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right subclavian is developed from the seventh right somatic inter-segmental artery. The ventral root of the left fourth arch and the arch itself form the arch of the aorta. The fifth arch, as already stated, is quite transitory; it leaves no remains, therefore it is not necessary to speak of any part of the ventral stem as its ventral root. The ventral part of the sixth arch on the right side becomes the extra-pulmonary part of the corresponding pulmonary artery. On the left side it practically disappears. On the right side the dorsal part disappears, but on the left side it persists, till birth, as the ductus arteriosus, which connects the pulmonary artery with the aorta, and after birth it is converted into the ligamentum arteriosum. The truncus arteriosus is cleft into two parts by a spiral septum; one part, which remains continuous with the ventral roots of the fourth arches and therefore with the innominate artery and the aortic arch, becomes the ascending aorta, and the other, which remains associated with the sixth arches, becomes the stem of the pulmonary artery.

1st arch

2nd arch

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External carotids

Internal carotid




Dorsal root of
left 3rd arch
Left common

Arch of aorta
(4th arch)

vian artery

Arch of aorta


Arch of aorta

Left pulmon. ary artery Dorsal aorta

Whilst the changes mentioned have been taking place in the cephalic part of the arterial system, the primitive dorsal aortæ have fused together from a point immediately caudal to their seventh dorsal branches to a point immediately cranial to the origins of Left subcla- the umbilical branches from their ventral aspects, and their ventral branches have fused together into single stems, some of which have been converted into the coeliac, the superior mesenteric, and the inferior mesenteric arteries. The fusion of the dorsal aortæ commences in embryos about 2.5 mm. long. When the embryo has attained a length of 5 mm. the fusion has extended to the caudal ends of the aorta and the single stem is continued into the rudimentary tail as the caudal artery, which afterwards becomes the middle sacral artery. After the fusion is completed the umbilical arteries spring from the ventral aspect of the single dorsal aorta. (For the further history of the arterial system, see under Vascular System.)

Dorsal aorta


origin of the positions of the first and second arches,

the dorsal roots of the third arches on both sides,
and the dorsal roots of the fourth and fifth arches
on the right side are shown in dotted lines. The
positions of the fifth arches are not shown.

The Primitive Venous System.—The first definite venous trunks to appear are the umbilical veins, returning blood from the chorion, and the vitelline

veins, which convey the blood from the yolk-sac. Two umbilical veins enter the body-stalk and unite to form the vena umbilicalis impar (Fig. 81). This divides, at the posterior margin of the umbilicus, into the right and left lateral umbilical veins, which run round the lateral margins of the umbilical orifice, in the lateral margins of the body wall of the embryo. At the cranial margin of the umbilicus they turn medially, enter a transverse bar of mesoderm which forms the caudal boundary of the pericardium and is known as the septum transversum, and pass through it into the caudal end of the heart (Figs. 81, 82).

The blood from the yolk-sac passes, for a short time, into the lateral umbilical veins at the posterior margin of the umbilicus. This is a very transitory arrangement, and it is soon replaced by the formation of two proper vitelline veins, one on cach side, which ascend, along the vitello-intestinal duct, to the cranial margin

of the umbilicus, where they enter the septum transversum, in which each vitelline vein joins the corresponding umbilical vein, forming a common vitello-intestinal trunk, which enters the sinus venosus (Fig. 81).

This trunk also receives the primitive head vein, or anterior cardinal vein, which returns the blood from the cranial part of the embryo (Fig. 82).

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A little later two veins are formed, one on each side, which return blood from the body wall and the primitive limbs. They are the posterior cardinal veins, and as soon as they are established they join the caudal ends of the anterior cardinal veins to form the ducts of Cuvier, which then open directly into the posterior part of the heart which is called the sinus venosus (Fig. 83). Shortly afterwards the common stems of the vitello-umbilical veins are absorbed into the sinus venosus, forming its right and left horns. When this has happened six veins open into

the sinus venosus, three on each side-the two ducts of Cuvier, the two vitelline veins, and the two lateral umbilical veins (Fig. 83).

The anterior cardinal veins and their tributaries, and cross anastomoses which form between them, are transformed into some of the cranial blood sinuses, the internal jugular veins, the innominate veins, and the cephalic (upper) part of the superior vena cava. The right duct of Cuvier becomes the caudal part of the superior vena cava, and the left is converted into the oblique vein of the left atrium (O.T. oblique vein of Marshall) (Fig. 88).

A portion of the abdominal part of the right posterior cardinal vein is replaced by the right subcardinal vein, and from this and a transverse anastomosis between it and the opposite subcardinal vein is formed that part of the inferior vena cava which extends from the renal veins to the liver, and a part of the left renal vein. From the remains of the cardinal veins and transverse anastomoses between them are formed (1) the azygos, the hemiazygos, and the accessory hemiazygos veins; (2) the inferior vena cava, caudal to the renal veins; (3) the common iliac veins; (4) the hypogastric veins; and (5) the parts of the left lumbar veins which pass dorsal to the aorta (Fig. 88).

The cephalic end of the inferior vena cava is formed from the cephalic extremity of the right vitelline vein and a caudal outgrowth from it which unites with the right subcardinal vein (Fig. 88).

Details of the history of the transformations of the cardinal veins, the vitelline and umbilical veins, and the formation of the cranial part of the inferior vena cava are given in the account of the further stage of the development of the vascular system.

The Primitive Heart. The primitive heart is formed in the dorsal wall of the pericardium, ventral to the fore-gut, by the fusion of the caudal parts of the primitive ventral aortæ, and shortly after its formation it is divided into five primitive chambers. The most caudal of the five, which receives the main primitive veins, is the sinus venosus, the second is the atrium, the third the ventricle, the fourth is the bulbus cordis, and the fifth and most cranial is the truncus aorticus, which discharges its contents into the ventral roots of the aortic arches (Fig. 84).

During the period which intervenes between the time when the embryo is 8 mm. and 17 mm. long, that is between the fifth and the eighth weeks, the greater part of the sinus venosus is absorbed into the atrium; the ventricle and the atrium are each divided into right and left chambers by the formation of an interatrial and an interventricular septum; the bulbus cordis is absorbed partly into the ventricle and partly into the truncus aorticus, and the truncus aorticus is separated into the ascending part of the aorta and the stem of the pulmonary artery. When these changes are completed the heart consists of right and left atria and right and left ventricles. The ventricles are entirely separated from one another by the interventricular septum, but there is an orifice of communication between the right and left atria (Fig. 88).

The right atrium receives blood from the superior and inferior vena cavæ, and from the walls of the heart, by the coronary sinus, which is a remnant of the transverse part and left horn of the sinus venosus. The blood which enters through the superior vena cava and by the coronary sinus, passes through the right atrioventricular orifice into the right ventricle, but the whole, or the greater part, of the blood which enters by the inferior vena cava passes through the foramen ovale, which lies in the interatrial septum, into the left atrium.

The blood which enters the right ventricle is ejected into the pulmonary artery. A small portion of it passes by the right and left branches of the artery into the lungs, and is returned to the left atrium by the pulmonary veins, but by far the greater part passes through the ductus arteriosus into the aorta, which it enters at a point immediately beyond the origin of the left subclavian artery (Fig. 88).

The blood which enters the left atrium, through the foramen ovale, mixes, in the left atrium, with the blood which is returned by the pulmonary veins; then it passes through the left atrio-ventricular orifice into the left ventricle, by which it is forced into the aorta. Some of this blood passes into the innominate artery, and so, by its right subclavian branch, to the right upper

extremity, and by its right common carotid branch to the right side of the head and neck; another part enters the left common carotid artery and is distributed to the left side of the head and neck, and some passes, through the left subclavian artery, to the left upper limb. The remainder mixes with the blood which enters the aorta, from the right ventricle, through the pulmonary artery and the ductus arteriosus. Part of this mixed blood is distributed to the body and the viscera, and the lower limbs, and the remainder passes through the umbilical arteries to the placenta (Fig. 88).

The Fatal Circulation. When the foetal circulation is thus fully established, purified oxygenated blood, returning from the placenta, enters the body of the fetus by the umbilical vein and passes to the liver. Some of it enters the liver, but the greater part passes, through a channel called the ductus venosus, to the inferior vena cava, where it mixes with the venous blood returning from the lower limbs and the abdominal region, including the liver. This mixed, but, as contrasted with the blood in the superior vena cava, comparatively pure blood enters the right atrium and passes through it and through the foramen ovale into the left atrium, thence to the left ventricle and through the left ventricle into the aorta. A portion of this comparatively pure blood is distributed to the head and neck and the upper limbs. The remainder unites with the stream of venous blood poured into the aorta through the ductus arteriosus. Part of it is distributed to the body and the lower limbs, and part is sent to the placenta to be purified and oxygenated (Fig. 88).


The remaining part of the blood stream is formed by the blood returned from the head and neck, the upper part of the body and the upper limbs, by the superior vena cava, and from the walls of the heart by the coronary sinus. is the most venous and impure blood in the body. After entering the right atrium it passes into the right ventricle, and thence into the pulmonary artery. very small part of it is passed to the lungs, by the right and left branches of the pulmonary artery; the remainder goes through the ductus arteriosus into the aorta, where, beyond the origin of the left subclavian artery, it mixes with the much purer blood which entered the aorta from the left ventricle.


At birth, when the placental circulation ceases, the lungs become the organs through which oxygen enters and carbonic acid leaves the blood; the foramen ovale in the interatrial septum closes, and the ductus arteriosus is obliterated. The course of the circulation and the condition of the blood in the different regions is, therefore, considerably altered.

On account of the cessation of the placental circulation all the blood which enters the right atrium is entirely venous, and, as the foramen ovale is closed, it all passes into the right ventricle, which forces it into the pulmonary artery. As the ductus arteriosus is closed, all the blood which enters the pulmonary artery must now pass through the lungs, where it is aerated, and whence it is returned, by the pulmonary veins, as oxygenated blood, to the left atrium. It passes from the left atrium to the left ventricle, which forces it through the aorta and its branches to all parts of the head, neck, body, and limbs; and now, for the first time, all parts receive blood of the same quality.


It has already been pointed out that there are two parts of the cœlom, the extra-embryonic and the intra-embryonic. Both are clefts separating an outer from an inner layer of mesoderm.

The Extra-embryonic Colom.-The extra-embryonic celom appears in the primary mesoderm and separates it into a parietal and a visceral layer. The parietal layer covers the inner surface of the trophoblast and forms with it the chorion. It covers also the outer surface of the amnion. The visceral layer covers the outer surface of the extra-embryonic portion of the wall of the entodermal cavity.

The extra-embryonic and intra-embryonic parts of the coelom are at first separate from one another (Fig. 36), then they become continuous, for a time, in the region of the umbilical orifice (Fig. 37), but are separated from one another again

when the umbilical orifice closes. The extra-embryonic portion is entirely obliterated when the outer surface of the expanding amnion fuses with the inner surface of the chorion (compare Figs. 77 and 78).

Pericardial cœlom

The Intra-embryonic Colom.-The intra-embryonic cœlom appears as a series of cleft-like spaces in the margin of the embryonic mesoderm. The spaces fuse together to form a n-shaped cavity (Fig. 89) which separates the peripheral part of the embryonic mesoderm into a parietal or somatic, and a visceral or splanchnic, layer. The bend of the n-shaped cavity lies in the margin of the cephalic part of the embryonic region, and it has no direct communication with the extraembryonic cœlom, but the greater part of each stem of the cavity, on account of the disappearance of its lateral wall, soon opens, laterally, into the extraperitoneal canal embryonic cœlom.







The transverse portion of the n-shaped cavity, which extends across the cephalic end of the embryonic area and connects the two limbs together, is the pericardial cavity. The adjacent part of each lateral limb of the cavity is the pleuro-pericardial canal, it becomes a pleural cavity, and the remaining portions of the two limbs unite ventrally, as the umbilical orifice closes, to form the single peritoneal cavity.

As the head fold forms, the pericardial part of the cavity is carried ventrally and caudally into the ventral wall of the fore-gut (Fig. 90). The mesoderm which originally formed its peripheral boundary, but which now lies in the cephalic boundary of the umbilical orifice, becomes thickened, and forms the septum transversum (Figs. 90, 91, 93).

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SEPARATION OF THE VARIOUS PARTS. D from above; A, B, and C at levels of line A, B, and C in Fig. D.

At the cephalic end of its dorsal wall, on each side, the pericardial cavity is still continuous with the two lateral parts of the coelom; and each lateral part, which

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