Book - Quain's Embryology 9

From Embryology

Development of the Heart and Blood-Vessels

Development of Particular Organs and Systems

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Sharpey W. Thomson A. and Schafer E.A. Quain's Elements of Anatomy. (1878) William Wood and Co., New York.

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1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital
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Development of the Heart

Origin of the Heart

(Simple Tubular Form)

The heart takes its origin in the form of an elongated sac or dilated tube in the substance of a thickening of the splanchno-pleure layer of the mesoblast, in the ventral aspect of the cephalic portion of the primitive alimentary canal, immediately in front of the fovea cardiaca. Doubts have existed as to the exact mode of production of the cavity of the organ, but the observations of Aflanasieff and Klein, and especially those of Foster and Balfour, appear to show that the substance in which the first rudiments of the heart arise is produced by a thickening of the lower wall of the mesoblastic layer of the primitive intestine, and that the cavity is formed by a solution of continuity or liquefaction of tliis substance in such a manner that, while the outer cells constitute the foundation of the commencing fibrous walls,, a deep set of cells very soon or from the first arrange themselves in the form of an endo-vascular lining of the cavity. The oi-gan has at first the form of an elongated sac or dilated tube of symmetrical shape, widening out behind into two lateral orifices, each of which is connected with an omphalo-mesenteric vein of its own side bringing the nascent blood back from the vascular area, while the anterior part of the rudimental heart leads into two arterial vessels, one of which arches over each side of the primitive pharynx and turns backwards below the proto-vertebrge to form one of the two primitive aortic tubes. From each of these last the omphalo-mesenteric arteries pass off into the vascular area.

According to recent observations by Kolliker and by Hensen (loc. cit.) a still earlier condition of the heart has been perceived in the embryo of mammals, in which there are two separate tubes hollowed out of the lateral parts of the cephalic fold. Each of these tubes is connected with a vein or entering vessel posteriorly, and an artery or out-going vessel anteriorly : these slowly come together and unite by fusion in the middle, in a limited space at first, and then more and more till the single tubular heart results. Each tube is in relation with the pleuroperitoneal cavity of its own side, and when the median fusion takes place the union of these two becomes the pericardium.

Fig. 584. Diagrammatic longitudinal section through the Axis op an Embryo.

The section is supposed to be made at a time when the head-fold has commenced, but the tail-fold has not yet appeared. A, epiblast ; B, mesoblast ; C, hypoblast ; FSo, fold of the somatopleure ; Sp, and FiSp, fold of the splanchopleure ; Avi, commencing (head) fold of the amnion ; NO, neural canal, closed in front, but still open behind ; Ch, notochord, in front uncleft mesoblast in the ba«e of the cranium ; D, the commencing foregut, or alimentary canal ; Ht, heart ; pp, pleuro-peritoneal cavity.

The rudimental heart in the form now described, exists in the chick at the thirty-sixth hour of incubation, and already, while still consisting of formative cells not differing greatly from those composing the other parts of the mesoblast, begins to exhibit motions of alternating systole and diastole, by slow contractions which begin behind and pass forward to the anterior extremity of the tube ; and a small quantity of imperfectly formed blood is propelled through the cavity.

The elongation which the tubular heart now undergoes causes it to lose the symmetrical form ; and its middle part now becomes detached from the lower side of the alimentary canal, and projects downwards (or forwards in the body) with an inclination to the right side of the embryo.

The heart is now found to be surrounded on the ventral aspect by a median cavity, which is a part of the pleuro-peritoneal space intervening between the wall of the heart as splanchno-pleure, and the somato-pleure forming the thoracic wall. This cavity becomes the pericardial sac.

As the development of the tubular heart progresses, the bend increases, and the venous is doubled back upon the arterial end. The tube also becomes divided by two slight constrictions into three

Fig. 585. Human Embryos at different early stages of development, showing the heart in its tubular condition.

A, upper half of the body of a human embryo of three weeks, viewed from the abdominal side (from Coste) ; a, frontal plate ; b, protovertebrse, on which, the primitive aortae are lying ; 3, the middle of the tube of the heart, below it the place of entrance of the great veins, above it the aortic bulb.

B, lateral view of a human embryo more advanced than that last referred to, and somewhat imperfectly developed (from A. Thomson) ; a, the frontal part of the head ; h, the vertebral column ; v, the wide communication of the umbilical vesicle or yolk-sac with the intestine ; u, communication with the allantois or urachus ; 2, auricular part of the heart connected with the veins posteriorly ; 8, ventricular part of the bent tube ; 4, the aortic bulb ; near the extremities of the tube the divided pericardium is seen portions, of which that originally posterior and receiving the veins is the widest, and constitutes the primitive auricle ; the middle one, next in width and most strongly bent upon itself, becomes the ventricular portion ; and the third, situated anteriorly and retaining most the simple tubular form, is the arterial or aortic bulb. This tubular stage of the rudimental heart has been observed in the human embryo by Coste and Allen Thomson (see fig. 585, A and B).

Fig. 586. Diagrammatic Outlines of the Heart and First Arterial Vessels of the Embryo, as seen from the Abdominal Surface.

A, at a period corresponding to the 36th or SStli hour of incubation in the chick • B, and C, at the 48th hour of incubation ; 1, 1, primitive veins ; 2, auricular part of the heart ; 3, ventricular part ; 4, aortic bulb ; 5, 5, the primitive aortic arches and their continuation as descending aorta ; these vessels are still separate in their whole extent in A, but at a later period, as shown more fully in C, have coalesced into one tube in a part ot the dorsal region ; in B, below the upper 5, the second aortic arch is formed, and farther down the dotted lines indicate the position of the succeeding arches to the number of rive in all ; 5', 5', the continuation of the main vessels in the body of the embryo ; 6, 6, the omphalo-mesenteric arteries imssing out of the body of the embryo into the vascular area of the germinal membrane.

Division into Single Auricle, Ventricle, and Arterial Bulb

By a continued increase of the inflection of the heart-tube, a change in the relative position of the several parts is effected, so that the auricular cavity comes to be placed above or behind (dorsally) and to the left of the ventricular part, the veins being carried forwards along with it, while the arterial bulb is attached by its extremity in front to the neck of the embryo immediately behind the lower visceral plates. There is as yet only a single passage through the heart, but the distinction of the auricular and ventricular cavities becomes more apparent, both by an increase in the diameter of each, and by the constriction which separates them, and by the much greater thickness acquired by the walls of the ventricular and bulbous parts as compared with the auricular portion.

The three parts of the heart have now the appearance of being very closely twisted together. The ventricular part becomes considerably wider transversely, and the auricular part shows two projecting pouches, one on each side of the arterial bulb, which are the first indications of the future auricular appendages. At the same time the constriction between the auricular and ventricular parts increases considerably, and the constricted part elongating produces what has been called the canalis auricularis.

Division of the Cavities - Ventricles

The next series of changes in the developing heart consists in the division of each original single cavity of the ventricle, auricle, and arterial bulb into two compartments, so as to form the right and left ventricles and auricles, and the stems of the -pulmonary artery and aorta. The first of these changes occurs in the ventricular portion, and is to be seen in progress on the fourth day in the chick, and the sixth and seventh week in the human embryo. The ventricular chamber of the heart, increasing considerably in breadth, that part of it which ultimately becomes the apex of the heart is thrown towards the left side, and in most mammals, and especially in the human embryo, a blmit cleft or depression appears betAveen this and the right part of the ventricle, which causes an external division into two portions corresponding to the future right and left ventricles ; and if the interior of the ventricular cavity be examined at this time, there is perceived a crescentic partition rising from the anterior or lower border of the right wall and projecting into the cavity, at first narrow and placed opposite the external notch, but gradually growing more and more towards the auriculo-ventricular aperture. As development progresses the external division becomes more or less effaced, when the apex of the heart formed by the left ventricle becomes more pointed, and the whole heart takes more of the conical form which belongs to its more advanced condition ; but the depression is still perceptible as the interventricular groove of the adult heart, which, as is well known, varies considerably in depth in different cases. In some animals, as the rabbit, the temporary external division of the ventricles is greater than in the human embryo, while in others, as in ruminants, there is very little of the external notching, and in them, as in birds, the heart very early assumes the conical form. The dugong presents a remarkable example of the persistence of the complete external separation of the ventricles, and there appears to be a tendency to the occasional occurrence of the same in the seal.

Fig. 587. Head op the Embryo of the Dog with the Heart seen from below (from Kolliker, after Bischoff). Magnified.

a, cerebral hemispheres : h, eyes ; c, midbrain ; d, inferior maxillary plates ; e, superior maxillary processes ; /, /', /", second, third, and fourth branchial or visceral plates ; g, right, h, left auricle of the heart ; h, right, i, left ventricle ; 1, aortic or arterial bulb, with three paii's of aortic or vascular arches protruding from it.

The internal septum of the ventricles continuing to rise between the right and left divisions of the cavity, reaches at last the base where it is placed in relation with both the auriculo-ventricular orifice and the root of the arterial bulb ; but at this place there remains for a time a communication over the still free border of the septum between the right and left ventricles, wdiich is interesting, as this is the seat of the abnormal communication between the right and left ventricles in almost all cases of malformation of the heart presenting that condition.

Division of the Auricles

Although the auricular cavity presents externally some appearance of being divided into two at a period antecedent to the partition of the ventricles, in consequence of the formation of the right and left auricular appendages before mentioned, the internal division of the cavity does not take place till some time later, as on the fifth and sixth days in the chick, and in the eighth week in the human embryo. The auricular septum commences as an internal fold proceeding from the anterior wall of the common cavity, and starting from the septum of the ventricles, it grows backwards towards the entrance of the common vein or sinus, but stops short of it some distance. For a time, therefore, the veins enter the back part of the auricular cavity in common. It is proper to explain, however, that, by the time at which the auricular septum is forming, the venous sinus has been modified so as to produce three veins entering the auricle at its back part. Of these, two correspond with the right superior cava and the inferior cava veins, and the third to the left superior cava and connected with what afterwards becomes the coronary sinus. For a time, all the three vessels open so as to communicate freely with the whole auricular cavity. But changes now occur which cause the left superior cava and the inferior cava to be directed towards the left side, while the right superior cava is placed more immediately in connection with the right part of the auricular cavity.

The auricular septum,in extending itself backwards, is not completed, but leaves an oval deficiency in its lower and middle part, as the foramen ovale, and the inferior cava opens immediately behind this. Some time later in the human embryo, or in the course of the tenth or eleventh weeks, two new folds make their appearance in the auricles posteriorly. One of these constituting the Eustachian valve, of a crescentic form, is placed to the right of the entrance of the inferior vena cava, and in the angle between it and the orifice of the left superior cava (or great coronary sinus), and besides separating these two veins, and thus throwing the opening of the left superior cava into communication with the right auricle, this fold, as it runs forward into the annulus ovalis or border of the anterior auricular septum, has the effect of deepening the entrance of the inferior cava into a groove which lies close to the foramen ovale, and directs the blood entering by that vessel through the passage into the left auricle.

Fig. 588. Shows the position and form of the heart in the human Embryo from the Fourth to the Sixth week.

A, upper half of the body of a human embryo of Dearly four weeks old (from Kolliker after Coste) ; B and C, anterior and posterior views of the heart of a human embryo of six weeks (from Kolliker after Ecker) ; a, frontal lappet ; h, mouth ; c, below the lower jaw and in front of the second and third branchial arches ; d, upijer limb ; e, liver ; /, intestine cut short ; 1, superior vena cava ; 1', left superior cava or brachio-cephalic connected with the coronary vein ; 1", opening of the inferior vena cava ; 2, 2', right and left auricles ; 3, 3', right and left ventricles ; 4, aortic bulb.

The other fold referred to advances from the posterior wall of the common auricle to meet the anterior auricular septum, but yet to the left of the border of the foramen ovale. To this border, however, it adheres as it grows forwards, and thus gradually fills up the floor of the fossa ovalis. Up to the middle of foetal life, this posterior septum being incomplete, there is a direct passage from right to left through the foramen ; but, after that period, the fold in question, having advanced beyond the anterior border of the annulus ovalis and lying to the left, it does not adhere to this or the fore part of the annulus, but leaves a passage between, and appears as a crescentic fold in the left auricle, which, as it passes beyond the annulus, constitutes a very perfect valve against the return of blood from the left into the right auricle.

Division of the Arterial Bulb

The third important change occurring in the heart belongs to the arterial bulb, by which there are developed from this tube the first parts or main stems of the pulmonary artery and the ao'-ta. Within the thick walls of this arterial tube there is at first only a single cylindrical cavity, continued from the originally single ventricle ; but, a short time after the partition of the ventricular cavity has commenced, or in the seventh week of the human embryo, a division of the bulb by an independent process begins to take place. This consists in the projection inwards of a lateral fold of the wall on the two sides, affecting, however, only the inner and middle coats, and not perceptible externally ; so as to divide the cavity of the bulb into two channels, which may be described as respectively anterior and posterior, but which from the spiral direction taken by the folds are somewhat twisted on each other, so that the channel which at the ventricular end is placed anteriorly becomes connected with the right ventricle and forms the pulmonary stem, and that which is placed posteriorly becomes connected with the left ventricle and forms the commencement of the aorta. In the distant portion of the bulb, however, the pulmonary channel is situated to the left and posteriorly, and the aortic channel is to the right and most forwards, and at this end these channels are respectively connected with different aortic arches, giving rise to the permanent pulmonic and systemic vessels in the manner afterwards described.

It is further to be noted that the partition of the bulb begins at the remote extremity, and progresses towards the ventricles. There is a time, therefore, during which the ventricular septum, and the septum of the bulb, advancing towards each other, are incomplete and disunited ; and from the difference in their direction it is obvious that there must be a peculiar twist of one or both, in order that they may finally unite so as to become continuous.

Fig. 589. View op the Front and Right Side of the Fcetal Heart, at four months, the right auricle being laid open (from Kilian).

a, the right auriculo-ventricular opening ; h, a probe passed iip the vena cava inferior and throiigh the fossa ovalis and foramen ovale into the left auricle ; r, vena cava inferior ; c, Eustachian valve ; v, valve of the foramen ovale ; s', vena cava superior.

Fig. 590. View of the Posterior and left surface of the Heart of a Foetus of four months, the left Auricle being opened (from Kilian).

o, left auriculo-ventricxilar orifice ; c, inferior vena cava, through which a probe b, is passed from below, and thence by the foramen ovale into the left auricle ; e, left auricular appendage laid open ; o, valve of the foramen ovale seen to be attached to the left side of the annulus ovalis of the septum.

The completion of the partition of the aortic and pulmonary stems is afterwards effected by the progress of the division from within outwards through the external walls of the tubes ; but the two vessels remain united externally by a common envelope of pericardium.

The remarkable cases sometimes observed of abnormal transposition of the two great arterial stems from their natural connection with their respective ventricles may be explained by reference to the history of the development of the parts of the heart before given.

Formation of the Valves

The formation of the auriculo-ventricular and semilunar valves begins during the time of the changes previously described by the projection of thick folds from the inner wall of the heart. In the case of the semilunar valves the trifid division is early perceived, but the cavities or sinuses within the valves are late of being developed. In the auriculo-ventricular valves there is at first an entire or annular projecting fold of the inner substance round the orifice, and this becomes gradually divided into segments, and the chordae tendinae are gradually produced by perforation of the valve plate. (See Tonge in Proceed. Eoy. Soc, 1868.)

The manner in which the pulmonary veins, which are formed separately in the lungs, come to be connected with the left auricle has not yet been ascertained.

No further important changes occur in the internal structure of the heart, but there are some which affect the external form and thickness of its walls. In early foetal life the size of the heart bears a considerably greater proportion to that of the body than at a later period. At birth it is still proportionally large. For some time the auricular portion remains more voluminous than the ventricular, but in the latter half of foetal life the permanent proportion is more nearly established. The walls of both ventricles are also thicker than in after life, and it is especially deserving of notice that the wall of the right is up to near the time of birth quite as thick as that of the left, — a peculiarity which may be connected with the orifice of the right ventricle to propel the blood of the foetus through the extended course of the ductus arteriosus, the descending aorta and the placental circulation.

Development of the Blood-Vessels

The Principal Arteries. The Aorta

The most interesting part of this history is that relating to the development of the aorta and the larger vessels arising from it. The double condition of the main trunk of "the aorta has already been referred to as existing in the chick up to near the end of the second day. About the fortieth hour the inedian fusion or coalescence of the two vessels begins to take place in the dorsal region, by their external union, at first in a very limited space, and very soon afterwards by the formation of a perforation through their united walls. The union of the two vessels which begins in the dorsal region extends itself backwards towards the lumbar vertebrae, and when it reaches the place where the omphalo-mesenteric arteries pass out on each side, these vessels, each of which was originally the continuation merely of one of the aortas, appear now as branches of a single and median aorta. The iliac vessels are the next large vessels formed from the hinder part of the aorta. The first vessels belonging to these trunks are not, however, those of the lower limbs, for these are not yet formed ; but the umbilical or hypogastric arteries, developed at a very early period in connection with the allantois, and subsequently attaining to a large size along with the growth of the placenta. As the limbs are formed, arteries are developed in them, and these are branches of the main aorta ; but they are for a long time comparatively small, w'hile the umbilical arteries speedily attain a large size, so that, even up to the conclusion of foetal life, they appear to form the principal part of the two large vessels into which the aorta divides at its lower extremity. The middle sacral artery may be looked upon as the continuation of the median stem of the aorta, and probably originates from a double vessel in the same manner as the aorta itself.

Fig. 591. Transverse Section through the Dorsal Region op an Embryo-Chick of the Second Day (from Foster and Balfour, after His). M, medullary canal ; Pv, proto-vertebral column ; w, rudiment of Wolffian duct in the intermediate mass; Ch, notocliord ; Ao, one of the two aortas ; A, epiblast ; C, hypoblast ; BC, splanchnopleure ; Pp, pleuroperitoneal space.

Fig. 592. Transverse section through the Dorsal region op an Embryo Chick, end of Third Day (from Foster and Balfour).

Am, amnion ; m p, muscle plate ; CV, cardinal vein; Ao, dorsal aorta at the point where its two roots begin to join ; Ch, notochord (the line does not quite reach it) ; Wd, Wolffian duct; Wb, commencement of formation of Wolffian body ; ep, epiblast ; so, somatopleure ; Sp, splanchnopleure ; hy, hypoblast. The section passes through the place where the alimentary canal {hy) communicates with the yolk-sac.

The double state of the main aorta when first formed in the foetus was discovered by Serres, and described bj him in his 4tli Memoir on Transcendental Anatomy (Annal. des Scien. Nat., 18130), but was doubted by Von Baer, as Serres's observations did not show the relation of the primitive trunks of the aorta to the pharyngeal vascular arches. The fact of the original double condition was, however, placed beyond doubt by Allen Thomson (Edin. New Philos. Journal, ISoO) by the method of tranverse sections, then fii-st employed as a means of embryological investigation, and the process of median union was traced in full detail. The relation of this process to the occurrence of a permanent double canal in the aorta as a malformation, as described by Vrolik. Schroder van der Kolk and Cruveilhier. and observed in at least one case by Allen Thomson, has already been referred to in vol. i., p. 350.

According to Serres, the vertebral arteries within the cranium are originally separate, and the basilar artery results from their mesial union or fusion in the same manner as occurs in the aorta, and the median union of the anterior cerebral arteries in the forepart of the Circle of Willis is another example of the same process. It seems probable that the internal cross band observed by John Davy in the interior of the basilar artery (Researches Physiol, and Anatom., lSo9, p. 301) may be a remains of the septum or united walls of the two vertebral arteries.

Aortic or Branchial Arches

The two primitive arterial arches which lead into the dorsal aorta from the arterial bulb of the rudimentary heart, at the time of the establishment of the first circulation, are the most anterior of a series of five pairs of vascular arches which are developed in succession round this part of the pharynx ; and which, since their discovery by Eatlike in 1825 (Oken's Isis, 1825) have been regarded with much interest, as corresponding with those vessels which are the seat of development of the subdivided blood-vessels of the gills in fishes and amphibia. These vascular arches thus exhibit in the amniota, along with the branchial or pharyngeal clefts and visceral plates, a typical resemblance to the structure of gills, although no full development of these respiratory organs occurs in such animals, but they furnish by their various transformations the basis of formation of the permanent pulmonary and aortic stems and the main vessels to which they give rise.

The form and position of the primitive aortic arches, up to the time of their transformation into permanent vessels, is nearly the same in reptiles, birds and mammals ; and the main differences in the seat and distribution of the large permanent vessels are to be traced to changes in the openness and extent of growth of the several arches. The five pairs Of arches do not all co-exist at the same time, for they are developed in succession from before backwards, and by the third day of incubation, or by the corresponding period of the fourth week in the human embryo, when the posterior arches have been formed, already a part of the anterior arches, beginning with the first one, has become obliterated. Each of the first four branchial arches occupies a place in the substance of the pharyngeal or visceral plates, and in front of one of the pharyngeal clefts. The first or anterior is therefore situated in the inferior maxillary plate, and in front of the tympano-Eustachian, or first pharyngeal cleft ; and the fifth arterial arch is placed behind the fourth pharyngeal cleft, and in the substance of the neck, in which there is no distinct bar or plate in the higher animals, but which is the seat of a developed branchial bar in some aquatic animals.

The vessels forming the arterial arches are given off on each side in succession from two short canals, into which the primitive arterial bulb divides immediately in front of the place where it joins the neck. These may be named the lower (ventral) or anterior aortic roots ; and similarly, when they have passed round the wall of the pharynx, the branchial arches unite in succession into a vessel on each side, thus forming the upper (dorsal) or posterior aortic roots.

On the third and fourth days in the chick, and from the fourth to the sixth week in the human embryo, there are still three complete pairs of arterial arches passing round the pharynx, and connected both before and behind with the anterior and posterior aortic roots previously mentioned. The transformations of these arches were in part traced by Von Baer and various other observers, but the fuller knowledge of their changes is due to the later researches of Eatlike (Mem. of Vienna Acad., 1857), and although some 'points are still left in doubt, their history may now be given from these observations, and the supplemental illustration derived from the investigation of the various examples of congenital malformation, the greater number of which are manifestly related to variations in the natural mode of transformation. This will be explained by reference to the diagram in fig. 593.

Fig. 593. Diagram of the Aortic or Branchial Vascular Arches of the Mammal, with their transformations giving rise to the permanent arterial Vessels (accordiug to Rathke, slightly altered).

A, Primitive arterial stem or aortic Lnlb, now divided into A, the ascending part of the aortic arch, and P, the pulmonary ; a, the right ; a', the left aortic root ; A', the descending aorta. On the right side, 1, 2, 3, 4, 5, indicate the five Ijranchial primitive arterial arches ; on the left side, I, II, III, IV, the four branchial clefts, which, for the sake of clearness, have been omitted on the right side. It will be observed, that while the fourth and fifth pairs of arches rise from the part of the aortic bulb or stem, which is at first undivided, the first, second, and third pairs are branches above c, of a secondary stem on each side. The permanent systemic vessels are represented in deep shade, the pulmonary arteries lighter ; the parts of the primitive arches, which have only a temporary existence, are drawn in outline only, c, placed between the permanent common carotid arteries ; ce, the external carotid arteries ; ci, c'l, the right and left internal carotid arteries ; s, the right subclavian rising from the right aortic root beyond the fifth arch ; r, the right vertebral from the same opposite the fourth arch ; v', s', the left vertebral and subclavian arteries rising together from the left or permanent aortic root opposite the fourth arch ; P, the pulmonary arteries rising together from the left' fifth arch ; d, the outer or back part of the left fifth arch, forming the ductus arteriosus ; j)n, 2^n , the right and left pneumogastric nerves, descending in front of the aortic arches, with their recurrent branches represented diagrammatically as passing behind, with a view to illustrate the relations of these nerves respectively to the right subclavian arteiy (4) and the arch of the aorta and ductus arteriosus {d).

From these researches it appears that the permanent vessels owe their formation to the persiscence of certain of the foetal arches or parts of them, while other arches or portions of them become obliterated and disappear. Thus it is ascertained that in mammals the main aortic arch, which in the adult passes to the left of the trachea and gullet, is formed by the persistence of the fourth embryonic arterial arch of the left side, which not only remains patent, and becomes connected with the aortic stem of the arterial bulb, but keeps pace by its increased width and the development of its walls with the rate of growth in the other parts of the body, so that it soon surpasses all the rest of the arches in its width of calibre and thickness of its walls. In birds, however, the permanent aortic arch passes to the right of the trachea and gullet, and it is formed by the persistence of the fourth embryonic arch of the right side ; while, in all reptiles, as there are two permanent aortic arches, it is by the persistence of both the right and left foetal arches that the two aortas are produced, the right being that which is most directly connected with the systemic or left ventricle.

The pulmonary arteries of mammals would appear by Ratlike's observations to be developed in connection with only one foetal arterial arch, viz., the fifth of the left side, from the middle part of which they appear as branches, and the whole fifth arch of the right side undergoes rapid atrophy and ultimate obliteration. The first part of the left fifth arch, becoming the common pulmonary artery, is connected with that division of the arterial bulb which is separated as the pulmonary stem ; but the remote part of this arch also remains fully patent, and undergoing equally with the rest of it full development, continues to lead into the left root of the aorta as ductus arteriosus Botalli, which serves to convey the blood from the right ventricle of the foetal heart into the descending aorta, but becomes obliterated at the time of birth.

This duct is therefore in mammals due to a persistent condition of the fifth left branchial arch. But, in birds and reptiles, it appears that the process of transformation is somewhat different, for in them the right and left pulmonary arteries (excepting in those serpents in which there is only one lung developed) are formed in connection with the respective right and left fifth branchial arches, and there are thus two ductus arteriosi during foetal life, the short one of the right side corresponding to that which is left in mammals, and the longer one of the left side passing round the pharynx into the left aortic root. Both of these arches are obliterated at the time of the exclusion of the bird from the egg ; but in some reptiles the ductus arteriosi remain permanently open during life.

The subclavian and vertebral arteries were shown by Ratlike to spring from the posterior aortic roots at a place between the junction of the fourth and fifth arches. In mammals, the vessels on the left side are from the first in direct connection Avith the aortic root at the place which they permanently occupy ; but upon the right side, as the fourth arch and the aortic root are obliterated posteriorly, the passage for blood from the aortic stem into the subclavian trunk is formed by the persistence of the forepart of the fourth right arch as far as the place where it meets the origin of the subclavian and vertebral arteries.

The common carotid trunks, occupying the region which afterwards becomes the neck, but which is at first absent or extremely short, are formed by the anterior divisions of the aortic roots ; while the external carotid artery is due to the persistence of a channel in the continuation of each anterior aortic root, and the internal carotid artery arises from the persistence of the crossing third arch and the upper part of the posterior aortic root.

Thus it falls out that, in man and a certain number of mammals, an innominate artery is formed on the right side by the union of the first part of the fourth right aortic arch leading into the right subclavian, and the right anterior aortic root which forms the common carotid ; while, on the left side, the carotid and subclavian vessels rise separately from the permanent aortic arch in consequence of the distance lying between them in the original foetal condition.

It does not come within the scope of this chapter to describe the further steps of development of these vessels, nor to enter into an explanation of the manner in which abnormal position of the arch of the aorta and its branches, or of the pulmonary arteries, may be supposed to arise. For further information on this subject the reader is referred to the short account of the varieties given in the description of the blood-vessels in the first volume of this work, as well as to the third volume of Henle's Handbuch, and to the special works of Tiedemann and Eichard Quain on the Arteries.

Development of the Great Veins

In the early embryo, before the development of the allantois, a right and a left omphalo-mesenteric vein bring back the blood from the walls of the umbilical vesicle, and unite to form a short trunk, the meatus venosus, which is continued into the auricular extremity of the rudimentary heart.

In the first commencement of the placental circulation, or in the fourth week of foetal life, two umbilical veins are seen coming from the placenta, and uniting to form a short trunk, which opens into the common omphalo-mesenteric vein. Very soon the right omphalo-mesenteric vein and right umbilical vein disappear. In connection with the common trunk of the umbilical and omphalo-mesenteric veins, two sets of vessels make their appearance in the yomig liver. Those furthest from the heart, named vena; Iwpatlca advehentcs, become the right and left divisions of the portal vein ; the others are the hepatic veins, venae kcpaticce. Tcvclwnifs. The portion of vessel intervening between those two sets of veins forms the ductnn rcnoKiix, and the part above the hepatic vein, being subsequently joined by the ascending vena cava, forms the upper extremity of that vein. Into the remaining or left omphalo-mesenteric vein, open the mesenteric and splenic veins. The part above the latter forms the trank of the portal vein ; and the portion of vessel between the union of this with the umbilical vein and the origin of the venae hepaticae advehentes is so altered that the portal trunk opens into the commencement of the right vena advehens.

At the time of the commencement of the placental cu-culation, two short transverse venous trunks, the ducts of Cuvier, open, one on each side, into the auricle of the heart. Each is formed by the union of a superior and an inferior vein, named the itrimitive jugular and the cardinal.

The primitive jugular vein receives the blood from the cranial cavity by channels in front of the ear, which are subsequently obliterated : in the greater part of its extent it becomes the external jugiilar vein : and near its lower end it receives small brandies, which, grow to be the external jugular and subclavian veins. The cardinal veins are the primitive vessels which return the blood from the Wolffian bodies, the vertebral column, and the parietes of the trunk. The inferior vena cava is a vessel of later development, whichopens into the trunk of the umbilical and omphalo-mesenteric veins, above the vense heijaticas revehentes. The iliac veins, which unite to form the inferior vena cava, communicate with the cardinal veins. The inferior extremities of the cardinal veins are persistent as the internal iliac veins. Above the iliac veins the cardinal veins are obliterated in a considerable part of their course ; their upper portions then become continuous with two new vessels, the j;osfe?-ipr vertchral veins of Ratlike, which receive the lumbar and intercostal twig.

Fig. 594. Diagrams illustrating the development of the Great Veins (after Kollliker).

A, plan of the principal veins of the fretus of about four weeks, or soon after the first formation of the vessels of the hver and the vena cava inferior.

B, veins of the liver at a somewhat earlier period.

C, principal veins of the foetus at the time of the first estabhshment of the placental circulation.

D, veins of the liver at the same period.

dc, the right and left ducts of Cuvier ; ca, the right and left cardinal veins ; j. j,ihe jugular veins ; s, the subclavian veins ; az, the azygos vein ; u, the umbilical or left umbilical vein ; u', in B, the temporary right umbilical vein ; o, the omphalo-meseuteric vein ; o', the right omphalo-mesenteric vein ; m, the mesenteric veins ; p, the portal vein ; p', p', the vente advehentes ; I, the ductus venosus ; V, I', the hepatic veins ; c'l, vena cava inferior ; il, the division of the vena cava inferior into common iliac veins ; cr, the external iliac or crural veins ; h, the hypogastric or internal iliac veins, in the line of continuation of the primitive cardinal veins.

In C, li, in dotted lines, the transverse branch of communication between the jugular veins which forms the left innominate vein ; ri, the right innominate vein ; ca, the remains of the left cardinal vein by which the superior intercostal veins fall into the left innominate vein ; above lo, the obliquely crossing vein by which the hemiazygos joins the azygos vein.

Figure 594. Development of the Great Veins

As development proceeds, the direction of the ducts of Cuvier is altered by the descent of the heart from the cervical into the thoracic region, and becomes continuous with that of the primitive jugular veins. A communicating branch makes its appearance, directed transversely from the junction of the Jeft subclavian and jugular veins, downwards, and across the middle line to the right jugular ; and further down in the dorsal region between the posterior verteljral veras a communicating branch passes obliquely across the middle line from right to left. The communicating branch between the primitive jugular veins is converted into the left innominate vein. The portion of vessel between the right subclavian vein and the termination of the communicating branch becomes the right innominate vein. The portion of the primitive jugular vein below the communicating vein, together with the right duct of Cuvier. forms the vena cava superior, while the cardinal vein opening into it is the extremity of the great vena azygos. On the left side, the portion of the primitive jugular vein placed below the communicating branch, and the cardinal and i^osterior vertebral veins, together wdth the cross branch between the two posterior vertebral veins, are converted into the left superior intercostal and left superior and inferior azygos veins. The variability in the adult arrangement of these vessels depends on the various extent to which the originally continuous vessels are developed or atrophied at one point or another. The left duct of Cuvier is obliterated, except at its lower end, which always remains pervious as the coronal sinus. Even the adirlt, traces of the existence of this vessel can always be recognised in the form of a fibrous band, or sometimes even a narrow vein, which descends obliquely on the left auricle ; and in front of the root of the left lung there remains a small fold of the serous membrane of the pericardium, the re.sfii/ial fold of the pericardium, so named by Marshall, to whom is due the first full elucidation of the nature and relations of the left primitive vena cava.

Figure 595. A and B. Vestige of Left Superior Cava and a Case of its Persistence

Fig. 595. A and B. — Diagrammatic Outlines of the Vestige of the Left Superior Cava and of a Case of its Persistence (sketched after Marshall).

A, brachio-cephalic veins with the superior intercostal, azygos, and principal cardiac veins.

B, the same in a case of persistence of the left superior cava, showing its communication with the sinus of the coronary vein.

The views are supposed to be from before, the parts of the heart being removed or seen through.

1, 1', the internal jugular veins ; 2, 2', subclavian veins ; 3, right innominate ; 3', right or regular superior cava ; 4, in A, the left innominate ; in B, the transverse or communicating vein between the right and left superior vena cavae ; 5, in A, the opening of the superior intercostal vein into the innominate ; 5', vestige of the left superior cava or duct of Cuvier ; 5, 5', in B, the left vena cava superior abnormally persistent, along with a contracted condition of 4, the communicating vein : 6, the sinus of the coronaiy vein ; 6', branches of the coronary veins ; 7, the superior intercostal trunk of the left side, or left cardinal vein ; 8, the principal azygos or right cardinal vein ; 7', 8', some of the upper intercostal veins ; 9, the opening of the inferior vena cava, with the Eustachian valve.

The left duct of Cuvier has been observed persistent as a small vessel in the adult. Less frequently a right and left innominate vein open separately into the right am'icle. an arrangement which is also met witli in birds and in certain mammals, and which results from the vessels of the left side being developed similarly to those of the right, while the cross branch remains small or absent. (Quain on the Arteries, plate 58, figs. 9 and 10.)

Figure 596. View of the Foetal Heart and Great Vessels

Fig. 596. View of the Foetal Heart and Great Vessels, from the left side to show the vestige of the Left Superior Cava Vein in situ. (This figure is planned after one of Marshall's, and slightly altered according to an original dissection.)

a, right auricle ; b, left auricle and pulmonary veins ; c, the conns arteriosus of the right ventricle ; d, the left ventricle ; c, descending aorta ; + , vestigial fold of the pericardium ; /, arch of the aorta, with a part of the pericardium remaining superiorly ; r/, main pulmonary artery and ductus arteriosus ; [/', left pulmonary artery ; 1,1', right and left internal jugular veins ; 2, 2', subclavian veins ; 3, 3', right innominate and superior vena cava ; 4, left innominate or communicating vein ; 5, 5', remains of the left superior cava and duct of Cuvier, passing at + in the vestigial fold of the pericardium, joining the coronary sinus, 6, below, and receiving above the superior intercostal vein, 7 ; 7', 7', the upper and lower intercostal vein, joining into one.

A case is recorded by Gruber, in which the left vena azygos opened into the coronary sinus, and was met by a small vein descending from the union of the subclavian and jugular. (Reichert and Dubois. RejTnond's Archiv, 18(54, p. 729.) In this case, the jugular veins had been developed in the usual manner, while the left vena azygos continued to pour its blood into the duct of Cuvier.

(Consult Kolliker, Entwickelungsgeschichte, p. 41-1, et seq. ; J. Marshall on the Development of the great Anterior Veins in Man and Mammalia, in Phil. Trans., part i., 1850 ; and Wenzel Gruber, Uber die Sinus Communis und die Valvulaj der Vense Cardiaca3, &c., in Mem. de I'Acad. imper. des Scien. de St. Petersbourg, 1SG4 ; and in Virchow's Archiv, Jan. 1SG6.)

Peculiarities of the Foetal Organs of Circulation

It may be useful here to recapitulate shortly the peculiarities of structure existing in the advanced stage of the formation of the foetal organs of circulation with reference to their influence in determining the course of the blood during intra-uterine life, and the changes which occur in them in consequence of the establishment of pulmonary respiration at birth.

The so-called foramen ovale retains the form of a free oval opening in the septum auricularum up to the fourth month, but in the course o"f that month and the next there takes place the growth from below and on the left side of a flat plate or curtain, which advancing upwards fills up the floor of the fossa ovalis, adheres to its left borders as far as its anterior part, and then becoming free and passing beyond the anterior border of the fossa, converts the aperture into an oblique passage or slit over the valvular margin of the fold, so that in the last three and a half months the arrangement is completed, by which blood may readily pass from the right into the left auricle, but not in a contrary direction.

The Eustachian Valve constitutes a crescentic fold of the lining structure of the heart, which is situated to the right of the opening of the inferior vena cava and fossa ovalis, deepens that fossa, and directs the blood entering it from the inferior cava towards the opening of the foramen ovale ; while it throws the opening of the great coronary vein into connection with the right auricle, into which the superior vena cava also opens.

The formation at an early period of foetal life of the transverse vein of the neck uniting the left with the right brachio-cephalic veins, carries the whole of the blood returning from the head and neck, together with that from the main azygos, into the stream entermg the heart by the superior cava.

The ductus arteriosus passes from the main pulmonary artery into the aorta, at the hollow part of the arch, a short distance beyond the place of origin of the left subclavian artery. It is nearly of the same width with the pulmonary stem, while the right and left pulmonary arteries are of comparatively small size, so long as the lungs have not been expanded by air in rfespiration.

Umbilical Vessels

Besides the usual branches of the descending aorta intended to supply the abdominal viscera and the lower limbs, two large vessels, named hypogastric or iimhilical arteries, are prolonged from the common iliacs, and passing out of the abdomen, proceed along the umbilical cord, coiling round the umbilical vein to reach the placenta. The commencement of each of these vessels afterwards forms the tmnk of the corresponding- internal iliac artery, but, from their size, they might be regarded in the fojtus as the continuations of the common iliac arteries into which the aorta divides. From the placenta the blood is returned by the umbilical vem. which, after entering the abdomen, communicates by one branch with the portal vein of the liver, and is continued by another, named ductus rcnosus, into one of the hepatic veins, through which it joins the main stem of the vena cava inferior.

Figure 597. Semi-diagrammatic view of the Organs of Circulation in the Foetus from Before

Fig. 597. Semi-diagrammatic view of the Organs of Circulation in the Foetus from Before (from Luschka, modified, and from Nature).

a, front of the thyroid cartilage ; 6, right side of the thyroid body ; c, trachea ; d, surface of the right lung turned outwards from the heart ; e, diai^hragm below the apex of the heart ; /, right lobe of the liver, dissected to show ramifications of the portal and hepatic veins ; /', the middle part and left lobe of the liver in the same manner, .showing branches of the umbilical veins and ductus venosus ; g, right, fj', left kidney ; ij" , suin-arenal bodies ; h, right, li, left ureter ; i, portion of the small intestine tunied towards the side, to show the veins from it going to the portal vein ; Ic, urinary bladder ; I, is placed below the umbilicus, which is turned towards the left of the fcetus, and points by a line to the urachus ; m, rectum, divided and tied at its upjier part.

A, A. right auricle of the heart opened to show the foramen ovale : a probe, introduced through the large divided right hepatic vein and vena cava inferior, is seen passing through the fossa ovalis into the left auricle : at the lower part of the fossa ovalis is seen the Eustachian valve, to the right and inferioi'ly the auriculo-ventricular orifice ; B, the left auricular appendix ; C, the surface of the right ventricle ; D, placed on the inner surface of the left lung, i)oints to the left ventricle.

1, ascending part of the arch of the aorta ; 1', back part beyond the ductus arteriosus ; 1", abdominal aorta; 2, stem of the pulmonary artery ; 2', the place of division into right and left pulmonary arteries and root of the ductus arteriosus : the left pueumo-gastric nerve is seen descending over the arch of the aorta ; 3, superior vena cava ; 3', right, 3", left innominate vein ; 4, stem of the inferior vena cava, between the junction of the hepatic vein and the right auricle ; 4', continuation of the vena cava inferior below ; 5, umbilical vein within the body of the fojtus ; 5 x , without the bo'ly, in the umbilical cord ; 5', 5', ductus venosus ; between 5 and 5', the direct branches of the umbilical vein to the liver ; 5", 5", hepatic veins, tlu-ough one of which a probe is passed into the fossa ovalis and through the foramen ovale ; 6, vena portD3 ; 6', its left branch joining the umbilical vein; (i", its right branch; 7, placed on the right iliac vein, points to the right common iliac ai-tery; 7', left common iliac artery; 8, right, 8', left umbilical arteries coming from the internal iliac arteries ; 8 x , umbilical arteries without the body, in the umbilical cord ; 9, 9', external iliac arteries ; 10, placed below the right renal vessels ; 11, inferior mesenteric artery, above the root of which .ore seen the two spermatic arteries.

Course of the Blood in the Foetus

The right auricle of the foetal heart receives its blood from the two venre cavfe and the coronary vein. The blood brought by the superior cava is simply the venous blood returned from the head and upper half of the body ; whilst the mferior cava, which is considerably larger than the superior, conveys not only the blood from the lower half of the body, but also that which is returned from the placenta through the umbilical vein. This latter stream of blood reaches the vena cava inferior, partly by a direct passage — the ductus venosus, and partly by the hepatic veins, which bring to the vena cava inferior all the blood circulating through the liver, whether derived from the supply of placental blood entering by the umbilical vein, or proceeding from the vena portfe or hepatic artery.

The blood of the superior vena cava, descending in front and to the right of the Eustachian valve, and mixed with a small portion of that from the inferior cava, passes on into the right ventricle, and is thence propelled into the trunk of the pulmonary artery. A small part of it is then distributed through the branches of that vessel to the lungs, and retuins by the pulmonary veins to the left auricle ; but, as these vessels remain comparatively undilated up to the time of birth, by far the larger part passes through the ductus arteriosus into the dorsal aorta, entering that vessel beyond the place of origin of the arteries of the head and upper limbs, and, mixed perhaps with a small quantity of the blood flowing into the aorta from the left ventricle, is distributed in part to the lower half of the body and the viscera, and in part is conveyed along the umbilical arteries to the placenta. From these several organs it is returned by the vena cava inferior, the venaj portsB, and the umbilical vein : and, as already noticed, reaches the right auricle through the trank of the inferior cava.

Of the blood entering the heart by the inferior vena cava, only a small part is mingled with that of the superior cava, so as to pass into the right ventricle ; by far the larger portion, directed by the Eustachian valve through the foramen ovale, flows from the right into the left auricle, and thence, together with the small quantity of blood returned from the lungs by the pulmonary veins, passes into the left ventricle, from whence it is sent into the arch of the aorta, to be distributed almost entu-ely to the head and upper limbs. A small portion of it, may, however, flow on into the descending aorta, and join the fuller stream of blood from the ductus arteriosus. From the upper half of the body the blood is returned by the branches of the superior cava to the right auricle, from which its course into the right ventricle and pulmonary trunk has been already traced.

There is probably a considerable difference in the early and more advanced stages of foetal life, m the distribution of the stream of blood entering the heart by the vena cava inferior. In the early stages, a lai-ge part of the cuiTcnt being directed into the left, but in the three last months, and as the foetus approaches maturity, more and more of the blood of the inferior cava joins the stream from the superior cava ; and, indeed, the coui-se of the blood, and the relative position of the veins, as well as other original peculiarities of the foetal heart, become gradually altered, in preparation, as it were, for the important changes which take place at birth. It seems also probable that very little of the blood propelled from the left ventricle passes into the descending aoi-ta beyond the ductus arteriosus diu-ing those months of foetal life in which the peculiarities of the circulation are most complete.

From the preceding account of the course of the blood in the foetus, it will be seen that, whilst the modified blood from the placenta is principally conveyed to the upper or cephalic half of the foetus, the lower half of the body is chiefly supplied with the blood which has already circulated through the head and upper limbs. The larger portion of the blood, however, which passes into the descending aorta, is sent out of the body to the placenta. This duty is principally performed by the right ventricle, which after birth is charged with an office somewhat analogous, in having to propel the blood through the lungs. But the passage of the blood through the vessels of the umbilical cord and placenta is longer and subject to greater resistance than that of the pulmonary chculation, and the right ventricle of the foetus, although probably aided by the left in the placental circulation, also takes a large shave in the systemic through the lower half of the body ; and this, ijerhaps, may be the reason wliy the wall of the right equals in thickness that of the left ventricle in the foetus.

Sabatier was the first to call attention particularly to the action of the Eustachian valve in separating the currents of blood entering the right auricle by the superior and inferior venae cavae. (Traite d'Anat., vol. ii., p. 22i.) This separation, as well as that occm-ring between the currents passing through the aortic arch and the ductus arteriosus into the descending aorta, were illustrated experimentally by John Reid. (See art. '• Heart," in Cyclop, of Anat. and Phj'siol., and Edin. Med. and Sui-g. Journal, ISo.j.) A striking confirmation of the extent to which the last mentioned division of the two currents of the foetal blood may take place, without distm'bance of the chculation up to the time of bii-th, is afforded by the examples of malformation in which a complete obliteration has existed in the aortic trunk immediately before the place of the union of the ductus arteriosus with the posterior part of the aortic arch.

Changes in the Circulation at Birth

The changes which occur in the organs of circulation and respiration at birth, and lead to the establishment of their permanent condition, are more immediately determined by the inflation of the, lungs with air in the first respiration, the consequent rapid dilatation of the pulmonary blood-vessels with a greater quantity of blood, and the interruption to the passage of blood through the placental circulation. These changes are speedily accompanied by shrinking and obliteration of the ductus arteriosus, in the space between the division of the right and left pulmonary arteries and its junction with the aorta, and of the umbilical arteries from the hypogastric trunk to the place of their issue from the body by the umbilical cord ; — by the cessation of the passage of blood through the foramen ovale, and somewhat later by the closure of that foramen, and by the obliteration of the umbilical vein as far as its entrauce into the liver, and of the ductus venosus within that organ.

The process of obliteration of the arteries appears to depend at first mainly on the contraction of the coats, but this is very soon followed by a considerable thickening of their substance, reducing ra^mlly their internal passage to a minute tube, and leading in a short time to final closure, even although the vessel may not present externally any considerable diminution of its diameter. It commences at once, and is perceptible after: a few respirations have occurred. It makes rapid progress upon the first and second days, and by the third or fourth days the passage through the umbilical arteries is usually completely interrupted. The ductus arteriosus is rarely found open after the eighth or tenth day, and by three weeks it has in almost all instances become completely impervious.

The process of closure in the veins is slower, there not being the same thickening or contraction of their coats ; but they remain empty of blood and collapsed, and by the sixth or seventh day, are generally closed.

Although blood ceases at once to pass through the foramen ovale from the moment of birth, or as soon as the left auricle becomes filled with the blood returning from the lungs, and the pressure within the two auricles is equalised, yet the actual closure of the foramen is more tardy than any of the other changes now referred to. It is gradually effected by the union of the forepart of the valvular fold forming the floor of the fossa ovalis with the margin of the annulus on the left side ; but the crescentic margin is generally perceptible in the left auricle as a free border beyond the place of union and not unfrequently the union remains incomplete, so that a probe may be passed through the reduced aperture. In many cases a wider aperture remains for more or less of the first year of infancy, and in certain instances there is such a failure of the union of the valve as to allow of the continued passage of venous blood, especially when the circulation is disturbed by over-exertion, from the right to the left auricle, as occurs in the malformation attending the morbus coeruleus.

1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital

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