Book - Quain's Embryology 9
DEVELOPMENT OF THE HEART
Origin of the Heart
Simple Tubular Foi'in. — The heart takes its origin in the form of an elongated sac or dilated tube in the substance of a thickening of the splanehuo-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 isleui-operitoneal cavity of its own side, and when the median fusion takes place the union of these two becomes the pericardium.
Fig. 584. — Diagrammatic longitcdixal section through the Axis op an Embrto.
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 splanclmopleure ; Avi, commencing (head) fold of the amnion ; NO, neural canal, closed in front, but still open behind ; C/i, notochord, in front uncleft mesoblast in the ba«e of the cranium ; D, the commencing foregut, or alimentary canal ; Ht, heai-t ; jU^, pleuro-i>eritoneal 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) ; «, frontal plate ; h, protovertebrse, on which, the primitive aortiB 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
Fig. 586.— Diagrammatic Outlines of the Heart and First Arterial Vessels OP 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 clown 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.
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).
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 ]:»y 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 consideral:)ly, and the constricted part elongating produces what has been called the canalis auricular is.
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 septTim,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 OP 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 externallj^ ; 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 somcAvhat 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 chordse 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 w^hich affect the external form and thickness of its walls. In early foetal Hfe 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.
