, from their junction with the vena cava inferior to the caudal veins, forms a rhomboidal figure. The iliac vein joins the outer angle of this figure, and is thus in direct communication with the inferior vena cava, but it is also connected with a longitudinal vessel on the outer border of the kidneys, which receives transverse vertebral veins and transmits their blood to the kidneys, thus forming a renal portal system. The anterior limbs of the rhomboid formed by the cardinal veins soon atrophy, so that the blood from the hind limbs can only pass to the inferior vena cava through the renal portal system. The posterior parts of the two cardinal veins (uniting in the Urodela directly with the unpaired caudal vein) still persist. The iliac veins also become directly connected with a new vein, the anterior abdominal vein, which has meanwhile become developed. Thus the iliac veins become united with the system of the vena cava inferior through the vena renalis advehens on the outer border of the kidney, and with the anterior abdominal veins by the epigastric veins.

[233] This statement of Götte's is opposed to that of Rathke for
the Amniota, and cannot be considered as completely established.

The visceral venous system begins with the development of two vitelline veins, which at first join the sinus venosus directly. They soon become enveloped in the liver, where they break up into a capillary system, which is also joined by the other veins from the viscera. The hepatic system has in fact the same relations as in Fishes. Into this system the anterior abdominal vein also pours itself in the adult. This vein is originally formed of two vessels, which at first fall directly into the sinus venosus, uniting close to their opening into the sinus with a vein from the truncus arteriosus. They become prolonged backwards, and after receiving the epigastric veins above mentioned from the iliac veins, and also veins from the allantoic bladder, unite behind into a single vessel. Anteriorly the right vein atrophies and the left continues forward the unpaired posterior section.

A secondary connection becomes established between the anterior abdominal vein and the portal system; so that the blood originally transported by the former vein to the heart becomes diverted so as to fall into the liver. A remnant of the primitive connection is still retained in the adult in the form of a small vein, the so-called vena bulbi posterior, which brings the blood from the walls of the truncus arteriosus directly into the anterior abdominal vein.

The pulmonary veins grow directly from the heart to the lungs.

For our knowledge of the development of the venous system of the Amniota we are mainly indebted to Rathke.

Reptilia. As an example of the Reptilia the Snake may be selected, its venous system having been fully worked out by Rathke in his important memoir on its development (No. 300).

The anterior (external jugular) and posterior cardinal veins are formed in the embryo as in all other types (fig. 370, _vj_ and _vc_); and the anterior cardinal, after giving rise to the anterior vertebral and to the cephalic veins, persists with but slight modifications in the adult; while the two ductus Cuvieri constitute the superior venæ cavæ.

The two posterior cardinals unite behind with the caudal veins. They are placed in the usual situation on the dorsal and outer border of the kidneys.

With the development of the vena cava inferior, to be described below, the blood from the kidneys becomes mainly transported by this vessel to the heart; and the section of the posterior cardinals opening into the ductus Cuvieri gradually atrophies, their posterior parts remaining however on the outer border of the kidneys as the venæ renales advehentes[234].

[234] Rathke's account of the vena renalis advehens is thus
entirely opposed to that which Götte gives for the Frog, but my
own observations on the Lizard incline me to accept Rathke's
statements, for the Amniota at any rate.

[FIG. 370. ANTERIOR PORTION OF THE VENOUS SYSTEM OF AN EMBRYONIC
SNAKE. (From Gegenbaur; after Rathke.)

_vc._ posterior cardinal vein; _vj._ jugular vein; _DC._ ductus
Cuvieri; _vu._ allantoic vein; _v._ ventricle; _ba._ truncus
arteriosus; _a._ visceral clefts; _l._ auditory vesicle.]

While the front part of the posterior cardinal veins is undergoing atrophy, the intercostal veins, which originally poured their blood into the posterior cardinal veins, become also connected with two longitudinal veins--the posterior vertebral veins--which are homologous with the azygos and hemiazygos veins of Man; and bear the same relation to the anterior vertebral veins that the anterior and posterior cardinals do to each other.

These veins are at first connected by transverse anastomoses with the posterior cardinals, but, on the disappearance of the front part of the latter, the whole of the blood from the intercostal veins falls into the posterior vertebral veins. They are united in front with the anterior vertebral veins, and the common trunk of the two veins on each side falls into the jugular vein.

The posterior vertebral veins are at first symmetrical, but after becoming connected by transverse anastomoses, the right becomes the more important of the two.

The vena cava inferior, though considerably later in its development than the cardinals, arises fairly early. It constitutes in front an unpaired trunk, at first very small, _opening into the right allantoic vein_, close to the heart. Posteriorly it is continuous with two veins placed on the inner border of the kidneys[235].

[235] The vena cava inferior does not according to Rathke's
account unite behind with the posterior cardinal veins, as it is
stated by Götte to do in the Anura. Götte questions the accuracy
of Rathke's statements on this head, but my own observations are
entirely in favour of Rathke's observations, and lend no support
whatever to Götte's views.

The vena cava inferior passes through the dorsal part of the liver, and in doing so receives the hepatic veins.

The portal system is at first constituted by the vitelline vein, which is directly continuous with the venous end of the heart, and at first receives the two ductus Cuvieri, but at a later period unites with the left ductus. It soon receives a mesenteric vein bringing the blood from the viscera, which is small at first but rapidly increases in importance.

The common trunk of the vitelline and mesenteric veins, which may be called the portal vein, becomes early enveloped by the liver, and gives off branches to this organ, the blood from which passes by the hepatic veins to the vena cava inferior. As the branches in the liver become more important, less and less blood is directly transported to the heart, and finally the part of the original vitelline vein in front of the liver is absorbed, and the whole of the blood from the portal system passes from the liver into the vena cava inferior.

The last section of the venous system to be dealt with is that of the anterior abdominal vein. There are originally, as in the Anura, two veins belonging to this system, which owing to the precocious development of the bladder to form the allantois, constitute the allantoic veins (fig. 370, _vu_).

These veins, running along the anterior abdominal wall, are formed somewhat later than the vitelline vein, and fall into the two ductus Cuvieri. They unite with two epigastric veins (homologous with those in the Anura), which connect them with the system of the posterior cardinal veins. The left of the two eventually atrophies, so that there is formed an unpaired allantoic vein. This vein at first receives the vena cava inferior close to the heart, but eventually the junction of the two takes place in the region of the liver, and finally the anterior abdominal vein (as it comes to be after the atrophy of the allantois) joins the portal system and breaks up into capillaries in the liver[236].

[236] The junction between the portal system and the anterior
abdominal vein is apparently denied by Rathke (No. 300, p. 173),
but this must be an error on his part.

In Lizards the iliac veins join the posterior cardinals, and so pour part of their blood into the kidneys; they also become connected by the epigastric veins with the system of the anterior abdominal or allantoic vein. The subclavian veins join the system of the superior venæ cavæ.

The venous system of Birds and Mammals differs in two important points from that of Reptilia and Amphibia. Firstly the anterior abdominal vein is only a foetal vessel, forming during foetal life the allantoic vein; and secondly a direct connection is established between the vena cava inferior and the veins of the hind limbs and posterior parts of the cardinal veins, so that there is no renal portal system.

Aves. The Chick may be taken to illustrate the development of the venous system in Birds.

On the third day, nearly the whole of the venous blood from the body of the embryo is carried back to the heart by two main venous trunks, the anterior (fig. 125, _S.Ca.V_) and posterior (_V.Ca_) cardinal veins, joining on each side to form the short transverse ductus Cuvieri (_DC_), both of which unite with the sinus venosus close to the heart. As the head and neck continue to enlarge, and the wings become developed, the single anterior cardinal or jugular vein (fig. 371, _J_), of each side, is joined by two new veins: the vertebral vein, bringing back blood from the head and neck, and the subclavian vein from the wing (_W_).

On the third day the posterior cardinal veins are the only veins which return the blood from the hinder part of the body of the embryo.

[FIG. 371. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK AT THE
COMMENCEMENT OF THE FIFTH DAY.

_H._ heart; _d.c._ ductus Cuvieri. Into the ductus Cuvieri of each
side fall _J._ the jugular vein, _W._ the vein from the wing, and
_c._ the inferior cardinal vein; _S.V._ sinus venosus; _Of._
vitelline vein; _U._ allantoic vein, which at this stage gives off
branches to the body-walls; _V.C.I._ inferior vena cava; _l._ liver.]

About the fourth or fifth day, however, the vena cava inferior (fig. 371, _V.C.I._) makes its appearance. This, starting from the sinus venosus not far from the heart, is on the fifth day a short trunk running backward in the middle line below the aorta, and speedily losing itself in the tissues of the Wolffian bodies. When the true kidneys are formed it also receives blood from them, and thenceforward enlarging rapidly becomes the channel by which the greater part of the blood from the hinder part of the body finds its way to the heart. In proportion as the vena cava inferior increases in size, the posterior cardinal veins diminish.

The blood originally coming to them from the posterior part of the spinal cord and trunk is transported into two posterior vertebral veins, similar to those in Reptilia, which are however placed dorsally to the heads of the ribs, and join the anterior vertebral veins. With their appearance the anterior parts of the posterior cardinals disappear. The blood from the hind limbs becomes transported directly through the kidney into the vena cava inferior, without forming a renal portal system[237].

[237] The mode in which this is effected requires further
investigation.

On the third day the course of the vessels from the yolk-sack is very simple. The two vitelline veins, of which the right is already the smaller, form the ductus venosus, from which, as it passes through the liver on its way to the heart, are given off the two sets of _venæ advehentes_ and _venæ revehentes_ (fig. 371).

With the appearance of the allantois on the fourth day, a new feature is introduced. From the ductus venosus there is given off a vein which quickly divides into two branches. These, running along the ventral walls of the body from which they receive some amount of blood, pass to the allantois. They are the _allantoic_ veins (fig. 371, _U_) homologous with the anterior abdominal vein of the lower types. They unite in front to form a single vein, which becomes, by reason of the rapid growth of the allantois, very long. The right branch soon diminishes in size and finally disappears. Meanwhile the left on reaching the allantois bifurcates; and, its two branches becoming large and conspicuous, there still appear to be two main allantoic veins. At its first appearance the allantoic vein seems to be but a small branch of the vitelline, but as the allantois grows rapidly, and the yolk-sack dwindles, this state of things is reversed, and the less conspicuous vitelline appears as a branch of the larger allantoic vein.

[FIG. 372. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK DURING THE
LATER DAYS OF INCUBATION.

_H._ heart; _V.S.R._ right vena cava superior; _V.S.L._ left vena
cava superior. The two venæ cavæ superiores are the original 'ductus
Cuvieri,' they open into the sinus venosus. _J._ jugular vein;
_Su.V._ anterior vertebral vein; _In.V._ inferior vertebral vein;
_W._ subclavian; _V.C.I._ vena cava inferior; _D.V._ ductus venosus;
_P.V._ portal vein; _M._ mesenteric vein bringing blood from the
intestines into the portal vein; _O.f._ vitelline vein; _U._
allantoic vein. The three last mentioned veins unite together to
form the portal vein; _l._ liver.]

On the third day the blood returning from the walls of the intestine is insignificant in amount. As however the intestine becomes more and more developed, it acquires a distinct venous system, and its blood is returned by veins which form a trunk, the _mesenteric vein_ (fig. 372, _M_) falling into the vitelline vein at its junction with the allantoic vein.

These three great veins, in fact, form a large common trunk, which enters at once into the liver, and which we may now call the _portal vein_ (fig. 372, _P.V_). This, at its entrance into the liver, partly breaks up into the _venæ advehentes_, and partly continues as the ductus venosus (_D.V_) straight through the liver, emerging from which it joins the vena cava inferior. Before the establishment of the vena cava inferior, the venæ revehentes, carrying back the blood which circulates through the hepatic capillaries, join the ductus venosus close to its exit from the liver. By the time however that the vena cava has become a large and important vessel it is found that the venæ revehentes, or as we may now call them the _hepatic veins_, have shifted their embouchment, and now fall directly into that vein, the ductus venosus making a separate junction rather higher up (fig. 372).

This state of things continues with but slight changes till near the end of incubation, when the chick begins to breathe the air in the air-chamber of the shell, and respiration is no longer carried on by the allantois. Blood then ceases to flow along the allantoic vessels; they become obliterated. The vitelline vein, which as the yolk becomes gradually absorbed proportionately diminishes in size and importance, comes to appear as a mere branch of the portal vein. The ductus venosus becomes obliterated; and hence the whole of the blood coming through the portal vein flows into the substance of the liver, and so by the hepatic veins into the vena cava.

Although the allantoic (anterior abdominal) vein is obliterated in the adult, there is nevertheless established an anastomosis between the portal system and the veins bringing the blood from the limbs to the vena cava inferior, in that the caudal vein and posterior pelvic veins open into a vessel, known as the coccygeo-mesenteric vein, which joins the portal vein; while at the same time the posterior pelvic veins are connected with the common iliac veins by a vessel which unites with them close to their junction with the coccygeo-mesenteric vein.

Mammalia. In Mammals the same venous trunks are developed in the embryo as in other types (fig. 373 A). The anterior cardinals or external jugulars form the primitive veins of the anterior part of the body, and the internal jugulars and anterior vertebrals are subsequently formed. The subclavians (fig. 373 A, _s_), developed on the formation of the anterior limbs, also pour their blood into these primitive trunks. In the lower Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc.), the two ductus Cuvieri remain as the two superior venæ cavæ, but more usually an anastomosis arises between the right and left innominate veins, and eventually the whole of the blood of the left superior cava is carried to the right side, and there is left only a single superior cava (fig. 373 B and C). A small rudiment of the left superior cava remains however as the sinus coronarius and receives the coronary vein from the heart (figs. 373 C, _cor_ and 374, _cs_).

[FIG. 373. DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS SYSTEM OF
MAMMALS (MAN). (From Gegenbaur.)

_j._ jugular vein; _cs._ vena cava superior; _s._ subclavian veins;
_c._ posterior cardinal vein; _v._ vertebral vein; _az._ azygos
vein; _cor._ coronary vein.

A. Stage in which the cardinal veins have already disappeared. Their
position is indicated by dotted lines.
B. Later stage when the blood from the left jugular vein is carried
into the right to form the single vena cava superior; a remnant
of the left superior cava being however still left.
C. Stage after the left vertebral vein has disappeared; the right
vertebral remaining as the azygos vein. The coronary vein remains
as the last remnant of the left superior vena cava.]

The posterior cardinal veins form at first the only veins receiving the blood from the posterior part of the trunk and kidneys; and on the development of the hind limbs receive the blood from them also.

As in the types already described an unpaired vena cava inferior becomes eventually developed, and gradually carries off a larger and larger portion of the blood originally returned by the posterior cardinals. It unites with the common stem of the allantoic and vitelline veins in front of the liver.

[FIG. 374. DIAGRAM OF THE CHIEF VENOUS TRUNKS OF MAN. (From
Gegenbaur.)

_cs._ vena cava superior; _s._ subclavian vein; _ji._ internal
jugular; _je._ external jugular; _az._ azygos vein; _ha._ hemiazygos
vein; _c._ dotted line shewing previous position of cardinal veins;
_ci._ vena cava inferior; _r._ renal veins; _il._ iliac; _hy._
hypogastric veins; _h._ hepatic veins.

The dotted lines shew the position of embryonic vessels aborted in
the adult.]

At a later period a pair of trunks is established bringing the blood from the posterior part of the cardinal veins and the crural veins directly into the vena cava inferior (fig. 374, _il_). These vessels, whose development has not been adequately investigated, form the common iliac veins, while the posterior ends of the cardinal veins which join them become the hypogastric veins (fig. 374, _hy_). Owing to the development of the common iliac veins there is no renal portal system like that of the Reptilia and Amphibia.

Posterior vertebral veins, similar to those of Reptilia and Birds, are established in connection with the intercostal and lumbar veins, and unite anteriorly with the front part of the posterior cardinal veins (fig. 373 A)[238].

[238] Rathke, as mentioned above, holds that in the Snake the
front part of the posterior cardinals completely aborts. Further
investigations are required to shew whether there really is a
difference between Mammalia and Reptilia in this matter.

On the formation of the posterior vertebral veins, and as the inferior vena cava becomes more important, the middle part of the posterior cardinals becomes completely aborted (fig. 374, _c_), the anterior and posterior parts still persisting, the former as the continuations of the posterior vertebrals into the anterior vena cava (_az_), the latter as the hypogastric veins (_hy_).

Though in a few Mammalia both the posterior vertebrals persist, a transverse connection is usually established between them, and the one (the right) becoming the more important constitutes the azygos vein (fig. 374, _az_), the persisting part of the left forming the hemiazygos vein (_ha_).

The remainder of the venous system is formed in the embryo of the vitelline and allantoic veins, the former being eventually joined by the mesenteric vein so as to constitute the portal vein.

The vitelline vein is the first part of this system established, and divides near the heart into two veins bringing back the blood from the yolk-sack (umbilical vesicle). The right vein soon however aborts.

The allantoic (anterior abdominal) veins are originally paired. They are developed very early, and at first course along the still widely open somatic walls of the body, and fall into the single vitelline trunk in front. The right allantoic vein disappears before long, and the common trunk formed by the junction of the vitelline and allantoic veins becomes considerably elongated. This trunk is soon enveloped by the liver.

The succeeding changes have been somewhat differently described by Kölliker and Rathke. According to the former the common trunk of the allantoic and vitelline veins in its passage through the liver gives off branches to the liver, and also receives branches from this organ near its anterior exit. The main trunk is however never completely aborted, as in the embryos of other types, but remains as the ductus venosus Arantii.

With the development of the placenta the allantoic vein becomes the main source of the ductus venosus, and the vitelline or portal vein, as it may perhaps be now conveniently called, ceases to join it directly, but falls into one of its branches in the liver.

The vena cava inferior joins the continuation of the ductus venosus in front of the liver, and, as it becomes more important, it receives directly the hepatic veins which originally brought back blood into the ductus venosus. The ductus venosus becomes moreover merely a small branch of the vena cava.

At the close of foetal life the allantoic vein becomes obliterated up to its place of entrance into the liver; the ductus venosus becomes a solid cord--the so-called round ligament--and the whole of the venous blood is brought to the liver by the portal vein[239].

[239] According to Rathke the original trunk connecting the
allantoic vein directly with the heart through the liver is
aborted, and the ductus venosus Arantii is a secondary connection
established in the latter part of foetal life.

Owing to the allantoic (anterior abdominal) vein having merely a foetal existence an anastomosis between the iliac veins and the portal system by means of the anterior abdominal vein is not established.

BIBLIOGRAPHY _of the Venous System_.

(498) J. Marshall. "On the development of the great anterior veins." _Phil. Trans._, 1859.

(499) H. Rathke. "Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Säugethieren." _Meckel's Archiv_, 1830.

(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbelthiere." _Bericht. üb. d. naturh. Seminar. d. Univ. Königsberg_, 1838.

_Vide_ also Von Baer (No. 291), Götte (No. 296), Kölliker (No. 298), and Rathke (Nos. 299, 300, and 301).

_Lymphatic System._

The lymphatic system arises from spaces in the general parenchyma of the body, independent in their origin of the true body cavity, though communicating both with this cavity and with the vascular system.

In all the true Vertebrata certain parts of the system form definite trunks communicating with the venous system; and in the higher types the walls of the main lymphatic trunks become quite distinct.

But little is known with reference to the ontogeny of the lymphatic vessels, but they originate late in larval life, and have at first the form of simple intercellular spaces.

The lymphatic glands appear to originate from lymphatic plexuses, the cells of which produce lymph corpuscles. It is only in Birds and Mammals, and especially in the latter, that the lymphatic glands form definite structures.

_The Spleen._ The spleen, from its structure, must be classed with the lymphatic glands, though it has definite relations to the vascular system. It is developed in the mesoblast of the mesogastrium, usually about the same time and in close connection with the pancreas.

According to Müller and Peremeschko the mass of mesoblast which forms the spleen becomes early separated by a groove on the one side from the pancreas and on the other from the mesentery. Some of its cells become elongated, and send out processes which uniting with like processes from other cells form the trabecular system. From the remainder of the tissue are derived the cells of the spleen pulp, which frequently contain more than one nucleus. Especial accumulations of these cells take place at a later period to form the so-called Malpighian corpuscles of the spleen.

BIBLIOGRAPHY _of Spleen_.

(501) W. Müller. "The Spleen." _Stricker's Histology._

(502) Peremeschko. "Ueb. d. Entwick. d. Milz." _Sitz. d. Wien. Akad. Wiss._, Vol. LVI. 1867.

_Suprarenal bodies._

In Elasmobranch Fishes two distinct sets of structures are found, both of which have been called suprarenal bodies. As shewn in the sequel both of these structures probably unite in the higher types to form the suprarenal bodies.

One of them consists of a series of paired bodies, situated on the branches of the dorsal aorta, segmentally arranged, and forming a chain extending from close behind the heart to the hinder end of the body cavity. Each body is formed of a series of lobes, and exhibits a well-marked distinction into a cortical layer of columnar cells, and a medullary substance formed of irregular polygonal cells. As first shewn by Leydig, they are closely connected with the sympathetic ganglia, and usually contain numerous ganglion cells distributed amongst the proper cells of the body.

The second body consists of an unpaired column of cells placed between the dorsal aorta and unpaired caudal vein, and bounded on each side by the posterior parts of the kidney. I propose to call it the interrenal body. In front it overlaps the paired suprarenal bodies, but does not unite with them. It is formed of a series of well-marked lobules, etc. In the fresh state Leydig (No. 506) finds that "fat molecules form the chief mass of the body, and one finds freely imbedded in them clear vesicular nuclei." As may easily be made out from hardened specimens it is invested by a tunica propria, which gives off septa dividing it into well-marked areas filled with polygonal cells. These cells constitute the true parenchyma of the body. By the ordinary methods of hardening, the oil globules, with which they are filled in the fresh state, completely disappear.

The paired suprarenal bodies (Balfour, No. 292, pp. 242-244) are developed from the sympathetic ganglia. These ganglia, shewn in an early stage in fig. 380, _sy.g_, become gradually divided into a ganglionic part and a glandular part. The former constitutes the sympathetic ganglia of the adult; the latter the true paired suprarenal bodies. The interrenal body is however developed (Balfour, No. 292, pp. 245-247) from indifferent mesoblast cells between the two kidneys, in the same situation as in the adult.

The development of the suprarenal bodies in the Amniota has been most fully studied by Braun (No. 503) in the Reptilia.

In Lacertilia they consist of a pair of elongated yellowish bodies, placed between the vena renalis revehens and the generative glands.

They are formed of two constituents, viz. (1) masses of brown cells placed on the dorsal side of the organ, which stain deeply with chromic acid, like certain of the cells of the suprarenals of Mammalia, and (2) irregular cords, in part provided with a lumen, filled with fat-like globules[240], amongst which are nuclei. On treatment with chromic acid the fat globules disappear, and the cords break up into bodies resembling columnar cells.

[240] These globules are not formed of a true fatty substance,
and this is also probably true for the similar globules of the
interrenal bodies of Elasmobranchii.

The dorsal masses of brown cells are developed from the sympathetic ganglia in the same way as the paired suprarenal bodies of the Elasmobranchii, while the cords filled with fat-like globules are formed of indifferent mesoblast cells as a thickening in the lateral walls of the inferior vena cava, and the cardinal veins continuous with it. The observations of Brunn (No. 504) on the Chick, and Kölliker (No. 298, pp. 953-955) on the Mammal, add but little to those of Braun. They shew that the greater part of the gland (the cortical substance) in these two types is derived from the mesoblast, and that the glands are closely connected with sympathetic ganglia; while Kölliker also states that the posterior part of the organ is unpaired in the embryo rabbit of 16 or 17 days.

The structure and development of what I have called the interrenal body in Elasmobranchii so closely correspond with that of the mesoblastic part of the suprarenal bodies of the Reptilia, that I have very little hesitation in regarding them as homologous[241]; while the paired bodies in Elasmobranchii, derived from the sympathetic ganglia, clearly correspond with the part of the suprarenals of Reptilia having a similar origin; although the anterior parts of the paired suprarenal bodies of Fishes have clearly become aborted in the higher types.

[241] The fact of the organ being unpaired in Elasmobranchii and
paired in the Amniota is of no importance, as is shewn by the
fact that part of the organ is unpaired in the Rabbit.

In Elasmobranch Fishes we thus have (1) a series of paired bodies, derived from the sympathetic ganglia, and (2) an unpaired body of mesoblastic origin. In the Amniota these bodies unite to form the compound suprarenal bodies, the two constituents of which remain, however, distinct in their development. The mesoblastic constituent appears to form the cortical part of the adult suprarenal body, and the nervous constituent the medullary part.

BIBLIOGRAPHY _of the Suprarenal bodies_.

(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilien." _Arbeit. a. d. zool.-zoot. Institut Würzburg_, Vol. V. 1879.

(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick. d. Nebennieren." _Archiv f. mikr. Anat._, Vol. VIII. 1872.

(505) Fr. Leydig. _Untersuch. üb. Fische u. Reptilien._ Berlin, 1853.

(506) Fr. Leydig. _Rochen u. Haie._ Leipzig, 1852.

_Vide_ also F. M. Balfour (No. 292), Kölliker (No. 298), Remak (No. 302), etc.