Book - Text-Book of the Embryology of Man and Mammals 17-1
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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The Development of the Blood-vessel System
The very first fundament of the blood-vessels and the blood has already been treated of in the first part of this text-book. We will therefore here concern ourselves with the special conditions of the vascular system, with the origin of the heart and chief blood-vessels, and with the special forms which the circulation presents in the various stages of development, and which are dependent on the formation of the foetal membranes. In this I shall treat separately, both for the heart and for the rest of the vascular system, the first fundamental processes of development and the succeeding alterations, from which the ultimate condition is finally evolved.
The first Developmental Conditions of the Vascular System
Development of the Heart
The vascular system of Vertebrates can be referred back to a very simple fundamental form namely, to two blood-vessel trunks of which the one runs above and the other below the intestine in the direction of the longitudinal axis of the body. The dorsal trunk, the aorta, lies in the attachment of the dorsal mesentery, by means of which the intestine is connected to the vertebral column ; the other trunk, on the contrary, is imbedded in the ventral mesentery, as far, at least, as such a structure is ever established in the Vertebrates ; it is almost completely metamorphosed into the heart. The latter is therefore nothing else than a peculiarly developed part of a main blood-vessel provided with especially strong muscular walls.
In the first fundament of the heart there are two different types to be distinguished, one of which is present in Selachians, Ganoids, Amphibia, and Cyclostomes, the other in Bony Fishes and the higher Vertebrates Reptiles, Birds, and Mammals.
In the description of the first type, I select as an example the development of the heart in the Amphibia, concerning which a detailed account has very recently been published by RABL.
Fig. 297. Cross section through the region of the heart of an embryo of Salamandra maculosa, in which the fourth visceral arch is indicated, after RABL. d, Epithelium of the intestine ; cm, visceral middle layer ; ep, epidermis ; Ih, anterior part of the body-cavity (pericardio-thoracic cavity) ; end, endocardium ; p, pericardium ; vhg, mesocardium anterius.
In Amphibia the heart is established very far forward in the embryonic body, underneath the pharynx or cavity of the head-gut (figs. 297, 298). The embryonic body-cavity (IK) reaches into this region, and in cross sections appears upon both sides of the median plane as a narrow fissure. The lateral halves of the body-cavity are separated from each other by a ventral mesentery (vhy), by means of which the under surface of the pharynx is united with the wall of the body. If we examine the ventral mesentery more closely, we observe that in its middle the two mesodermic layers from which it has been developed separate from each other and allow a small cavity (h) to appear, the primitive cardiac cavity. This is stir rounded by a single layer of cells, which is afterwards developed into the endocardium (end).* Outside of the latter the adjacent cells of the middle germ-layer are thickened ; they furnish the material out of which the cardiac musculature (the myocardium) and the superficial membrane of the heart (pericardium viscerale) arise. The fundament of the heart is attached above [dorsally] to the pharynx (d) and below to the body-wall by the remnant of the mesentery, which persists as a thin membrane. We designate these two parts as the suspensory ligaments of the heart, as back [dorsal] and front [ventral] cardiac mesenteries (hhg, vhy), or as mesocardium posterius and anterius. At this time there is nothing to be seen of a pericardial sac, unless we should designate as such the anterior [ventral] region of the bodycavity, from which, as the further course of development will show, the pericardium is chiefly derived.
In the second type, the heart arises from distinct and widely separated halves, as the conditions in the Chick and the Rabbit most distinctly teach.
In the Chick the first traces of the fundament may be demonstrated at an early period, in embryos with four to six primitive segments. They appear here at a time when the various germ-layers are still spread out flat, at a time when the front part of the embryonic fundament first begins to be elevated as the small cephalic protuberance, and the cephalic portion of the intestine is still in the first phases of development. As has already been stated, the intestinal cavity in the Chick is developed by the folding together and fusion of the intestinal plates [splanchnopleure]. If one examines carefully the ridge of an intestinal fold in the very process of being formed (fig. 299 A df) t one observes that its visceral middle layer is somewhat thickened, composed of large cells, and separated from the entoblast by a space filled with a jelly-like matrix. In the latter there lie a few isolated cells, which subsequently the yolk, so that the two endothelial sacs of the heart are in contact ; they subsequently fuse. They lie in a cardiac suspensorhun formed by the visceral middle layers, the mesocardium, on which one can distinguish an upper [dorsal] and an under part mesocardium superius(-f )and inferius (*). By means of this mesocardium the primitive body-cavity is temporarily divided into two portions.
- Relative to the origin of the endothelial sac of the heart, compare the observations given on page 186.
Fig. 298. Cross section from the same series as that from which fig. 297 was drawn, after KABL.
, Epithelium of the intestine ; vm, visceral, pm, parietal middle layer ; hhg, posterior, vlig, anterior mesocardium ; end, endocardium ; h, cavity of the heart ; Ik, ventral part of the body-cavity ; ep, epidermis. +++++++++++++++++++++++++++++++++++++++
Fig. 299. Three diagrams to illustrate the formation of the heart in the Chick.
.', Xeural tube; m, mesenchyma of the head ; d, intestinal cavity ; df, folds of the intestinal plate [splanchnopleure], in which the endothelial sacs of the heart are established ; h, endothelial sac of the heart ; ch, chorda ; Ih, bodycavity ; ale, outer, ik, inner germ-layer ; 'nilc 1 , parietal middle layer ; ink", visceral middle layer, from the thickened portion of which the musculature of the heart is developed ; dn, intestinal suture, in which the two intestinal folds are fused ; db, part of the entoblast which has become detached from the epithelium of the cephalic portion of the intestine at the intestinal suture and lies on the yolk ; + dorsal mesocardium ; * ventral mesocardium.
A, The youngest stage shows the infolding of the splanchnopleure, by means of which the cephalic part of the intestine is formed. In the angles of the intestinal folds the two endothelial sacs of the heart have been established between the inner germ-layer and the visceral middle layer.
B, Somewhat older stage. The two folds (A df) have met in the intestinal suture (dn), so that the two endothelial sacs of the heart lie close together in the median plane below the head-gut.
C, Oldest stage. The part of the entoblast which lines the head-gut (<?) has become separated at the intestinal suture (B dn) from the remaining part of the entoblast, which (J6) lies upon +++++++++++++++++++++++++++++++++++++++
surround n, small cavity, the primitive cardiac cavity (A). These cells issuine more of an endothelial character. While the intestinal folds grow toward each other, the two endothelial tubes become enlarged and ])iish the thickened part of the visceral middle layer before them, so that the latter forms a low, ridge-like elevation into the primitive body-cavity. hi the embryos of higher Vertebrates also, just as in the Amphibia, this stretches forward into the embryonic fundament as far as the last visceral arch, and has here received the special name of neck-cavity or parietal cavity.
In older embryos (fig. 299 7>) the edges of the two folds have met in the median plane, and consequently the two cardiac tubes have moved close together. A process of fusion then takes place between the corresponding parts of the two intestinal folds.
First the entoblastic layers fuse, and in this way is produced (fig. 299 7>) beneath the chorda dorsalis (ck) the cavhVy of the head-gut (d), which then detaches itself from the remaining part of the entoblast (fig. 299 G db) ; the latter is left lying on the yolk and becomes the yolk-sac. Under the cavity of the head-gut the two cardiac sacs have come close together, so that their cavities are separated from each other by their own endothelial walls only. By the breaking through of these there soon arises from them (7i) a single cardiac tube. On the side toward the body-cavity this is covered by the visceral middle layer (mk 2 ), the cells of which are distinguished in the region of the fundament of the heart by their great length and furnish the material for the cardiac musculature, while the inner endothelial membrane becomes only the endocardium.
The whole fundament of the heart lies, as in the Amphibia, in a ventral mesentery, the upper [dorsal] part of which, extending from the heart to the head-gut (fig. 299 G +), can here also be called the dorsal suspensory of the heart or mesocardium postering, and the lower [ventral] part (*) mesocardium anterius. In the Chick, when the cardiac tube begins to be elongated and bent into an S- shaped form, the mesocardium anterius quickly disappears.
Similar conditions are furnished by cross sections through Rabbit embryos 8 or 9 days old. In the latter the paired fundaments of the heart are indeed developed still earlier and more distinctly than in the Chick, even at a time when the entoderm is still spread out flat and has not yet begun to be infolded. Upon cross sections one sees (fig. 301), in a small region at some distance from the median plane, the splanchnopleure separated from the somatopleure by a small fissure (pfi), which is the front end of the primitive body-cavity. At this place the visceral middle layer (ahh) is also raised up somewhat from the entoderm (sw), so that it causes a projection into the bodycavity (ph). Here there is developed between the two layers a small cavity, which is surrounded by an endothelial membrane (lhh\ the primitive cardiac sac. At their first appearance the halves of the heart lie very far apart. They are to be seen both in the very slightly magnified cross section (fig. 300) and also in the surface view of an embryo Rabbit (fig. 302) at the place indicated by h. They afterwards move toward each other in the same manner as in the Chick by the infolding of the splanchnopleure, and come to lie on the under side of the head-gut, where they fuse and are temporarily attached above and below by means of a dorsal and ventral mesentery. Concerning the processes of development just sketched the question may be raised : What relation do the paired and the unpaired fundaments of the heart sustain to each other ? It is to be answered to this, that the impaired fundament of the heart, ivhich is present in the lower Vertebrates, is to be regarded as the original form. The double heart-formation, however aberrant it at first sight it appears, can be easily referred back to this.
Figs. 300, 301. Cross section through the head of an embryo Rabbit of the same age as that shown in fig. 302. From KOLLIKER. Fig. 301 is a part of fig. 300 more highly magnified.
Fig. 300. h, h', Fundaments of the heart ; sr, cesophageal groove.
Fig. 301. rf, Dorsal groove; mp, medullary plate; no, medullary ridge ; h, outer germ-layer; d<l, inner germ-layer ; dd', its chordal thickening ; sp, undivided middle layer ; hp, parietal, dfp, visceral middle layer ; -ph, pericardial part of the body-cavity ; ahh, muscular wall of the heart ; ihh, endothelial layer of the heart ; mes, lateral undivided part of the middle layer ; sw, intestinal fold, from which the ventral wall of the pharynx is formed.
A single cardiac tube cannot bn developed in the higher Vertebrates, because at the time of its formation a headgut does not yet exist, but only the fundament of it is formed in the still flat entoderm. The parts which will subsequently form the ventral wall of the head -gut, and in which the heart is developed, are still two separated territories ; they still lie at some distance from the median plane at the right and at the left. If therefore it is necessary for the heart to be formed at this early period, it must arise in the separated regions, which by the process of infolding are joined into a single ventral tract. The vessel must arise as two parts, which, like the two intestinal folds, subsequently fuse.
Whether the heart is formed either case it has for a time the form of a straight sac lying ventral to the head -gut and composed of two tubes one within the other, which are separated by a large space assumably filled with a gelatinous matrix. The inner, endothelial tube becomes the endocardium ; the outer tube, which is derived from the visceral middle layer, furnishes the foundation for the myocardium and the pericardial membrane that immediately invests the surface of the heart.
Fig. 302. Embryo Rabbit of the ninth day, seen j n one wav or t ie other, in n it from the dorsal side, after KOLLIKER. Magnified 21 diameters.
The axial (stem-) zone (stz) and the parietal zone (2?z) are to be distinguished. In the former 8 pairs of primitive segments have been formed at the side of the chorda and neural tube.
p, Area pellncida ; rf, dorsal groove ; r/t, fore brain; ab, optic vesicle; mil, mid -brain; Mi, hind-brain ; uw, primitive segment ; stz, axial zone ; pz, parietal zone ; h, heart ; ph, pericardial part of the body-cavity ; rd, margin of the anterior intestinal portal showing through the overlying structures ; af, fold of the aninion ; vo, \ena omphalomesenterica.
The First Developmental Conditions of the Large Vessels. Vitelline Circulation, Allantoic and Placental Circulation
At both ends, in front and behind, the heart is continuous with the trunks of blood-vessels, which have been established at the same time with it. The anterior or arterial end of the cardiac tube is elongated into an unpaired vessel, the truncus arteriosus, which continues the forward course under the head-gut, and is divided in the region of the first visceral arch into two arms, which embrace the head-gut on the right and left and ascend within the arch to the dorsal surface of the embryo. Here they bend around and run backward in the longitudinal axis of the body to the tail-end. These two vessels are the primitive aortcv (figs. 107, 116 ao) ; they take their course on either side of the chorda dorsalis, above the entoderm and below the primitive segments. They give off lateral branches, among which the arterice omplialomesentericw are in the Amniota distinguished by their great size. These betake themselves to the yolk-sac and conduct the greatest portion of the blood from the two primitive aortas into the area vasculosa, where it goes through the vitelline circulation.
In the Chick, the conditions of which form the basis of the following account (fig. 303), the two vitelline arteries (R.Of.A, L.Of.A) quit the aortaj at some distance from their tail-ends, and pass out laterally from the embryonic fundament between entoderm and visceral middle layer into the area pellucida, traverse the latter, and distribute themselves in the vascular area. They are here resolved into a fine network of vessels, which lie, as a cross section (fig. 116) shows, in the mesenchyme between the entoderm and the visceral middle layer, and which are sharply bounded at their outer edge (toward the vitelline area) by a large marginal vessel (fig. 303 S.T), the sinus terminalis. The latter forms a ring which is everywhere closed, with the exception of a small region which lies in front, at the place where the anterior amniotic sheath has been developed.
From the vascular area the blood is collected into several large venous trunks, by means of which it is conducted back to the heart. From the front part of the marginal sinus it returns in the two vence vitellinai anter lores, which run in a straight line from in front backwards and also receive lateral branches from the vascular network. From the hind part of the sinus terminalis the blood is taken up by the venae vitellinie posteriores, of which the one of the left side is larger than the one of the right ; the latter afterwards degenerates more and more. From the sides likewise there come still larger collecting vessels, the vense vitellinre laterales. All the vitelline veins of either side now unite in the middle of the embryonic body to form a single large trunk, the vena omphalomesenterica (A'.O/and L.Of), which enters the posterior end of the heart (//).
Fig. 303. Diagram of the vascular system of the yolk-sac at the end of the third day of incubation, after BALFOUB.
The whole blastoderm has been removed from the egg and is represented as seen from below. Hence what is really at the right appears at the left, and vice vtrsd. The part of the area opaca in which the close vascular network has been formed is sharply terminated at its periphery by the sinus terminalis, and forms the vascular area ; outside of the latter lies the vitelline area. The immediate neighborhood of the embryo is free from a vascular network, and now, as previously, is distinguished by the name area pellucida.
H. Heart; A A, aortic arches; Ao, dorsal aorta; L.Of.A, left, R.Of.A, right vitelline artery; S. T, sinus terminalis ; L.Of, left, R.Of, right vitelline vein ; S. V, sinus venosus ; D.C, diictus Cuvieri ; S.Ca.V, superior, V. Ca, inferior cardinal vein. The veins are left in outline; the arteries are black.
The motion of the blood begins to be visible in the case of the Chick as early as the second day of incubation. At this time the blood is still a clear fluid, which contains only few formed components. For the most of the blood-corpuscles still continue to lie in groups on the walls of the tubes, where they constitute the previously described blood-islands (fig. 114), which cause the redbesprinkled appearance of the vascular area. The contractions of the heart, by which the blood is set in motion, are at first slow and then become more and more rapid. On the average, according to PREYER, the strokes then amount to 130 150 per minute. However, the frequency of pulsations is largely dependent upon external influences; it increases with the elevation of the temperature of incubation and diminishes at every depression of it, as well as when the egg is opened for study. At the time when the heart begins to pulsate, no muscle-fibrillse have been demonstrated in the myocardium ; from this results the interesting fact that purely protoplasmic, still undifierentiated cells are in a condition to make strong rhythmical contractions.
At the end of the third or fourth day the vitelline circulation in the Chick is at its highest development ; it has undergone some slight changes. We find instead of a single vascular network a double one, an arterial and a venous. The arterial network, which receives the blood from the vitelline arteries, lies deeper, nearer to the yolk, while the venous spreads itself out above the former and is adjacent to the visceral middle layer. The circulating blood is distinguished by the abundance of its blood-corpuscles, the bloodislands having entirely disappeared.
The function of the vitelline circulation is twofold. First it serves to provide the blood with oxygen, opportunity for acquiring which is afforded by the whole vascular network being spread out at the surface of the egg. Secondly it serves to bring nutritive substances to the embryo. The yolk-el einents below the entoblast are disassociated, liquefied, and taken up into the blood-vessels, by which they are carried to the embryo, where they serve as nutrition for the rapidly dividing cells. Thus far the embryonic body increases in size at the expense of the yolk-material in the yolksac, which becomes liquefied and absorbed.
The system of vitelline blood-vessels in Mammals agrees in general with that of the Chick, and is distinguished from the latter only in some unimportant points, which do not need to be discussed. However, this question certainly arises* What signification has a vitelline circulation in Mammals (fig. 134 ds) in which the egg is furnished with only a small amount of yolk-material ? Two things are here to be kept in mind ; first, that the eggs of Mammals were originally provided with abundant yolk-material, like those of Reptiles (compare p. 222), and, secondly, that the blastodermic vesicle, which arises after the process of cleavage, becomes greatly distended by the accumulation within it of a fluid very rich in albumen, furnished by the walls of the uterus. Out of this vesicle likewise the vitelline blood-vessels undoubtedly take up nutritive material and convey it to the embryo, until a more ample nutrition is provided by means of the placenta.
In addition to the vitelline blood-vessels there arises in the higher Vertebrates a second system of vessels, which is distributed in the foetal membranes outside the embryo and for a time is more developed than the remaining vessels of the embryo. It serves for the allantoic circulation of Birds and Reptiles and the placenta! circulation of Mammals.
When in the Chick the allantois (PI. I., fig. 5 al] is evaginated from the front [ventral] wall of the hind-gut, and as an ever increasing sac soon grows out of the body-cavity through the dermal umbilicus into the coalom of the blastodermic vesicle between the serosa and the yolk-sac, there appear in its walls two blood-vessels, which grow forth from the ends of the two primitive aortse the umbilical vessels, or arterice umbilicales. The blood is again collected from the fine capillary network, into which these vessels have been resolved, into the two umbilical veins (veme \ umbilicales), which, after having arrived at the navel, pass on to the two Cuvierian ducts (see p. 577) and pour their blood into these near the entrance of the latter into the sinus venosus. The terminal part of the right vein soon atrophies, whereas the left receives the lateral branches of the right side and is correspondingly developed into a larger trunk. This now also loses its original connection with the ductus Cuvieri, since it effects with the left hepatic vein (vena hepatica revehens) an anastomosis, which continually becomes larger and finally carries the whole stream of blood. Together with the left hepatic vein the left umbilical vein then empties directly into the sinus venosus at the posterior margin of the liver (HOCHSTETTER).
The umbilical and vitelline veins undergo opposite changes in calibre during development : while the vitelline circulation is well developed, the umbilical veins are inconspicuous stems ; afterwards, however, with the increase in the size of the allantois they enlarge, whereas the venae omplialomesentericae undergo degeneration and in the same proportion as the yolk-sac by the absorption of the yolk becomes smaller and loses in significance.
So far as regards the purpose of the umbilical circulation, it subserves in Reptiles and Birds ihe function of respiration. For the allantois, when it has become larger, in the Chick for example, applies itself closely to the serosa and spreads itself out in the vicinity of the air-chamber and underneath the shell, so that the blood circulating in it can enter into an exchange of gases with the atmospheric air. It loses its importance for respiration in the egg only at the moment \vhen the Chick with its beak breaks through the surrounding embryonic membranes, and breathes directly the air contained in the air-chamber. For the conditions of the circulation are now altered throughout the whole body, since with the beginning of the process of respiration the lungs are in a condition to take up a greater quantity of blood, resulting in a degeneration of the umbilical vessels (compare also p. 584).
The umbilical or placental circulation in Mammals (fig. 139 Al) plays a still more important role ; for here the tw r o umbilical arteries convey the blood to the placenta. After the blood has been laden in this organ with oxygen and nutritive substances, it flows back again to the heart, at first through two, afterwards through a single umbilical vein (p. 584).
The further Development of the Vascular System up to the Mature Condition
The Metamorphosis of the Tubular Heart into a Heart with Chambers
As has been .shown in a preceding section, the heart of a Vertebrate originally has for a short time the form of a straight sac, which sends off at its anterior end the two primitive aortic arches, while it receives at its posterior end the two omphalomesenteric veins. The sac lies far forward immediately behind the head on the ventral side of the neck (fig. 304 /*,), in a prolongation of the body-cavity (the parietal or cervical cavity). It is here attached by means of a mesentery of only brief duration, which stretches from the alimentary canal to the ventral wall of the throat, and which is divided by the cardiac sac itself into an upper [dorsal] and an under part, or mesocarclium posterius and anterius.
During the first period of embryonic development the heart is distinguished by a very considerable growth, especially in the longitudinal direction ; consequently it soon ceases to find the necessary room for itself as a straight sac, and is therefore compelled to bend itself into an S-shaped loop (lig. 304). It then takes such a position in the neck that one of the bends of the S, which receives the vitelline veins or, let us say briefly, the venous portion, conies to lie behind and at the left ; the other or arterial portion, which sends off' the aortic arches, in front and at the right (fig. 305).
But this initial position is soon altered (figs. 305, 313) by the two curves of the S assuming another relation to each other. The venous portion moves headwards, the arterial, on the contrary, in the opposite direction, until both lie approximately in the same transverse plane. At the same time they become turned around the longitudinal axis of the embryo, the venous loop moving dorsally, the arterial, on the contrary, ventrally. Seen from in front [ventral aspect] one hides the other, so that it is only in a side view that the S-shaped curvature of the cardiac sac is distinctly recognisable.
By the increase in the size of this viscus the anterior part of the bodycavity is already greatly distended, and becomes still more so in later stages, when there is produced a very thin-walled elevation, that projects out to a great distance (figs. 157 h, 314). Inasmuch as the heart completely fills the cavity, and is covered in by only the thin, transparent, and closely applied wall of the trunk, the niembrana reuniens inferior of KATHKE, it appeal's as though at this time the heart were located entirely outside of the body of the embryo.
After the completion of the twisting, there is effected a division of the S-shaped sac into several successive compartments (figs. 306, 308). The venous portion, which has become broader, and the arterial part are separated from each other by a deep constriction (ok} and can now be distinguished as atrium (vli) and ventricle, while the constricted region between the two may be indicated, by a designation introduced by HALLER, as auricular canal (ok). The atrium thereby acquires a striking form, since its two lateral walls develop large out-pocketings (ho), the auricles of the heart (auriculae corclis) ; the free edges of the latter, which in addition soon acquire notches, are turned forward, and subsequently enfold more and more the arterial part of the heart, the truncus arteriosus (Ta), and a part of the surface of the ventricle.
Fig. 304. Head of a Chick incubated 58 hours, seen from the dorsal face, after MIHALKOVICS. Magnified 40 diameters.
The brain is divided into 4 vesicles: prli, primary fore-brain vesicle ; iiih, mid-brain vesicle ; kh, hindbrain vesicle ; nil, after-brain vesicle; an, optic vesicle ; k, heart (seen through the Jast brainvesicle) ; -co, vena omphalomesenterica ; us, primitive segment ; rm, spinal cord ; x, anterior wall of brain, which is evanii'ated to form the cerebrum.
The auricular canal (fig. 308 o)is in embryos a well-distinguished narrowed place in the cardiac tube. Owing to the great flattening of its endothelial tube in the sagittal direction, its walls almost coming into contact, the passage between atrium and ventricle is reduced to a narrow transverse fissure. It is here that the atrioventricular valves are afterwards developed.
Fig. 305. Heart of a human embryo, the body of which was 2 - 15 mm. long (embryo Lg), after His. [Compare fig. 313.] K, Ventricle ; Ta, truncus arteriosus ; V, venous end of the S-shaped cardiac sac.
Fig. 306. Heart of a human embryo that was 4'3 mm. long, neck measurement (embryo 1), after His. k, Ventricle ; Ta, truncus arteriosus ; ok, canalis auricularis ; vh, atrium with the heart-auricles ho (auriculas cordis).
The fundament of the ventricle at first presents the form of a curved tube (figs. 305, 306 k), which however soon changes its form. For at a very early period there is observable on its anterior [ventral] and posterior surfaces a shallow furrow running from above downward, the sulcus interventricularis (fig. 307 si), which allows a left and a right half of the ventricle to be distinguished externally. The latter is the narrower, and is continued upward into the truncus arteriosus (Ta), the beginning of which is somewhat enlarged and designated as bulbus. Between bulbus and ventricle lies a place that is only slightly constricted, called the /return Halleri ; it was recognised even by the older anatomists, then remained for a time little regarded, and now has been again described as noteworthy by His. For it marks the place at which subsequently the semilunar valves are established.
During the externally visible changes of form, some alterations are also progressing in the finer structure of the walls of the heart. As previously remarked, the fundament of the heart consists in the beginning of two sacs, one within the other an inner endothelial tube lined with flat cells, and an outer muscular sac consisting of cells with abundant protoplasm and derived from the middle germ-layer. The two are completely separated from each other by a considerable space, which is probably filled with gelatinous substance.
The endothelial tube is in general a tolerably faithful copy of the muscular sac, yet the narrower and wider regions are more sharply marked off from one another in the former than in the latter ; "as regards its form, it sustains such a relation to the whole heart as it would if it were a greatly shrivelled, internal cast of it " (His). In the muscular sac distinct traces of muscle-fibres can be recognised even at the time when the S-shaped curvature makes its appearance. At later stages in the development differences appear between atrium and ventricle. In the atrium the muscular wall is uniformly thickened into a compact plate, with the inside of which the endothelial tube is in immediate contact. In the ventricle, on the contrary, there occurs a loosening, as it were, of the muscular wall. There are formed numerous small trabeculse of muscular cells, which project into the previously mentioned space between the two sacs and become united to one another, forming a large-meshed network (fig. 311 A). The endothelial tube of the heart, by forming out-pocketings, soon comes into intimate contact with the trabeculse, and envelops each one of them with a special covering (His). Thus there arise in the spongy wall of the ventricle numerous spaces lined with endothelium, which toward the surface of the heart end blindly, but which communicate with the central cavity and like this receive into them the stream of blood.
The embryonic heart of Man and Mammals resembles in its first condition that which has been described up to this point the heart of the lowest Vertebrates, the Fishes. In the former as in the latter it consists of a region, the atrium, which receives the venous blood from the body, and of another, the ventricle, which drives the blood into the arterial vessels. Corresponding to this condition of the heart, the whole circulation in embryos of this stage and in Fishes is still a single and a single one. This becomes changed in the evolution of Vertebrates, as in the embryonic life of the individual, with the development of the lungs, upon the appearance of which a doubling of the heart and of the blood-circulation is introduced.
The cause of such a change is clear, from the topographical relation of the two lungs to the heart, the former arising in the immediate vicinity of the heart by evagination of the fore-gut (fig. 314 la). The lungs therefore receive their blood from an arterial stem lying very near the heart, from the fifth [sixth] pair of aortic arches that arise from the trimcus arteriosus. Similarly they give back again the venous pulmonary blood directly to the heart through short stems, the pulmonary veins, which, originally united into a single collecting trunk (BoRN, ROSE), open into the atrium at the left of the great venous trunks. Therefore the blood that flows directly out of the heart into the lungs also flows directly back again to the heart. Herein is furnished the prerequisite for a double circulation. This comes into existence when the pulmonary and the body currents are separated from each other by means of partitions throughout the short course of the vascular system which both traverse in common (viz., atrium, ventricle, and trimcus arteriosus).
The process of separation begins in the vertebrate phylum with the Dipnoi and Amphibia, in which pulmonary respiration appears for the first time and supplants bronchial respiration. In the amniotic Vertebrates it is accomplished during their embryonic development. Therefore we now have to follow out further the manner in which, in the case of Mammals and especially of Man, according to the recent investigations of His, BORN, and ROSE, the partitions are formed how atrium and ventricle are each divided into right and left compartments, and the truncus arteriosus into arteria pulmonalis and aorta, and how in this way the heart attains its definite form.
The partitions arise independently in each of the three divisions of the heart mentioned.
Let us first take into consideration the atrium, which is for a time the largest and most capacious region of the cardiac sac (fig. 308). In Man a separation into left and right halves (Iv and rv) is observable even in the fourth week, since there is then formed on its hinder [dorsal] and upper wall a perpendicular projection inward, the first trace of the atrial partition (vs) or septum atriorum.
The halves are even now distinguished by the fact that they receive different venous trunks. The vitelline and umbilical veins, as well as the Cuvierian ducts to be discussed later, empty their blood into the right compartment, not directly, however, and by means of separate orifices, but after they have united with one another in the vicinity of the heart to form a large venous sinus (sr) the sinus venosus or s. reunions. This is immediately adjacent to the atrium and communicates with it by means of a large opening in its posterior [dorsal] wall, which is flanked on the right and on the left by a large venous valve (*). Only one small vessel, which traverses the musculature of the heart obliquely, opens, near the atrial partition, into the left compartment ; it is the previously mentioned unpaired pulmonary vein, which is formed immediately outside the atrium by the union of four branches, two of which come from each of the two wings of the lung now being established.
In the further course of development the atrial partition grows from above downward until it reaches the middle of the atrial canal (fig. 309 si). In this manner two completely separated atria would have come into existence at a very early period, if there had not been formed in the upper part of the partition, while it was still growing downward, an opening, the future foramen ovale, which maintains a connection between the two chambers (fig. 309) up to the time of birth. The opening has arisen either from the septum atriorum having become thin and having broken through at a certain region, or from its having been incomplete at this place from the very beginning, as is the case with the Chick for example, where it is traversed by numerous small orifices. Afterwards the foramen ovale, adapting itself to the conditions of the circulation existing at the time, becomes still larger.
Fig. 308. Heart of a human embryo 10 mm. long, neck measurement ; posterior [dorsal] half of the heart, the front walls of which have been removed. After His.
ks, Partition of the ventricle ; Ik, left, rk, right ventricle ; ok, auricular canal ; Iv, left, re, right atrium ; sr, mouth of the sinus reunions ; vs, partition of the atrium (atrial crescent, His ; septum primum, BORN) ; * Eustachian valve ; Ps, septum spurium.
Fig. 309. Posterior (dorsal) half of the heart of a human embryo of the fifth week, cut open, after His.
ks, Ventricular partition ; Ik, left, rk, right ventricle ; si, lower [posterior] part of the atrial partition (septum intermedium, His) ; to, left, rv, right atrium ; sr, mouth of the sinus reunions ; vs, atrial partition (atrial crescent, His ; septum secundum, BORN) ; Ps, septum spurium ; * Eustachian valve.
The of the atrial partition has, moreover, the immediate result of separating the auricular canal into the left and right atrioventricular orifices (compare fig. 308 ok with fig. 309). The auricular canal, even very soon after its formation, undergoes important alterations both from without and within. At first visible from the outside (fig. 308 o&), it afterwards disappears from view (fig. 309) by being in a manner overgrown on all sides by the ventricle, and thereby incorporated in its walls, which enlarge upward and, in consequence of a vigorous growth of the musculature, acquire considerable thickness. The opening of the atrial canal into the ventricle, or the foramen atrioventriculare commune (fig. 310 A F.av.c), now has the form of a fissure extending from left to right, which is bounded on either side by two ridge-like lips (o.ek and u.ek}the atrioventricular lips of LINDES, or the endotbelial cushions of SCHMIDT. The ridges have arisen from a growth of the endocardium, and consist of a, gelatinous connective substance and an endothelial investment. The atrial partition, when it has grown down to the auricular canal, soon fuses along its free lower margin with these lips (fig. 309 si) ; the auricular canal is thereby divided into a left and a right atrioventricular opening, ostium atrioventriculare sinistruin and clextrum (fig. 310 B F.av.s and F.av.d], and at the same time both the dorsal and ventral endocardial ridges, which originally bound the opening, are divided in the middle (o.ek and n.ek). The dorsal components soon fuse with the corresponding pieces of the opposite [ventral] side, and thus there arise at the lower margin of the atrial partition (fig. 309 si) two new ridges, one of which projects into the left, the other into the right atrioventricular opening, which furnish the foundation of the median cuspidate valves.
The development of the atrial partition and the division of the auricular canal into the two atrioventricular openings are closely related processes, the former being the cause of the latter. This is clearly proved by pathological -anatomical conditions of arrested development of the heart. In all cases in which the formation of the atrial partition has been for any reason w r hatever interrupted and the lower part of it has been altogether wanting, there has always been only one atrioventricular opening (an ostium venosum commune) present (ARNOLD).
Before we progress further in the history of the development of the atrium, we must add an account of the metamorphoses which have taken place meanwhile in the territory of the ventricle and truncus arteriosus.
The ventricle begins to acquire its partition not much later than the atrium. By the end of the first month its musculature has become considerably thickened (fig. 311 A). Muscular trabeculre have arisen, which project far into the interior of the chamber and are joined to one another, so as to constitute a spongy tissue, the numerous fissures in which are continuous with the narrowed cavity of the heart and likewise allow the current of the blood to pass through them. At one place the musculature is especially thickened and forms a crescent-shaped fold projecting inward, the fundament of the ventricular partition (septum ventriculorum) (figs. 308, 309, 310 ks). This takes its origin from the lower and posterior [dorsal] wall of the ventricle, in the region which is marked externally by the previously mentioned sulcus interventricularis (fig. 307 si). Its free edge is directed upwards and grows toward the bulbus arteriosus and the atrioventricnlar opening. The latter originally lies more in the left half of the ventricle (fig. 310 A F.av.c), but it gradually moves over more to the right, and finally assumes such a position that the ventricular partition by its growth upwards meets it exactly in the middle and fuses with its edges directly opposite the atrial partition (figs. 309, 310 B).
Fig. 310. Two diagrams (after BORN) to elucidate the changes in the mutual relations of the ostium atrioventriculare and the ostium interventriculare, as well as the division of the ventricle and large arteries. The ventricles are imagined to have been divided into halves ; one looks into the posterior [dorsal] halves, in which, moreover, the cardiac trabeculse, etc., have been omitted for the sake of simplifying the view.
J, Heart of an embryo Rabbit, in which the head is 3-5 5-8 mm. long. The ventricle is divided by the ventricular partition (ks) into a left and a right half as far as the ostium interventriculare (Oi). The right end of the foramen atrioventriculare commune (F.av.c) extends into the right ventricle ; the endocardia! cushions (o.ek, u.ek) are developed.
B, Heart of an embryo Rabbit, head 7'5 mm. long. The endocardial cushions (o.ek, u.ek) of the foramen atrioventriculare commune are fused, and thereby the for. atrioventr. com. is now separated into a for. atrioventr. dextrum (F.av.d) and sinistrum (F.av.s). The ventricular partition (ks) has likewise fused with the endocardial cushions, and has grown forward as far as the partition (s) of the trunciis arteriosus. By the closure of the remnant of the ostium interventriculare (Oi) the septum membranaceum is formed.
rk, Right, Ik, left ventricle ; ks, ventricular partition ; Pu, arteria pulmonalis ; Ao, aorta ; s, partition of the truncus arteriosus ; Oi, ostium interventriculare ; F.av.c, foramen atrioventriculare commune ; F.ae.d and F.av.s, foramen atrioventriculare dextrum and sinistrum ; o.ek, u.ek, upper and lower endothelial or endocardial cushions.
The division of the ventricle in Man is completed as early as the seventh week. From the atrium, the two compartments of which are united by the foramen ovale, the blood is now conducted through a right and a left ostium atrioventriculare into completely separated right and left ventricles.
The two atrioventricular openings are narrow at the time of their origin ; they are in part surrounded by the previous!} mentioned endocardial ridges that project from the partition, in part by corresponding growths of the endocardium at their lateral circumference. The membranous projections are comparable with primitive pocketvalves, such as are also established in the bulbus arteriosus (GEGENBAUR) ; they constitute the starting-point for the development of the large atrioventricular valves, but furnish, as GEGENBAUR and BERNAYS have shown, only a part the membranous marginal thickening (mk l ) which subsequently disappears almost completely, whereas the compact main part of the valve arises from that portion of the thickened muscular wall of the ventricle itself that surrounds the atrioventricular opening (fig. 311 B mfc).
Fig. 311. Diagrammatic representation of the formation of the atrioventricular valves. A , Earlier, B, later condition. After GEGENBAUR. mk, Membranous valve ; mk\ the primitive part of the same ; cht, chordae tendinese ; v, cavity of the ventricle ; b, trabecular network of cardiac musculature ; 21111, papillary muscles ; tc, trabeculfe carnese.
As was previously stated, in the case of Man the wall of the ventricle during the first months consists of a close spongy network of muscular trabeculae, which are invested by the endocardium and the interstices of which communicate with the small central cavity (fig. 311 A). Such a spongy condition of the wall of the heart persists permanently in Fishes and Amphibia ; in the higher Vertebrates and Man, on the contrary, metamorphoses occur. Toward its external surface the wall of the heart becomes more compact, in that the muscular trabeculae become thicker and the spaces between them narrower, in some parts even disappearing entirely (fig. 311 B tc). The reverse of this process takes place toward the inside. In the vicinity of the atrioventricular opening the trabeculse become thinner and the interstices larger. In this way a part of the thick wall of the ventricle, which looks toward the atrium and encloses the opening, is undermined, as it were, by the blood-current. In this part the muscle-fibres afterwards become entirely rudimentary; there are formed from the interstitial connective-tissue substance tendinous plates, which with the endocardial cushions attached to their margins become the permanent atrioventricular valves (fig. 311 B ink). The latter therefore arise from a part of the spongy wall of the ventricle.
The remnants of the shrivelled muscular trabecuke (fig. 311 B cht), which are attached to the valve from below, become still more rudimentary in the immediate vicinity of the attachment : here also a part of the muscular fibres disappears entirely ; the connective tissue, on the contrary, is preserved, and is converted into the tendinous cords which, known under the name of chordca tendinete, serve to hold in place the valves. At some distance from the latter the trabeculse projecting into the ventricle preserve their fleshy condition and become the papillary muscles (pvi), from the apices of which the chordae tendinere arise. " Whatever of the primitive trabecular network still persists on the inner surface of the ventricle forms a more or less stout ineshwork of muscles, the fleshy pillars of the heart (tc), or trabeculfe carnese." In consequence of all these alterations the originally small cavity of the ventricle has become considerably enlarged at the expense of a part of its spongy wall. For the whole of the space which in fig. 311 B lies below the valves has been produced from the system of originally narrow spaces (fig. 311 A), and has been employed for the enlargement of the central cavity by the degeneration of the fleshy columns into slender tendinous cords.
It still remains for us to investigate the division of the truncus arteriosus and the final metamorphosis of the atrium.
At about the time when the formation of the partition in the ventricle takes place, the truncus arteriosus, which arises from it, becomes somewhat flattened, and thus acquires a fissure-like lumen. On the flat sides two ridge-like thickenings make their appearance (fig. 310 A and B s), grow toward each other, and by their fusion divide the cavity into two passages which are triangular in cross section. Now, too, the beginning of the internal separation makes itself visible externally as two longitudinal furrows, in the same way that the formation of a partition in the ventricle is indicated by the sulcus interventricularis. The two canals resulting from the division are the aorta and the pulmonary artery (Ao and Pu). For a time they continue to be surrounded by a common adventitia, then they become widely separated and also externally detached from each other. The whole process of separation in the truncus arteriosus takes place independently of the development of a partition in the ventricle, beginning as it does at first above and advancing from there downwards. Finally the aortic septum penetrates also into the cavity of the ventricle itself (fig. 310 It s and ks), there unites with the independently developed ventricular partition, furnishes the part known as pars membranacea (Oi), and thus completes the separation of the vessels leading out from the heart, the aorta falling to the lot of the left ventricle, the art. pulmonalis to the right.
The pars membranacea indicates therefore in the finished heart the place at which the separation between the right and left halves of tlu heart is completed (fig. 310 B Oi}. "It is, as it were, the keystone in the final separation of the primitive simple cardiac sac into the four secondary cardiac cavities, as they are formed in Birds and Mammals " (ROSE). From a comparative-anatomical point of view this place presents a special interest from the fact that in Eeptiles there exists here a permanent opening between the two ventricles, the foramen Pannizzse.
Even before the division of the truncus Fig. 3112. Diagram of the ar- arteriosus, the semilunar valves have become arangement of the arterial valves. From GEGENBAUR. established as Jour ridges, consisting ot A, Undivided truncus arteriosus gelatinous tissue with a covering of enclo with four fundaments of valves. B, Division into pui- thelium, at the contracted place which is monaiis (p) and aorta (>, designated as the /return Halleri. Two of each of which possesses three valves. them are halved at the time ot tne division of the truncus into aorta and art.
pulmonalis. For each vessel, therefore, there are now three ridges, which, owing to a shrivelling of the gelatinous tissue, assume the form of pockets. Their arrangement, to which GEGENBAUR has called attention, is intelligible from their method of development, as the accompanying diagram (fig. 312) shows. "By the division of the originally single bulbus arteriosus (A) into two canals (7>), the nodule-like fundaments of the four original valves are distributed in such a manner that the anterior [ventral] one and the anterior halves of the two lateral ones fall to the anterior arterial trunk (pulmonalis), the posterior and the posterior halves of the lateral ones to the posterior arterial trunk (aorta)." Finally, as regards the atrium, it is to be said that the sinus venosus, mentioned at p. 558, the mouth of the pulmonary vein, and the foramen ovale undergo important alterations.
The sinus venosus disappears as an independent structure, since it is gradually merged into the wall of the atrium. In consequence of this the great venous trunks, which originally emptied their blood into it and which have meanwhile been converted into the superior and inferior vense cavre and into the sinus coronarius (the details of which are given in section d), empty directly into the right half of the atrium, and here gradually separate farther and farther from one another. Of the two valves which surround, as was previously stated, the mouth of the sinus venosus, the left becomes rudimentary (figs. 308, 309) ; the right (*), on the contrary, persists at the mouth of the inferior vena cava and of the sinus coronarius, and is divided, corresponding to these, into a larger and a smaller portion, of which the former becomes the valvula Eustachii, the latter the valvula Thebesii.
The four pulmonary veins are united for a time into a common short trunk, which empties into the left half of the atrium. Subsequently the common terminal portion becomes greatly enlarged and merged with the wall of the heart, in the same way as the sinus venosus does. In consequence the four pulmonary veins then open separately and directly into the atrium.
The foramen ovale, the formation of whicli was previously described, maintains a broad communication between the two sides of the atrium during the entire embryonic life. It is bounded behind and below by the atrial partition, a connective-tissue membrane that subsequently receives the name of valvula foraminis ovalis (fig. 309 si). Also from above and in front there is formed a sharp limitation, since a muscular ridge projects inward from the atrial partition, the anterior atrial crescent or the limbus Vieussenii (?;s). Even in the third month all of these parts are distinctly developed ; the valvula foraminis ovalis already reaches nearly to the thickened margin of the anterior muscular crescent, but is deflected obliquely into the left half of the atrium, so that a broad fissure remains open and permits the blood of the inferior vena cava to enter into the left part of the atrium. After birth the margins of the anterior and posterior folds come into contact, and, with occasional exceptions, fuse completely. The posterior fold furnishes the membranous partition of the foramen ovale ; the anterior, with its thickened muscular margin, produces above and in front the linibus Vieussenii. With this the heart has attained its permanent structure.
While the cardiac sac undergoes these complicated differentiations, it changes its position in the body of the embryo and acquires at an period a. special investment, the pericardium. Tn connection with llic latter the diaphragm is formed as a partition between the thoracic and abdominal cavities. This is consequently the most suitable place at which to acquaint ourselves better with these important processes, a part of which are not easily understood. The most of the discoveries in this field we owe to the investigations of CADIAT, His, BALFOUR, USKOW, and others.
The Development of the Pericardial Sac and the Diaphragm. The Differentiation of the Primary Body-cavity into Pericardial, Thoracic, and Abdominal Cavities
Originally the body-cavity is widely extended in the body of the embryo, for it can be traced in the lower Vertebrates into the fundament of the head, where it furnishes the cavities of the visceral arches. After the latter have become closed, during which muscles arise from the cells composing their walls, the body-cavity extends forward as far as the last visceral arch and constitutes a large space (fig. 313), in which the heart is developed within the ventral mesentery (mesocardium anterius and posterius). REMAK and KOLLIKER named this space throatcavity ; His introduced the name parietal cavity. But it will be most appropriate if one designates it, after the permanent organs which are derived from it, as the pericardio - thoracic cavity. The more the cardiac tube is thrown into curves, the more extensive this cavity becomes, and it soon acquires in the embryo a comparatively enormous size. By this its front wall is protruded ventrally like a hernia between the head and the navel of the embryo (figs. 314, 157).
Fig. 313. Human embryo (Lg of His) 215 mm. long, neck measurement. Reconstruction figure, after His (" Menschliche Embryonen "). Magnified 40 diameters.
M/>, Oral sinus ; Ab, aortic bulb ; Vm, middle part of the ventricle ; Vc, vena cava superior or ductus Cuvieri ; Sr, sinus reunions ; Vii, vena nmbilicalis ; VI, left part of the ventricle ; //o, auricle of the heart ; D, diaphragm ; V.om, vena omphalomesenterica ; Lb, solid fundament of the liver ; Lbg, hepatic duct.
The peiicardio-thoracic cavity begins very early to be sharply marked ofT from the future abdominal cavity by a transverse fold (figs. 313, 314 s-f 0> which begins at the front [ventral] and Literal walls of the trunk, and the free edge of which projects dorsalwards and median wards (fig. 314 z-\-l) into the primitive body-cavity. It marks the course which the terminal part of the vena omphalomesenterica takes in order to reach the heart. Subsequently there are found imbedded in the fold all of the venous trunks which empty into the atrial sinus of the heart (figs. 313, 314), the omphalomesenteric and umbilical veins and the Cuvierian ducts (dc), which collect the blood from the walls of the trunk. Therefore the formation of the transverse fold is most intimately connected ivith the development of the veins. It takes the name of septum transversum (massa transversa, USKOW), and has the form of a transverse bridge of substance uniting the two lateral walls of the trunk (fig. 313), which inserts itself between the sinus venosus and the stomach, and is united with both as well as with the ventral mesentery. Its posterior portion (fig. 314 z + l) contains abundant embryonic connective tissue and blood-vessels, and constitutes a mass described as prekepatieus (Vorleber), since the two liver-sacs (fig. 313 Lb + Lbg) grow out from the duodenum into it and produce the hepatic cylinders. In proportion as this takes place, and the hepatic cylinders spread out from the ventral mesentery laterally into the septum transversum, the latter increases in thickness and now embraces two different fundaments, in front, a plate of substance in which the Cuvierian ducts and other veins run to the heart (the primary diaphragm) ; behind, the two lobes of the liver, which produce ridges that project into the body cavity.
By means of the septum transversum the pericardio-thoracic and the abdominal cavities are almost completely separated (fig. 314). There remain only two narrow canals (brh) (thoracic prolongations of the abdominal cavity, His), which establish a connection behind with the abdominal cavity at either side of the intestinal tube and its dorsal mesentery. The two canals (brh) receive the two fundaments of the lungs (Ig) when they grow out from the ventral wall of the intestinal tube. They afterwards become the two thoracic or pleura! cavities (brh), whereas the larger cavity communicating with them (hh), in which the heart has developed, becomes the pericardial chamber. The latter takes up the whole ventral side of the embryo ; the thoracic cavities, on the contrary, lie quite dorsal next to the posterior wall of the trunk.
How does the closure of these three originally communicating spaces take place, and how do they attain their altered, final position in relation to one another? The pericardia! sac is the first to be separated off. The impulse to separation is furnished by the Cuvierian ducts (fig. 314 dc). One portion of the latter runs down from the dorsum, where it arises by the confluence of the jugular and cardinal veins, along the lateral walls of the trunk to the transverse septum (fig. 314 dc} ; it thereby crowds the pleura into the pericardio-thoracic cavity, and in this manner produces the pleuro-pericardial fold. Since the latter is carried farther and farther inward, it continues to narrow the communication between the pericardial cavity (hli) and the two pleural cavities (brh} ; finally, it cuts off the communication entirely, when its free edge has grown [median wards] as far as, and has fused with, the mediastinum posterius, in which the rcsophagus lies. By this migration of the Cuvierian ducts is also explained the position of the superior vena cava, which later opens into the atrium from above, for it is derived from the Cuvierian duct. Originally located in the lateral wall of the trunk, its terminal part is afterwards enclosed in the mediastinum.
Fig. 314. Sagittal reconstruction of a human embryo 5 mm. long, neck measurement (embryo R, His), to elucidate the development of the pericardio-thoracic cavity and the diaphragm, after His.
ab, Bnlbus arberiosus ; brh, thoracic cavity (recessus parietalis, His) ; hh, pericardia! cavity ; dc, ductus Cuvieri ; dv, vena omphalomesenterica ; nr, umbilical vein ; vca, cardinal vein ; rj, jugular vein ; lg. lung ; z + I, fundament of the diaphragm and liver ; ilk, mandible.
After the closure of the pericardia! sac, the narrow, tubular thoracic cavities (fig. 314 Mi) continue for a time to remain in communication behind with the abdominal cavity. The fundaments of the lungs (ly] meantime grow farther into them, and their tips finally come in contact with the upper surface of the liver, which also has now become larger. Then a closure is effected at these places also. From the lateral and posterior walls of the trunk project folds (the pillars of USKOW), which fuse with the septum transversum, and thus form the dorsal part of the diaphragm. One can therefore distinguish a ventral older part and a dorsal younger one.
As GEGENBAUR points out, this explains the course of the phrenic nerve, which runs in front of [ventral to] the heart and lungs and approaches the diaphragm from in front.
Occasionally the fusion of the dorsal and ventral fundaments is interrupted on one side. The consequence of such arrested development is a diaphragmatic hernia i.e., a permanent connection between abdominal and thoracic cavities by means of a hernial orifice, through which loops of the intestine can pass into the thoracic chamber.
When the four large serous spaces of the body have been completely shut off from one another, the individual organs must still undergo extensive alterations of position, in order to attain their ultimate condition. The pericardial sac at first takes up the whole ventral side of the breast, and over a large area is connected with the anterior wall of the thorax and with the upper wall of the diaphragm. Moreover, the latter is united with the liver along its whole under surface. The lungs lie hidden in narrow tubes at the dorsal side of the embryo.
There are two factors that come into the account in this connection (fig. 315). With the increase in the extent of the lungs (/(/), the thoracic cavities (pl.p) extend farther ventrally, and thereby detach the wall of the pericardial sac (jpc), or the pericardium, on the one hand from the lateral and anterior walls of the thorax, and on the other from the surface of the diaphragm. Thus the heart (ht\ with its pericardial sac, is displaced step by step toward the median plane, where, together with the large blood-vessels (ao), the oesophagus (al), and the bronchial tubes, it helps to form a partition the mediastinum -between the greatly enlarged thoracic cavities. In front the pericardial sac then remains in contact with the wall of the thorax (st} and below with tho diaphragm for a little distance only.
The, second factor is the separation of the liver from the primary diaphragm, with ivhich it was united to form the septum transversum. This takes place as follows : At the margin of the liver the peritoneum, which originally covered only its under surface, grows over on to its upper surface, separating it from the primary diaphragm. A connection is retained near the wall of the trunk only. Thus is explained the development of the ligamentum coronarium hepatis, which was disregarded in the section which treated of the ligamentous supports of the liver (p. 330).
Fig. 315. Cross section through an advanced embryo of a Rabbit, to show how the pericardial cavity becomes surrounded by the pleural cavities, from BALFOUR. ht, Heart ; pc, pericardial cavity ; pl.p, thoracic or pleural cavity ; Ig, lung ; al, alimentary canal ; no, dorsal aorta ; ch. chorda ; r'p, rib ; st, sternum ; sp.c, spinal cord.
The diaphragm finally acquires its permanent condition by the ingrowth of muscles from the wall of the trunk into the connectivetissue lamella.
The Metamorphoses of the Arterial System
The development of the large arterial trunks lying in the vicinity of the heart is of great interest from a comparative-anatomical point of view. As in all Vertebrates at least five pairs of visceral arches are established on the two sides of the fore-gut (permanently in the gill-breathing Fishes, Dipnoi, and a part of the Amphibia, transitorily in the higher Vertebrates), so also there are developed at the corresponding places on the part of the vascular system five pairs of vascular arches* (fig. 316 1 ' 5 ). They take their origin from the truncus arteriosus (figs. 316, 317), which runs forward under the fore-gut, then follow along the visceral arches up to the dorsal surface of the embryo, and here unite on either side of the vertebral column into longitudinal vessels, the two primitive aortse (fig. 317 ad}. On this account they are called aortic arches, but they are more appropriately designated as visceral-arch vessels.
In the Vertebrates that breathe by means of gills, the vessels of the visceral arches become of importance in the process of respiration, and early lose their simple structure. From their ventral initial portions there arise numerous lateral branches running to the branchial lamella?, which have arisen in large numbers from the mucous membrane investing the visceral arches ; here they are resolved into fine capillary networks. From these the blood is re-collected into venous branches, which open into the upper end of the visceral-arch vessels. The larger the ventral and dorsal lateral branches, the more inconspicuous does the middle part of the vessel of the visceral arch become. At length it has separated into an initial part, the branchial artery, which is distributed to the branchial lamellae in numerous branches, and an upper part, the branchial vein, into which the blood is re-collected. The two are connected with each other by means of the close network only, which, from its superficial position in the mucous membrane, presents a suitable condition for the removal of the gases from the blood.
Since in the Amniota there are no branchial lamellae produced, branchial arteries and veins also fail to be developed, the vessels of the visceral arches retaining their original simple condition. But thoy are in part of only short duration; they soon suffer, by the complete degeneration of extensive portions, a profound metamorphosis, which is effected in a somewhat different manner in Reptiles, Birds, and Mammals. An exposition of the changes in the case of Man only will be given here.
- (The existence of six pairs of vascular arches has recently been shown to be the typical condition, the newly discovered pair, situated between the fourth and fifth pairs of RATHKE'S scheme (fig. 316), being of short duration in Amniota.)
Fig. 316. Diagram of the arrangement of the vessels of the visceral arches from an embryo of an amniotic Vertebrate.
1 5, First to fifth aortic arches ; a<l, aorta dorsalis ; ci, carotis interna ; ce, carotis externa ; v, vertebralis ; s, subclavia ; p, pulmonalis.
In human embryos only a few millimetres long, the truncus arteriosus, which emerges from the still single cardiac tube, is divided in the vicinity of the first visceral arch into a left and a right branch, which surround the pharynx, and are continuous above with the two primitive aortse. It is the first pair of aortic arches. In only slightly older embryos their number is rapidly increased by the formation of new connections between the ventral truncus arteriosus and the dorsal primitive aorta?. Soon a second, a third, a fourth, and, finally, a fifth pair make their appearance in the same sequence in which the visceral arches are established in the case of Man as well as the remaining Vertebrates.
Fig. 317. Development of the large arterial trunks, represented from embryos of a Lizard (A), the Chick (), and the Pig (C), after RATHKE. The first two pairs of arterial arches have in all cases disappeared . In A and B the third, fourth, and fifth pairs are still fully preserved ; in C only the two latter are still complete. p, Pulmonary artery arising from the fifth arch, but still joined to the dorsal aorta by means of a ductus Botalli ; c, external, c', internal carotid ; aJ, dorsal aorta ; o, atrium ; r, ventricle ; n, nasal pit ; m, fundament of the anterior limb.
The five pairs of vascular arches give off lateral branches to the neighboring organs at a very early period ; of these several acquire a great importance and become carotis externa and interna, vertebralis and subclavia as well as pulrnonalis. The carotis externa (fig. 316 ce and fig. 317 c) arises from the beginning of the first vascular arch, and is distributed to the region of the upper and lower jaws. The carotis iiiterna (tigs. 316 ci, 317 c') likewise arises from the first arch, but farther dorsally, at the point where the arch bends around to become continuous with the root of the aorta ; it conducts the blood to the embryonic brain and to the developing eye-ball (arteria ophthalmica). From the dorsal region of the fourth vascular arch (fig. 316 4 ) a branch is given off which is soon divided into two branches, one of which goes headwards to the medulla oblongata and the brain, the arteria vertebralis (v), whereas the other (s) supplies the upper limb (arteria subclavia). In the course of development these two arteries interchange relations in respect to calibre. In young embryos the vertebralis is by far the more important, while the subclavia is only a small inconspicuous lateral branch. But the more the upper extremity increases in size, the more the subclavia is elevated into the position of" the main trunk, and the more the vertebralis sinks to the rank of an accessory branch. Finally, from the fifth [sixth] arch there bud forth branches to the developing lungs (figs. 316, 317 p).
As the simple diagram shows, the fundament of the arterial trunks which arise from the heart is originally strictly symmetrical. But at an early period there occur reductions of certain vascular tracts even to their complete disappearance ; in this way the symmetrical arrangement is (jradually converted into an unsymmetrical one.
The accompanying diagram (fig. 318) in which the parts of the vascular course that degenerate are left free, and those which continue to be functional are marked by a heavy central line will serve to illustrate this metamorphosis.
First, as early as the beginning of the nuchal flexure, the first and second vascular arches with the exception of the connecting portions through which the blood flows to the carotis externa (b) disappear.
The third arch (c) persists, but loses its connection with the dorsal end of the fourth, and therefore now conveys all its blood toward the head into the carotis iiiterna (), of which it has now become the initial part.
The chief role in the metamorphosis is assumed by the fourth arid fifth arches (fig. 317 C). They soon exceed all other vessels in size, and as they lie nearest to the heart, they are converted into the two chief arteries which arise from it, the aortic arch and the arteria pulmonalis. An important modification is effected at the place of their origin from the tnmcus arteriosus when the latter is divided lengthwise by means of the development of the partition previously mentioned. The fourth arch (fig. 318 e) then remains in connection with the trunk (d) which arises from the left ventricle and receives blood exclusively from that source. The fifth arch (n), on the contrary, forms the continuation of that half (m) of the truncus arteriosus which emerges from the right ventricle. Thus the division of the blood into two separate currents initiated in the heart is also continued into the nearest vessels, but for a short distance only, since the fourth and fifth pairs of vascular arches (fig. 317) still empty their blood together into the aorta cominunis (ad), with the exception of a certain portion which runs through their accessory branches, in part to the head (c.c) and upper limbs, in part to the still diminutive lungs. Gradually, however, the, process of separation thus introduced is continued still farther into the region of the peripheral vessels and finally leads to the establishment of the entirely distinct major and minor circulations. The final condition is attained by the degeneration of certain portions of the vessels and the enlargement of others.
A preponderance of the vascular arches of the left side over those of the right is soon recognisable (fig. 318). The former continually increase in size, while those of the right side become less and less apparent and finally in places disappear altogether. They are retained only in so far as they conduct the blood to the lateral branches which, arising from them, go to the head, the upper limbs, and the lungs. Consequently of the right aortic arch there remains only the tract which gives rise to the right carotis cominunis (c) and the right subclavia (i-\-l). We designate its initial part as the arteria anonyma brachiocephalica. With this the permanent condition is now established. The remnant of the right fourth vascular arch appears as a side branch only of the aorta (e), which forms an arch 011 the left side of the body, and here gives rise to the carotis cominunis sinistra (c) and the subclavia sin. (A) as additional lateral branches.
The right half of the fifth [sixth] pair of vascular arches likewise undergot'H degeneration, except for the portion that conveys blood to the right lung. On the left side of the body, on the contrary, the pulmonary arch still persists for a long time and conducts blood into the left lung and also through the ductus arteriosus Botalli (n), into the aorta. After birth, in connection with pulmonary respiration, the duct of BOTALLI also degenerates. For the lungs, when they are expanded by the first act of inspiration, are in a condition to receive a greater quantity of blood. The consequence is that blood no longer flows into the ductus Botalli, and that the latter is converted into a connective-tissue cord, which extends between aorta and art. pulmonalis.
Fig. 318. Diagrammatic representation of the metamorphosis of the bloodvessels of the visceral arches in a Mammal, after RATHKE.
a, Carotis intei'na ; b, carotis externa ; c, carotis communis ; d, body or systemic aorta ; e, fourth arch of the left side ; /, dorsal aorta ; g, left, k, right vertebral artery ; /i, left subclavian artery ; i, right subclavian (fourth arch of the right side) ; I, continuation of the right subclavian ; m, pulmonary artery ; n, its ductus Botalli.
In addition to the regressive changes mentioned, there are effected meantime alterations of position in the large vascular trunks that arise from the heart. They move at the same time with the heart from the neck region into the thoracic cavity. In this fact lies the explanation of the peculiar course of the nervus laryngeus inf. or recurrens. At the time when the fourth vascular arch still lies forward in the region of its formation in the fourth visceral arch, the vagus sends to the larynx a small nerve branch, which, to reach its destination, passes below [caudad of] the vascular arch.
Fig. 319. Diagrammatic representation of the metamorphosis of the arterial arches in Birds, after RATHKE.
, Intez-nal, b, external, c, common carotid ; d, systemic aorta ; e, fourth arch of the right side (root of the aorta) ; /, rightsubclavian; g, dorsal aorta ; h, left subclavian (fourth arch of the left side) ; i, pulmonary artery ; k and I, right and left ductus Botalli of the pulmonary arteries.
When the latter migrates downwards, the nervus laryngeus must thereby be carried down with it into the thoracic cavity, and must form a loop, one portion of which, arising in the thoracic cavity from the vagus, bends around the arch of the aorta on the left side of the body (but around the subclavia on the right side of the body) to become continuous with the second portion, which takes the opposite or upward course to the region of its distribution.
The processes of development discussed also throw light on a series of abnormalities which are quite frequently observed in the large vascular trunks. I shall cite and explain two of the most important of these cases.
Occasionally in the territory of the vessels of the fourth visceral arches the original symmetrical condition is retained. The aorta is then divided in the adult into right and left vascular arches, which convey the blood into the unpaired aorta. From each of them there arises, as in the embryo, a separate carotis commnnis and subclavia.
Another abnormality is brought about by the development of the aortic arch of the right side of the body instead of that of the left, a condition which is met with in the class of Birds (fig. 319) as the normal state. This malformation is always connected with an altered position of the organs of the chest, a situs inversus viscerum. Of the other changes in the region of the arterial system the metamorphosis of the primitive aorta is to be mentioned before all others. As in the other Vertebrates (fig. 127 o), so in Man, there are formed a right and a left aorta ; but they subsequently move close together and fuse. This, again, explains an abnormality, which, it is true, has very rarely been observed in Man. The aorta is divided into right and left halves by means of a longitudinal partition ; the process of fusion, therefore, has not been fully effected.
The aorta gives off at an early period as branches the unpaired mesenterica sup. and rnesenterica inf. to the intestinal canal ; furthermore, near its posterior end, the two voluminous navel vessels, arteries unibilicales (fig. 139 Al). These run from the dorsal wall of the trunk along the sides of the pelvic cavity ventrally to that part of the allantois which is subsequently differentiated into urinary bladder and urachus, here bend upward and pass on either side of the latter in the abdominal wall to the navel, enter the umbilical cord, and are resolved in the placenta into a capillary network, from which the blood is re-collected into the veme unibilicales. During their passage through the pelvic cavity the umbilical arteries give off lateral branches that are at first inconspicuous, the iliacae internee, to the pelvic viscera, the iliacae externse to the posterior limbs now sprouting forth from the trunk as small knobs. The more the latter increase in size in older embryos, the larger do the iliacse externse and their continuations, the femorales, become.
After giving off the two umbilical arteries, the aorta becomes smaller and is continued to the end of the vertebral column as an inconspicuous vessel, the aorta caudalis or sacralis media.
At birth an important alteration occurs in this part of the arterial system also. With the detachment of the umbilical cord, the umbilical arteries can 110 longer receive blood ; they therefore waste away with the exception of the proximal portion, which has given off as lateral branches the internal and external iliacs, and is now designated as the iliaca communis. However, two connectivetissue cords result from the degenerating vessels, the ligamenta vesico-umbilicalia lateralia, which run to the navel on. the right and left of the bladder.
Metamorphoses of the Venous System
The older excellent works of KATHKE and the more recent meritorious investigations of His and HOCHSTETTER constitute the foundation of our knowledge in the difficult field with which we are now concerned. They show us that originally all of the chief trunks of the venous system, with the exception of the inferior vena cava, are established in pairs and sij HI metrically. This holds true not only for the vessels which collect the blood from the walls of the trunk and from the head, but also for the veins of the intestinal tube and the embryonic appendages which arise from it.
In the first place, so far as regards the veins of the body, the venous blood is collected from the head into the two jugular veins (fig. 320 vj and fig. 321 A je, ji), which run downwards along the dorsal side of the visceral clefts and unite in the vicinity of the heart with the cardinal veins (fig. 320 vca and fig. 321 A ca). The latter advance in the opposite direction, from below upwards, in the dorsal wall of the trunk, and collect the blood especially from the niesonephros. There arise from the confluence of the two veins the Cuvierian ducts (figs. 320, 321 A dc), from which are subsequently developed the two superior venae cavse. The veins of the trunk in Fishes exhibit a symmetrical arrangement like this throughout life.
In the earliest stages the Cuvierian ducts lie for some distance in the lateral wall of the pericardio-pleural cavity, where they run downwards from the dorsum to the front [ventral] wall of the trunk (fig. 320). On arriving at this point, they enter into the septum transversum, KOLLIKER'S mesocardiuni laterale, in order to reach the atrium of the heart. This important embryonic structure forms a point of collection for all the venous trunks emptying into the heart. In it there are joined to the Cuvierian ducts the veins from the viscera (fig. 313 V.om and Vu, fig. 320 dv and nv), the paired yolk veins and umbilical veins, all of which are joined into the common sinus venosus, which was previously (p. 558) mentioned apropos of the development of the heart, and which is situated directly between atrium and septum transversum.
The two vitelline veins (v. omphalomeseiitericre) return the blood from the yolk-sac ; they are the two oldest and largest venous trunks of the body, but they become inconspicuous in the same ratio as the yolk-sac shrinks to an umbilical vesicle. They run close together along the intestinal tube, and come to lie at the sides of the duodenum and stomach, where they are united to each other by transverse anastomoses even at a very early period.
The navel veins (vena? umbilicales) are also originally double. At first very small, they subsequently become, in contrast with the vitelline veins, more and more voluminous, as the placenta, from which they convey the blood back to the body of the embryo, is further developed. At the time of their first appearance the umbilical veins are found to be imbedded in the lateral wall of the abdomen (fig. 313 Vu), in which they make their way to the septum transversuni and the sinus venosus (sr).
Fig. 320. Sagittal reconstruction of a human embryo 5 mm. long, neck measurement (embryo R, His), to illustrate the development of the pericardio-thoracic cavity and the diaphragm, after His.
ab, Aortic bulb ; brh, thoracic cavity (recessus parietalis, His) ; kh, pericardial cavity ; tie, ductus Cuvieri ; dc, vitelline vein (v. omphalomesenterica) ; nv, umbilical vein ; vca, cardinal vein ; vj, jugular vein ; Ig, lung ; z + I, fundament of the diaphragm and the liver ; uk, lower jaw.
The inferior vena cava (fig. 321 A ci] is established later than any of these paired trunks. It makes its appearance as an inconspicuous, from the beginning unpaired, vessel (in the Rabbit on the twelfth day, HOCHSTETTER) on the right side of the aorta in the tissue between the two primitive kidneys ; caudalwards it is connected by lateral anastomoses with the cardinal veins. At the heart it opens into the sinus venosus.
From this primitive form of the venous system (fig. 321 A) is derived the ultimate condition in Man. There are three changes which are conspicuous in this connection. (1) The veins empty directly into the atrium instead of a venous sinus. (2) The symmetrical arrangement in the region of the Cuvierian ducts and the jugular and cardinal veins gives place to an unsymmetrical arrangement accompanied by a degeneration or stunting of some of the chief trunks. (3) With the development of the liver there is formed a special portal system.
Fig. 321. Diagram of the development of the venous system of the body.
dc, Ductus Cuvieri ; je, ji, vena jugularis externa, interna ; s, v. subclavia ; ch, v. hepatica revehens ; U, v. umbilicalis ; ci (ci 2 ), v. cava inferior ; ca (ca l , cor, ca 3 ), v. cardinalis ; ilcd, ilcs, v. iliaca communis dextra, sinistra ; ad, us, v. anonym a brachiocephalica dextra, sinistra ; cs, v. cava superior ; csd, v. cava superior dextra ; ess, rudimentary portion of v. cava superior sinistra ; cc, v. coronaria cordis ; o.z, v. azygos ; liz (kz 1 ), v. hemiazygos ; He, v. iliaca externa ; ill, v. iliaca interna ; /, v. renalis.
The alteration first mentioned is accomplished by the incorporation of the sinus venosus in the atrium. At first enclosed in the septum transversum, the sinus elevates itself above the upper surface of the latter, from which it detaches itself, and conies to lie as an appendage to the atrium in the anterior trunk-cavity. Finally it fuses completely with the heart and furnishes the smooth region of the atrial wall, which is destitute of the pectinate muscles (His).
There are in it separate openings for the two Cuvierian ducts the future venae cavae superiores and an opening distinct from them for the veins coming from the viscera below (the future cava inferior).
The metamorphoses in the region of the Cuvierian ducts begin with a change in their position. Their course from above downward becomes more direct. At the same time, like the sinus venosus, they emerge from the niveau of the transverse septum and lateral walls of the trunk into the body-cavity and carry before them the serous membrane, with which they are covered, as a crescentshaped fold, which contributes to the formation of the pericardial sac, and has been already described as the j)leuro-j)ericardial fold. By fusing with the mediastinum the Cuvierian ducts pass from the walls of the trunk into the latter and come to lie nearer together in the median plane. Of their affluents the jugular veins gradually predominate over the cardinal veins (fig. 322 B). There are three reasons for this. First, the anterior part of the body, and especially the brain, far outstrips in growth the posterior part ; secondly, there arises in this region a competitor of the cardinal veins, the inferior vena cava, which assumes in place of them the function of returning the blood. Thirdly, when the anterior limbs are established, the venae subclavise (s) empty into the jugulares. Consequently the lower portion of the jugular, from the entrance of the subclavia onward, now appears as the immediate continuation of the Cuvierian duct, and together with it is designated as superior vena cava (fig. 322 B csd).
There exists between the right and left sides a difference in the course of the superior venae cavae, which, as GEGENBAUR has pointed out, is the cause of the asymmetry that is developed in Man. While the right vena cava superior (fig. 322 B csd) descends more directly to the heart, the left (ess) describes a somewhat longer course. Its terminal portion is bent from the right to the left around the posterior [dorsal] wall of the atrium, where it is imbedded in the coronal furrow and receives the blood from the coronal vein (cc) of the heart.
In Reptiles, Birds, and many Mammals a stage of this kind, with two venae cavae superiores, becomes permanent ; in Man it exists only during the first months. Then there is a partial degeneration of the left vena cava superior. The degeneration is initiated by the formation of a transverse anastomosis (fig. 322 B as) between the right and left trunks. This conveys the blood from the left to the right side, where the conditions are more favorable for the return of the blood to the heart. In consequence of this the proximal end of the right cava becomes much larger, the left, on the contrary, proportionately smaller. Finally, there is a complete wasting away of the latter blood course (fig. 322 ess] as far as the terminal part (cc), which is lodged in the coronal groove. This part remains open, because the cardiac veins convey blood to it, and is now distinguished as sinus coronarius.
A process in many respects similar to this is repeated in the case of the cardinal veins (fig. 322 A ca). The latter collect the blood from the primitive kidneys and the posterior wall of the trunk, from the pelvic cavity and the posterior limbs. From the pelvic cavity they receive the vente hypogastrica? (Hi), and from the limbs the v. iliacae externae (He) and their continuation, the v. crurales. In this way the cardinal veins are at first, as in Fishes, the chief collecting trunks of the lower half of the body. Subsequently, however, they diminish in importance, since the inferior vena cava becomes the main collecting trunk instead of them.
Fig. 322. Diagram of the development of the venous system of the body.
ilc, Ductus Cuvieri ; je, ji, vena jugularis externa, interna ; s, v. subclavia ; r/<, v. hepatica revehens ; U, v. umbilical is ; cl (cl~), v. cava inferior; ca (ca l , ca' 2 , ccr), v. cardinal is ; ilcd, ilcs, v. iliaca communis dextra, sinistra ; ail, *, v. anonyma brachiocephalica dextra, sinistra ; cs, v. cava superior ; csd, v. cava superior dextra ; c.s-.s, nidimentary portion of v. cava superior sinistra ; cc, v. coronaria cord is ; az, v. azygos ; hz (/<:'), v. hemiazygos ; He, v. iliaca externa ; Hi, v. iliaca iuterna ; r, v. renalis.
It is only within the last few years that the development of the inferior vena cava has been (by HOCHSTETTER) explained. According to his investigations there are to be distinguished two tracts which are of dill'erent origin, a shorter anterior and a longer posterior. The former, as previously mentioned, makes its appearance as an inconspicuous vessel on the right side of the aorta in the tissue between the two primitive kidneys (fig. 322 A and B ci) ; the latter, on the contrary, is developed subsequently out of the posterior region of the right cardinal vein (fig. 322 B ci 2 ). The anterior, independently arising part of the inferior vena cava, soon after its establishment, unites with the two cardinal veins by means of transverse branches in the vicinity of the vena renalis (?*). In consequence of this increase of drainage territory, it soon increases considerably in calibre, and since it presents more favorable conditions for the conveyance of blood from the lower half of the body than the upper portion of the cardinal veins does, it finally becomes the chief conduit.
If the stage thus far described were to become the permanent condition (fig. 322 It), we should have an inferior vena cava, which forks in the region of the renal veins (r) into two parallel trunks, that descend at both sides of the aorta to the pelvis. Such cases, as is known, are found among the varieties of the venous system ; they are derived from the previously described stages of development as malformations by arrested growth. However, they are only rarely observed, for in the normal course of development there is established at an early period an asymmetry between the lower portions of the two cardinal veins, from the moment, indeed, when they have united themselves to the lower part of the inferior vena cava by means of anastomoses. The right portion acquires a preponderance, becomes enlarged, and finally alone persists (fig. 322 B, C), whereas the left lags behind in growth and withers. This results from two conditions. First, the right cardinal vein (ci 2 ) lies more in the direct prolongation of the vena cava inferior than does the left, and thus occupies a more favorable situation ; secondly, there is formed in the pelvic region an anastomosis (ilcs) between the two cardinal veins, which conducts the blood of the left hypogastrica and the left iliaca externa and cruralis to the right side. Owing to this anastomosis, which becomes the vena iliaca communis sinistra, the portion of the left cardinal vein lying between the renal veins and the pelvis (fig. 322 C c 3 ) is rendered functionless, and with the degeneration of the primitive kidney disappears. The right cardinal vein has now become for a certain distance a direct continuation of the inferior vena cava; it furnishes that portion of the latter which is situated between the renal veins and the division into the two ven;i> iliacse cornmunis (fig. 322 B and C ci 2 }.
While the abdominal part of the left cardinal vein (fig. 322 C e 3 ) succumbs and the corresponding region of the right cardinal vein produces the lower part of the inferior vena cava (ci 2 ), their thoracic portions persist in a reduced form, since they receive the blood from, the intercostal spaces (fig. 322 7? c). In this region occurs still another and last metamorphosis, by which likewise an asymmetry is brought about between the halves of the body. In the thoracic part of the body the original conditions of the circulation are altered by the degeneration of the left cava superior (fig. 322 C ess). The direct flow of the left cardinal vein to the atrium is thereby rendered more difficult, and finally ceases altogether, the tract designated by ca 2 undergoing complete degeneration. Meanwhile a transverse anastomosis (hz l ], which has been formed in front of the vertebral column and behind the aorta between the corresponding vessels of both sides, receives the blood of the left side of the body and transports it to the right side. In this manner the thoracic part of the left cardinal vein and its anastomosis become the left hemiazygos (hz and hz l ) ; the right and larger cardinal vein becomes the azygos (az).
Thus by many indirect ways, is attained the permanent condition of the venous system of the trunk, with its asymmetry and its preponderance of the venous trunks in the right half of the body.
A third series of metamorphoses, which we shall now take into consideration, concerns the development of a liver circulation.
The liver receives its blood in different stages of the embryonic development from various sources : for a time from the vitelline veins ; during a second period from the umbilical veins ; after birth, finally, from, the veins of the intestines the portal vein. This threefold alteration finds its explanation in the conditions of growth of the liver, the yolk-sac, and the placenta. As long as the liver is small, the blood corning from the volk-sac suffices for its nourishment. But when it increases greatly in size the yolk-sac, on the contrary, growing smaller other blood-vessels at this time, the umbilical veins, must supply the deficiency. When, finally, at birth the placental circulation ceases, the venous trunks of the intestinal canal, which meanwhile have become very large, can supply the needs.
These circumstances must be kept in mind, in order to comprehend the shifting conditions of circulation in the liver and the profound altrr.il ions lo which the venous trunks connected with it the vitelline, imihilic:il, and portal veins are naturally subjected in the changing supply of blood.
When the hepatic ducts grow out from the duodenum into the ventral mesentery and septum transversnm and send out shoots, they enclose the two vitelline veins accompanying the intestine, which are at tins place connected with each other by ring-like anastomoses (sinus annularis, His) which surround the duodenum (fig. 320 dv). They are supplied with blood by lateral branches given oft' from these veins. The more the liver increases in size, the larger do the lateral branches (venae hepaticse advehentes) become. Between the network of hepatic cylinders (fig. 187 lc] they are resolved into a capillary network (<y), from which at the dorsal margin of the liver large efferent vessels (vena? hepaticse revehentes) re-collect the blood and convey it back into the terminal portion of the vitelline vein, which empties into the atrium. In consequence of this the portion of the vitelline vein which lies between the vena3 hepaticse advehentes and revehentes continually becomes smaller, and finally atrophies altogether, since all the blood from the yolk-sac is employed for the hepatic circulation. The same process in the main is accomplished here as in the vessels of the visceral arches of gill -breathing Vertebrates, which upon the formation of branchial lamellae are converted into branchial arteries, branchial veins, and a capillary network interpolated between the two.
The two umbilical veins participate, even at an early period, in the hepatic circulation. Originally they run from the umbilical cord in the front [ventral] wall of the abdomen (fig. 313 Vu], from which they receive lateral branches, and then enter the sinus venosus (/Sr) above the fundament of the liver. They pursue, therefore, an entirely different course from that which they do later, when the terminal part of the umbilical vein is situated under the liver. According to His, this change in their course takes place in the following manner : The right umbilical vein in part atrophies (as also in the Chick, p. 552) and becomes, as far as it persists, a vein of the ventral wall of the abdomen. The left umbilical vein, on the contrary, gives off anastomoses in the septum transversum to neighboring veins, one of which makes its way under the liver to the sinus annularis of the vitelline veins, and thereby conducts a part of the placental blood into the hepatic circulation. Since by its rapid growth the liver demands a large accession of blood, the anastomosis soon becomes the chief course, and finally with the degeneration of the original tract receives all the blood of the umbilical veins. This, mingled with the blood of the yolk-sac, circulates through the liver in the vessels which took their origin from the vitelline veins in the venae hepaticre advehentes and revehentes. Then it flows into the atrium through the terminal part of the vitelline vein. The latter also receives the inferior vena cava, which at this time is still inconspicuous, and can therefore be designated even now, in view of the ultimate condition, as the cardiac end of the inferior vena cava.
During a brief period all of the placental blood must first traverse the hepatic circuit in order to reach the heart. A direct passage to the. inferior vena cava through the ductus venosus Arantii does not yet exist. But such an outlet becomes necessary from the moment when, by the growth of the embryo and the placenta, the blood of the umbilical veins has so increased in amount that the hepatic circulation is no longer able to contain it. There is then developed on the under surface of the liver out of anastomoses a more direct connecting branch, the ductus venosus Arantii (fig. 323 d.A), between umbilical vein (n.v) and inferior vena cava (c.i"). Thus is established and it persists until birth a condition by which the placental blood (n.v) is divided at the porta into two currents : one passing through the ductus venosus Arantii (d.A) into the inferior vena cava (c.i ") ; the other pursuing a round-about way, passing through the venae hepaticse advehentes (ha.s and ha.d) into the liver, here mingling with the blood brought to the liver through the vitelline vein (pf.a) from the yolk-sac and from the intestinal canal, which has in the meantime become enlarged, and finally passing through the venae hepaticse revehentes (h.r), also to reach the inferior vena cava (c.i").
Fig. 323. Liver of an 8-months human embryo, seen from the under surface, from GKGENBAUR. Lie, Left lobe of the liver ; r.le, right lobe ; n.r, umbilical vein ; d.A, ductus venosus Arantii ; pf.a, portal vein ; ha. s, ha.d, vena hepatica advehens sinistra and dextra ; h.r, vena hepatica revehens ; c.i', cava inferior; c.i", terminal part of the cava inferior, which receives the vente hepaticae revehentes (h.r.).
There is still something to be added concerning the development of tin' portal vein. It is to be seen in fig. 323 as an unpaired vessel. It. empties into the rig-lit aflerent hep.-itie vein, derives its roots from the region of the intestinal canal, and conveys the venous blood from the latter into the right lobe of the liver. It takes its origin from the two primitive vitelline veins.
According to the account of His, the two vitolline veins fuse along the tract where they run close together on the intestinal canal ; on the contrary, in the region where they run to the liver and are connected with each other to form two ring-like anastomoses, each of which encircles the duodenum, an unpaired trunk is formed by the atrophy of the right half of the lower [posterior] ring and the left half of the upper one. The portal vein thus formed therefore runs first to the left and backward [dorsad] around the duodenum, and then emerges on the right side of it ; it draws its blood partly from the yolk-sac and partly from the intestinal canal through the vena mesenterica. Afterwards the first source is exhausted with the regressive metamorphosis of the yolk-sac, but the other becomes more and more productive with the enlargement of the intestine, the pancreas, and the spleen, and during the last months of pregnancy conveys a large stream of blood to the liver.
The additional changes, which occur at birth, are easily comprehended (fig. 323). With the detachment of the after-birth the placental circulation ceases. The umbilical vein (n.v) conveys no more blood to the liver. That portion of its tract which extends from the umbilicus to the porta hepatis degenerates and becomes a fibrous ligament (the lig. hepato-umbilicale or lig. teres hepatis), Likewise the ductus Arantii (d.A) produces the well-known ligament enclosed in the left sagittal fissure (lig. venosum). The right and left venas hepaticre advehentes (ha.d, ha.s) again receive their blood, as in the beginning of the development, from the intestinal canal through the portal vein (pf.a).
Now that we have become acquainted with the details of the morphological changes, I close this section on the vascular system with a short sketch of the fcetal circulation of the blood. It is characteristic of this that no division into two separate circulations, into the major or systemic and the minor or pulmonary, has yet taken place ; further, that in most of the vessels neither purely arterial nor purely venous blood circulates, but a mixture of the two. Purely arterial blood is contained only in the umbilical veins as they come from the placenta, whence we will follow the circulation.
Having arrived at the liver, the current of the umbilical veins is divided into two branches. One stream goes directly through the ductns Arantii into the inferior vena cava, and is here mingled with the venous blood which returns to the heart from the posterior limbs and the kidneys. The other stream passes through the liver, where there is added to it the venous blood of the portal vein coming from the intestine ; by this circuitous course it also reaches, through the venae hepaticse revehentes, the inferior vena cava. From the latter the mixed blood flows into the right atrium, but, in consecjuence of the position of the Eustachian valve and because the foramen ovale is still open, the greater part of it passes through the latter into the left atrium. The other smaller part is again mingled with venous blood, which has been collected by the superior vena cava from the head, the upper limbs, and (through the azygos) from the walls of the trunk, and is propelled into the right ventricle and from there into the pulmonaiis. The latter sends a part of its highly venous blood to the lungs, the other part through the ductns Botalli to the aorta, where it is added to the arterial current coming from the left ventricle.
The blood of the left ventricle comes principally from the inferior cava, only a small part of it from the lungs, which pour their blood, which at this time is venous, into the left atrium. It is driven into the aortic arch and part of it is given off through lateral branches to the head and upper limbs (carotis communis, subclavia) ; the rest is carried on downwards in the aorta descendens, where the venous current of blood from the right atrium by the way of the cluctus Botalli is united with it. The mixed blood is distributed to the intestinal canal and the lower limbs, but the most of it reaches the placenta through the umbilical veins, where it is again made arterial.
In the distribution of the blood in the anterior and the posterior regions of the body a noteworthy difference is easily recognised. The former receives through the carotis and subclavia a more arterial blood, since to the stream in the aorta descendens is added the venous blood of the right ventricle through the ductus Botalli. Especially in the middle of pregnancy is this difference important. There has been an endeavor to refer to this fact the more rapid growth of the upper part of the body in comparison with the lower.
As this sketch has shown, there is everywhere a mingling of the different kinds of blood. This, it is true, is not uniform in the different months of embryonic life, because, indeed, the separate organs do not alter in size uniformly, and especially because the lungs during the later stages are in a condition to receive more blood, and finally because the foramen ovale and the ductus Botalli become narrower during the last months. On account of theso facts, less blood passes, even before birth, from the inferior vena, cava into the left atrium, and likewise less from the pulmonary artery into the descending aorta, than was the case in earlier months. Thus there is gradually introduced toward the end of pregnancy a separation into a right and a left heart, with their separate blood-currents (HASSE). But it is almost at a single stroke that this separation, in consequence of birth, becomes complete.
Great alterations are now brought about by the beginning of pulmonary respiration and by the cessation of the placental circulation. Both events cooperate to increase the blood-pressure in the left heart, and to diminish that in the right. The blood -pressure becomes reduced because no more blood runs into the right atrium from the umbilical vein and because the right ventricle must furnish more blood to the expanding lungs. In consequence of this the ductus Botalli (fig. 318 n) is closed and then converted into the ligamentuiu Botalli. Since, moreover, a greater quantity of blood now flows from the lungs into the left atrium, the pressure in the latter is increased, and since at the same time the pressure is diminished in the right atrium, the closure of the foramen ovale, owing to the peculiar valvular arrangements, is now effected. For the margin of the valvula foraminis ovalis applies itself firmly to the limbus Vieussenii and fuses with it.
By the closure of the oval foramen and the Botallian duct the division of the blood -current into a major, systemic circuit and a minor, pulmonary circuit, which was initiated before birth, is now completed.
Development of the Heart
- In the first fundament of the heart two different types can be distinguished in Vertebrates.
- First Type. In Gyclostomes, Selachians, Ganoids, and Amphibia the heart is developed from the beginning as an unpaired structure on the under [ventral] surface of the cavity of the head-gut, in the ventral mesentery, which is thereby divided into a mesocardium anterius and posterius.
- Second Type. In Birds and Mammals the heart is developed out of separate halves, which afterwards fuse with each other into a single tube, which then has the same position as in the first type.
- The second type is to be derived from the first, and is explainable as an adaptation to the great size of the yolk, in that the heart is established at a time when the splanchnopleure is still spread out flat upon the yolk and is not yet folded together to form the headgut.
- The cells which are united to form the endothelium of the heart are split off from a proliferating, thickened place of the entoderm.
- The heart is first established in all Vertebrates in the cervicocephalic region behind the last visceral arch.
- The posterior or venous end of the single cardiac tube receives the blood from the body through the oniphalomesenteric veins ; the anterior or arterial end gives off the blood to the body through the truncus arteriosus.
- In the arnniotic Vertebrates the single cardiac sac is converted by a series of metamorphoses (1) by flexures, constrictions, and changes of position, and (2) by the formation of partitions inside of it into a heart composed of two ventricles and two atria.
- The straight sac assumes the form of a letter S.
- The venous portion of the 8 comes to lie more dorsal, the arterial more ventral ; the two are marked off from each other by a constriction, the auricular canal, and are now to be distinguished as atrium and ventricle.
- The venous portion or the atrium forms lateral evaginations, the auricles of the heart, which surround from behind the truncus arteriosus.
- The formation of partitions, by which atrium, ventricle, and truncus arteriosus are divided into right and left halves, begins at three different places.
- First of all, the atrium is divided by an atrial partition into a right and a left half ; but the separation is incomplete, since there exists a passage in the partition, the foramen ovale, which remains open up to the time of birth.
- By its downward growth the atrial partition reaches the auricular canal (septum intermedium of His) and divides the opening in it into a right and left ostium atrioventriculare.
- The ventricle is divided into right and left halves by a partition (septum ventriculi) beginning at the apex of the heart ; the division is also indicated externally by the sulcus interventricularis,
- The truncus arteriosus is divided into pulmonary artery and aorta by the development of a special partition, which begins above, grows downward, arid joins the ventricular partition.
- The complete separation of the atria first takes place after birth by the permanent closure of the foramen ovaie.
- At the ostium atrioventriculare and at the ostium arteriosum the first fundaments of the valves are formed as thickenings of the endocardium (endocardia! cushions) projecting inward.
Development of the Chief Arterial Trunks of Man and Mammals
- From, the truncus arteriosus there arise five pairs of visceral arch vessels (aortic arches), which run along the visceral arches, embrace the head-gut laterally, and unite dorsally to form the two primitive aortas.
- The two vessels fuse at an early period to form the unpaired aorta lying under the vertebral column.
- In Mammals, of the five pairs of visceral-arch vessels the first and second degenerate ; the third furnishes the proximal part of the carotis interna ; the fourth arch becomes on the left side the aortic arch, on the right side the arteria anonyma brachiocephalica and the proximal part of the subclavia ; [the fifth early disappears ;] the fifth [sixth] arch gives off branches to the lungs, and becomes the pulmonary artery, but on the left side remains until the time of birth in open communication with the aortic arch through the ductus Botalli, whereas the corresponding portion 011 the right side atrophies.
- After birth the ductus Botalli is closed and converted into the ligament of the same name.
- From the aorta two pairs of large arterial trunks go to the fcetal membranes to the yolk-sac the vitelline arteries (arterise omphalornesentericse), to the allantois and placenta the umbilical arteries.
- The vitelline arteries subserve the vitelline circulation, and afterwards, with the reduction of the umbilical vesicle, degenerate.
- The umbilical arteries, which continually become larger with the increasing development of the placenta, arise from the lumbar portion of the aorta, pass forward [ventral] in the lateral wall of the pelvis, then at the side of the bladder and along the inner surface of the abdominal wall to the umbilicus and umbilical cord.
- The umbilical arteries give off the iliaca interna to the cavity of the pelvis, the iliaca externa to the lower limbs.
- After birth the umbilical artery degenerates into the ligamentuni vesico-unibilicale laterale, with the exception of its proximal part, which persists as the iliaca comniunis.
Development of the Chief Venous Trunks
- With the exception of the inferior vena cava, all venous trunks are established in pairs.
- The two jugulars collect the blood from the head, the two cardinals from the trunk, but especially from the primitive kidneys.
- The jugular and cardinal veins of either side unite to form the Cuvierian ducts, which pass transversely from, the lateral wall of the trunk to the posterior end of the heart, imbedded in a transverse fold of the front wall of the trunk, the septum transversum.
- The two vitelline veins collect the blood from the yolk-sac ; from the navel onward they run in the ventral mesentery to the septum transversum.
- The two umbilical veins collect the blood from the placenta ; from the attachment of the umbilical cord they run at first in the abdominal wall to the transverse septum.
- In the septum transversum the Cuvierian ducts and the vitelline and umbilical veins unite to form the sinus reunions, which subsequently disappears as an independent structure and is incorporated in the atrium.
- The cardinal veins diminish in importance (1) in consequence of the degeneration of the primitive kidneys, and (2) from the fact that the blood from the lower half of the body is conveyed back to the heart by the inferior vena cava.
- The upper part of the inferior vena cava arises as an unpaired, independent vessel between the two cardinal veins, and then, at the place where the renal veins empty in, unites with the right cardinal vein. The latter is in this way converted into the lower portion of the inferior cava.
- The Cuvierian ducts with the beginning of the jugular veins are designated as the venae cavse superiores.
- An asymmetry in the embryonic venous trunks, which are established in pairs, is brought about by the fact that the two superior vena3 cavse, and also at their middle the remnants of the two cardinal veins, are joined together by transverse trunks:
- Since through these cross anastomoses more and more of the blood, and finally the whole of it, is conveyed from the trunks of the left half of the body into those of the right half, the proximal part of the left superior vena cava, except a small portion, v/hich lies in the coronary groove of the heart, degenerates, receives the cardiac veins, and becomes the sinus.coronarius cordis. Likewise the cardiac end of the left cardinal vein disappears.
- From the paired fundaments of the venous trunks are formed the single superior vena cava, the sinus coronarius cordis, and the vena azygos and hemiazygos.
- The vitelline veins, which afterwards become unpaired, give rise, when the liver is developed, to the portal circulation (the venae hepaticee advehentes and revehentes).
- The umbilical veins, of which the right early degenerates, originally run in the abdominal wall above the liver to the sinus reunions ; then the left forms an anastomosis with the vitelline vein under the liver, whereby its current shares in the portal circulation.
- There arises out of an anastomosis between the umbilical vein and the cardiac end of the inferior vena cava on the under surface of the liver the ductus venosus Arantii, which results in the division of the blood of the umbilical vein into two currents.
- After birth the umbilical vein degenerates into the ligamentum teres hepatis, and the ductus venosus Arantii is obliterated ; the veme hepaticse advehentes now receive their blood from the terminal part of the original vitelline vein or the portal vein only, which collects the blood from the intestinal canal.
- The septum transversum, in which run the venous trunks on their way to the heart, is the starting-point for the development of the diaphragm and the pericardial sac, and forms at first an incomplete partition between the abdominal cavity and pleuro-pericardial cavity, which still communicate with each other on either side of the vertebral column.
- The pericardial sac is separated off from the thoracic cavity as follows : (1) the Cuvierian ducts or future superior vense cavse, instead of running transversely, run more and more obliquely from above downward, detach themselves from the septum transversum, and elevate the pleura into pericardial folds, which run from above downward and project inward ; (2) the margin of the pericardial fold fuses with the mediastinum posterius, in which are enclosed ossophagus and aorta, whereby the superior venae cavae are transferred to the mediastinum.
- The thoracic cavities have for a time the form of tubular spaces lying on the dorsal side of the heart and on either side of the spinal column ; they receive the developing lungs, and still communicate caudad with the abdominal cavity.
- The two thoracic cavities are separated from the abdominal cavity by the fusion of the dorsal rim of the septum transversum with peritoneal folds of the dorsal wall of the trunk (the pillars of USKOW).
- The diaphragm is composed of two parts, the ventral septum transversum, and a dorsal part, the pillars.
- Upon its first establishment the liver grows into the septum transversum, but subsequently detaches itself from the latter and remains united to the diaphragm by means of its peritoneal covering only, the coronal ligament.
Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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