Illustration: Figure 364Fig. 364. Diagram of the arrangement of the arterial arches in an embryo of one of the Amniota.(From Gegenbaur; afterRathke.)a.ventral aorta;a´´.dorsal aorta; 1, 2, 3, 4, 5. arterial arches;c.carotid artery.
Fig. 364. Diagram of the arrangement of the arterial arches in an embryo of one of the Amniota.(From Gegenbaur; afterRathke.)a.ventral aorta;a´´.dorsal aorta; 1, 2, 3, 4, 5. arterial arches;c.carotid artery.
In the Amniota the metamorphosis of the arteries is in all cases very similar. Five arches,viz.the mandibular, hyoid, and three branchial arches are always developed (fig. 364), but, owing to the absence of branchiæ, never function as branchial arteries. Of these the main parts of the first two, connecting the truncus arteriosus with the collecting trunk into which the arterial arches fall, always disappear, usually before the complete development of the arteries in the posterior arches.
The anterior part of the collecting trunk into which these vessels fall is not obliterated when they disappear, but is on the contrary continued forwards as a vessel supplying the brain, homologous with that found in Fishes. It constitutes the internal carotid. Similarly the anterior part of the trunk from which the mandibular and hyoid arteries sprang is continued forwards as a small vessel[230], which at first passes to the oral region and constitutes in Reptiles the lingual artery, homologous with the lingual artery of the Amphibia; but in Birds and Mammals becomes more important, and is then known as the external carotid (fig. 125). By these changes the roots of the external and internal carotids spring respectively from the ventral and dorsal ends of the primitive third artery,i.e.the artery of the first branchial arch (fig. 365,candc´); and thus this arterial archpersists in all typesas the common carotid,and the basal part of the internal carotid. The trunk connecting the third arterial arch with the system of the dorsal aorta persists in some Reptiles (Lacertilia,fig. 366A) as a ductus Botalli, but is lost in the remaining Reptiles and in Birds and Mammals (fig. 366B, C, D). It disappears earliest in Mammals (fig. 365C), later in Birds (fig. 365B), and still later in the majority of Reptiles.
The fourth arch always continues to give rise, as in the Anura, to the system of the dorsal aorta.
Illustration: Figure 365Fig. 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.
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. 366A 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 (eandd), is continuous with therightfourth arch, and also with the common carotid arteries (c); while a second springing from the right side of the ventricle is continuous with theleftfourth arch (handf). The right and left divisions of the fourth arch meet however on the dorsal side of the œsophagus to give origin to the dorsal aorta (g).
In Birds (fig. 366C) theleftfourth arch (h) loses its connection with the dorsal aorta, though the ventral part remains asthe 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. 366D) the truncus arteriosus is only divided into two, but theleft 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. 366A,i), Chelonians and Birds (fig. 366C,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. 366B,h) and Mammalia (fig. 366D,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 rightside. 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.
Illustration: Figure 366Fig. 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.andl.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.
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.andl.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.
Bibliographyof the Arterial System.
(496)H. Rathke. “Ueb. d. Entwick. d. Arterien w. bei d. Säugethiere von d. Bogen d. Aorta ausgehen.” Müller’sArchiv, 1843.(497)H. Rathke. “Untersuchungen üb. d. Aortenwurzeln d. Saurier.”Denkschriften d. k. Akad. Wien,Vol.XIII. 1857.
Videalso 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 infig. 367,v, and may be called thesubintestinal 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. Thedilated 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.
Illustration: Figure 367Fig. 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.
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, asubintestinal vessel.
From the subintestinal vein there is given off a branch which supplies the yolk-sack. This leaves the subintestinal vein closeto 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].
Illustration: Figure 368Fig. 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.
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 ductusvenosus, 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.
Illustration: Figure 369Fig. 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.
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.363and369), 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, thevena 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 thesuperior 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 theanteriorpart 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 theposteriorsection 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 longitudinalvessel 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, theanterior 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.
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,vjandvc); 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].
Illustration: Figure 370Fig. 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.
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].
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 theallantoic 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].
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 fœtal vessel, forming during fœtal 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 anteriorcardinal 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.
Illustration: Figure 371Fig. 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 fallJ.the jugular vein,W.the vein from the wing, andc.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.
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 fallJ.the jugular vein,W.the vein from the wing, andc.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].
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 ofvenæ advehentesandvenæ 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 theallantoicveins (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 twobranches 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.
Illustration: Figure 372Fig. 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.
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, themesenteric 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 theportal vein(fig. 372,P.V). This, at its entrance into the liver, partly breaks up into thevenæ advehentes, and partly continues as theductus 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 thehepatic 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 cavainferior, in that the caudal vein and posterior pelvic veins open into a vessel, known as thecoccygeo-mesentericvein, 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. 373A). 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. 373A,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. 373B and C). A small rudiment of the left superior cava remains however as thesinus coronariusand receives thecoronary veinfrom the heart (figs.373C,corand374,cs).
Illustration: Figure 373Fig. 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.
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 theblood 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.