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part of the cephalic limb of the loop become placed transversely (Fig. 827), but after a short period its ventricular extremity again passes caudally, until the original cranial limb of the loop. lies to the right and somewhat ventral to the original dorsal limb. Still later, the bulbus cordis is partly absorbed into the truncus arteriosus and partly into the ventricle; the right segment of the sinus venosus is absorbed into the atrium, and the left segment forms the coronary sinus. At a still later period the atrium is divided, by intra-atrial septa, into right and left atria. The atrio-ventricular canal becomes converted into the right and left atrioventricular apertures. . The ventricle, including the absorbed portion of the bulbus cordis, is

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separated into the right and left ventricles of the adult heart ; and the truncus arteriosus, and the part of the bulbus cordis which becomes incorporated with it, is separated into the ascending aorta and the pulmonary artery. Thus, from the embryonic heart are eventually produced the adult heart, the ascending aorta, the pulmonary artery, and the coronary sinus.

The main outlines of the transformation of the simple tubular heart of the young embryo into the four-chambered heart of the adult are, therefore, comparatively simple, but the details of the process are intricate and some of them are still imperfectly understood.

The Sinus Venosus. In embryos possessing fourteen mesodermal somites the primitive single heart, formed by the fusion of the caudal portions of the primitive ventral aortæ, receives at its caudal end two venous stems, each of which is the common termination of the lateral umbilical vein, the vitelline vein, and the anterior cardinal veins of the same side (Fig. 819). At a later period, after the heart has begun to fold on its longitudinal axis, the two common venous stems are absorbed into the heart and form its most caudal section, the sinus venosus, which consists of a right and a left cornu, united by an intermediate segment. In the meantime, however, the posterior cardinal veins have appeared and have united with the anterior cardinals. After this union occurs the portion of the anterior cardinal vein ventral to the point of union becomes the duct of Cuvier. At this period, therefore, each cornu of the sinus venosus receives three veins --the vitelline vein, the lateral umbilical vein, and the duct of Cuvier.

As development proceeds the left vitelline vein and the left lateral umbilical vein lose their connexion with the left horn of the sinus venosus and pour their blood into the liver, through which it reaches the cranial end of the right vitelline vein. The latter in the meantime has become the only outflow from the liver to the heart, and it ultimately forms the terminal part of the inferior vena cava. In association with these changes the right horn of the sinus venosus becomes considerably larger than the left horn (Fig. 828), and the left horn and the intermediate portion of the sinus become reduced to the form of a comparatively narrow channel which opens into the right horn, whilst the latter opens into the dorsal part of the atrial chamber, by a cleftlike aperture which is bounded by right and left lateral sips called the right and left venous valves (Figs. 829, 830).

As the right horn of the sinus venosus is absorbed into the right part of the dorsal portion of the atrium the caudal or lower portion of right venous valve becomes the valve of the inferior vena cava and the valve of the coronary sinus. The cephalic or upper portion is united with a septal projection into the cavity of the atrium called the septum spurium, and forms with it the crista terminalis of the completed heart, which indicates, therefore, the line of union of the right horn of the sinus with the atrium proper. The left venous valve becomes incorporated with the interatrial septum, and the angle in the posterior wall of the sinus venosus which indicates the line of union of the right cornua and the intermediate part of the sinus appears in the dorsal wall of the right atrium, where it unites with the dorsal part of the caudal or inferior portion of the septum secundum of the atrium, and takes part in the formation of the limbus ovalis.

Whilst these changes have been proceeding the right duct of Cuvier has become the lower part of the superior vena cava, the intermediate part of the sinus venosus and the left cornu are transformed into the coronary sinus, and the left duct of Cuvier has become the oblique vein of the left atrium. Thus when the changes mentioned are completed the right part of the atrium receives the superior and the inferior vena cava and the coronary sinus.

The Atrio-ventricular Canal.-In the early stages the atrio-ventricular canal opens through the dorsal wall of the ventricular chamber towards the left side, but, as the ventricle increases in size, the atrio-ventricular opening moves to the right till it occupies the middle part of the dorsal wall of the ventricle. Whilst the change in position is occurring

Trachea Esophagus

1 the atrio-ventricular canal becomes

Junction of right anterior and

Left posterior posterior cardinal veins

cardinal vein compressed into a transverse cleft which is bounded by a cranial (upper in adult position) and a caudal wall. On the middle part of each of those walls an endocardial thickening appears which is called Duct of Cuvieran endocardial cushion. Each Right horn of sinus venosus cushion is a flattened eminence,

Secondary foramen ovale

Left venous valve and when the two eminences meet and fuse the atrio-ventricular canal Right venous valve is divided into right and left Septum primum portions, of which the right por

Pericardial cavity tion forms a passage from the

Primary foramen ovale right part of the still incompletely

Cushion of atrio divided atrium into the right por

ventricular canal tion of the incompletely divided ventricle, and the left portion in

Interventricular septuma similar manner forms a channel of communication between the left part of the atrial chamber and the

Interventricular sulcus seft part of the ventricle. "The two parts of the primitively single Fig. 829.-SECTION OF THE HEART OF A HUMAN EMBRYO. atrio-ventricular canal become the

(Edinburgh University collection.) right and left atrio-ventricular apertures of the adult heart, and the margins of the apertures take part in the formation of the cusps of the atrio-ventricular valves.

The Division of the Atrium.—It has already been pointed out that as the tubular heart bends on its long axis the atrial chamber forms part of the dorsal limb of the loop. It lies, therefore, dorsal to the truncus arteriosus and the bulbus cordis, and retains that position throughout all the later stages of development; consequently, it forms the dorsal part or base of the fully developed heart. "It has been stated also that, as development proceeds and the atrial chamber expands, the right horn of the sinus venosus is absorbed into the right segment of the dorsal portion of the expanding atrium. Whilst this process of absorption of the sinus venosus is proceeding two other events are taking place : (1) The right and left margins of the atrium grow

ventrally round the sides Truncus arteriosus

of the truncus arteriosus 6th aortic arches.

Septum primum

and the adjacent part of the 4th aortic arches

bulbus cordis ; and (2) the Atrio-ventricular orifice

primitive atrium is divided

into the right and left perSeptum spurium.

Bulbus cordis manent atria by the forma

tion of two interatrial septa. Left duct of Cuvier Left anterior

By the ventral growth cardinal veins

of the right and left margins of the atrium the auricles of the right and left permanent atria are formed, and by the formation and fusion of the two interatrial septa the permanent inter

atrial septum of the fully deRight posterior cardinal vein

Ventricle veloped heart is established. Right duct of Cuvier

The septa by which the

atrial chamber is divided Right venous valve

are the septum primum and FIG. 830.—MODEL SHOWING THE INTERIOR OF THE RIGHT ATRIUM OF A the septum secundum ; so

HUMAN EMBRYO 5.5 MM. LONG. (Edinburgh University collection. named because the first apModelled by C. C. Wang.)

pears and partly disappears

before the second is formed. Both the septum primum and the septum secundum grow from the dorsal towards the ventral wall of the primitive atrium, the septum secundum to the right of the septum primum.

When the septum primum reaches the ventral wall of the atrium it fuses with the dorsal ends of the endocardial cushions of the atrio-ventricular canal. Until this fusion occurs an aperture, the primary foramen ovale, exists between the two atria and a number of perforations are formed in the more dorsal portion of the septum ; therefore the passage of blood from the inferior vena cava through the right part of the atrium into the left part is never prevented.

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The perforations eventually blend together to form the secondary foramen ovale of the interatrial septum. As soon as the septum primum is completed the primitive atrium is divided into the permanent right and left atria, each of which communicates through the secondary foramen

ovale, with the atrium of the Left part of atrio ventricular orifice Truncus arteriosus

opposite side, and with the Septum primum fusing with

corresponding portion of the

superior atrio-ventricular cushion Wall of left

ventricle through an atrioatrium

ventricular aperture which is

completely separate from its Wall of right

fellow of the opposite side.

As the ventral border of

the septum primum fuses with Septum primum fusing with

the endocardial cushions of the inferior atrio- atrio - ventricular canal, the ventricular

septum secundum appears, imcushion

mediately to the right of the

Septum
Left

primum

septum primum. It grows posterior

from the dorsal and cranial cardinal veins opened

(superior in adult) walls of the Right

atrium. As the septum secundum develops, the right cornu

of the sinus venosis is abInferior atrio

sorbed into the atrium, the ventricular cushion

left venous valve fuses with Right posterior cardinal veins opened

the septum primum or disRight duct of Cuvier opening into right horn of sinus venosus

appears, and the angle between

the right horn and the interFig. 831.-View OF THE INTERIORS OF THE RIGHT AND LEFT ATRIA OF mediate part of the sinus ap

A HUMAN EMBRYO 5.5 MM. LONG. (Edinburgh University collection. pears in the caudal part of the
Modelled by C. C. Wang.)

dorsal wall of the right atrium.

As the septum secundum increases in size its cephalic part grows first ventrally and then caudally and lastly dorsally; consequently the free border soon becomes concave and the concavity of its margin is directed dorsally.

Both extremities of the free margin of the septum secundum fuse with the right lateral surface of the septum primum, and the more ventrally situated part of the border, growing along the

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Fig. 832.- DEVELOPMENT OF THE HEART AND THE MAIN ARTERIES. Diagram of the heart, showing the formation of its septa, and of the cephalic portion of the arterial system.

septum primum, fuses with the angle between the right horn and the intermediate part of the sinus venosus forming with it the lower part of the limbus fossæ ovalis of the fully developed heart. The remainder of the limbus is formed by the thickened free margin of the septum secundum.

As soon as the septum secundum passes beyond the level of the foramen ovale that portion

of the dorsal part of the septum primum which is not yet fused with the septum secundum acts as a flap valve, permitting blood to pass from the right to the left atrium, but preventing its return. This condition persists until birth, when the thickened free margin of the septum secundum fuses with the right lateral face of the septum primum, and the foramen ovale of the fætus becomes the fossa ovalis of the child. The fossa ovalis is bounded ventrally and caudally (anteriorly and inferiorly) by the limbus fossæ ovalis, which is formed mainly by the originally free margin of the septum secundum, but to a small extent also by the angle between the right cornu and the intermediate part of the sinus venosus.

The Division of the Primitive Ventricle, the Bulbus Cordis, and the Truncus Arteriosus.- Two facts have already been pointed out with regard to the bulbus cordis ; firstly, that it forms part of the cranial or anterior limb of the early loop-shaped heart, and secondly, that it disappears by being absorbed partly into the ventricle and partly into the truncus arteriosus. The part absorbed into the ventricle enters into the formation of that part of the ventricle which afterwards becomes the right ventricle of the completed heart, and the part absorbed into the truncus arteriosus lengthens that trunk.

The division of the elongated truncus arteriosus into the pulmonary artery and the ascending aorta is a complicated process in which three factors are associated : (1) a proximal and (2) a distal set of endocardial swellings in the bulbar part of the truncus aorticus, which are known as the proximal and distal bulbar swellings, and (3) a septum called the aorto-pulmonary septum, which appears at the cephalic end of the truncus aorticus, growing from the angle between the orifices of the more dorsally situated sixth aortic arches, and the orifices of the two stems which form the ventral roots of the first four aortic arches. The three sets of elements are, at first, entirely distinct from one another, but ultimately they blend together to form a spiral septum by which the lumen of the truncus arteriosus is divided into two channels. One of the two channels communicates with the right ventricle and the other with the left ventricle. After the septum is completed it is cleft longitudinally into two parts, and so the truncus is divided into two vessels, the pulmonary artery which communicates with the right ventricle, and the aorta which is connected with the left ventricle.

The proximal bulbar swellings, which take part in the separation of the truncus into pulmonary artery and aorta, are prolonged into the ventricular region as the ventral part of the bulbus cordis is absorbed into the ventricle, consequently the septum which they form by their fusion is prolonged into the ventricular chamber, and helps to separate that cavity into two parts, by fusing with the dorsal end of the cephalic or anterior portion of the interventricular septum.

The interventricular septum appears as a semilunar ridge on the inner surface of the ventral part of the wall of the ventricle. At a later period its position is marked on the outer surface of the ventricle by the interventricular sulcus (Fig. 829) which persists in the completed heart. As the interventricular septum increases in height it fuses with the fused endocardial cushions which divided the atrio-ventricular canal into right and left parts, and with the ventral border of the bulbar septum which projects into the dorsal part of the ventricular chamber. The completed interventricular septum consists, therefore, of two distinct parts : a ventral part formed by the proper interventricular septum, and a dorsal part formed by the lower portion of the fused bulbar endocardial cushions. The two parts can be distinguished quite easily in the adult heart, for the interventricular septnm is muscular, and it forms by far the greater part of the permanent septum, whilst the bulbar part of the septum, being developed from endocardial thickenings, is membranous, and it forms the small pars membranacea of the permanent septum, which lies between the aortic vestibule of the left ventricle and the dorsal part of the right ventricle and the adjacent ventral part of the right atrium.

DEVELOPMENT OF THE VEINS. Simultaneously with the formation of the arteries, by which blood is distributed to the embryo and to all parts of the zygote, and in a similar manner, a series of vessels is developed by means of which the blood is returned to the heart. These vessels are the primitive veins ; they form two main groups which make their appearance at different periods.

The first group consists of (1) the vitelline veins which return blood from the yolk sac, and (2) the umbilical veins which return the blood from the placenta.

The second group consists of (1) the anterior cardinal veins, (2) the posterior cardinal veins, the (3) ducts of Cuvier, and (4) the subcardinal veins.

The ducts of Cuvier, one on each side, are the common stems by which the blood of the anterior and posterior cardinal veins is conveyed to the primitive heart. The anterior cardinal vein returns blood from the head and neck and fore limbs. The posterior cardinal veins carry blood from the body walls, the Wolffian bodies, and the hind limbs. The subcardinal veins also are closely associated with the return of blood from the Wolffian bodies.

The veins of the first group are largely transitory. The umbilical veins entirely disappear as blood channels, but a part of the left lateral umbilical vein is recognisable in the adult as the ligamentum teres of the liver. Portions of the intra-embryonic parts of the vitelline veins, and anastomoses which form between them, remain as the portal vein, its right and left branches, and the upper end of the inferior vena cava.

The anterior cardinal veins, their tributaries, and a transverse anastomosis which forms between them in the more cranialward or upper portion of the thoracic region, are converted into the (1) cavernous sinuses of the cranium, (2) the internal jugular veins, (3) the innominate veins, (4) the upper part of the superior vena cava, and (5) the upper part of the left superior the latter becomes the left vena advehens, persists until birth and, after the disappearance of the right lateral umbilical vein, it conveys the blood from the placenta to the liver, where part of the placental blood passes into the left vena advehens and so through the left vena revehens to the inferior vena cava, and part passes into the ductus venosus, by which it reaches that portion of the cranial part of the right vena revehens which becomes the cranial or upper end of the permanent inferior vena cava.

After birth when the placental circulation ceases the left lateral umbilical vein becomes the ligamentum teres of the liver.

Middle plexus
Posterior plexus

Anterior plexus

Posterior
stem

Anterior stem
Otic vesicle

Ophthalmic vein

Semilunar ganglion
Anterior cardinal vein

Middle stem

Primary head vein
Fig. 834.-DIAGRAM OF THE PRIMARY HEAD VEIN AND ITS TRIBUTARIES. (After Streeter.)

The Ductus Venosus. The ductus venosus is developed as the left lateral umbilical vein loses its direct connexion with the liver and becomes united to the left vena advehens. It is formed from the sinusoidal spaces of the rudimentary liver and connects the commencement of the left vena advehens with the cranial part of the right vena revehens. It forms the more direct channel by which blood from the placenta is passed to the heart through that part of the right vena revehens which becomes the upper end of the inferior vena cava. After birth its converted into the fibrous ligamentum venosum, which connects the left branch of the portal vein with the upper end of the inferior vena cava.

Middle and anterior plexuses

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Parts of primary head vein Fig. 835.— DIAGRAM OF THE HEAD VEins Of A 21 mm. EMBRYO. (After Streeter.) The Anterior Cardinal Veins. The anterior cardinal veins are the veins by which the blood is returned to the heart from the head and neck and, ultimately, from the upper extremities also, although the primitive veins of the upper extremities are, in the first place, tributaries of the posterior cardinal veins.

Each anterior cardinal vein may be separated into two parts—the intra-cranial and the extra-cranial.

In the majority of vertebrates the portion of each anterior cardinal vein which is situated in the head region, and which afterwards becomes intra-cranial, lies to the medial side of the otic vesicle and the 5th, 7th, 8th, 9th, 10th, and 11th cerebral nerves.

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